How Do Eels Reproduce? - CLT Livre

How Do Eels Reproduce?

How Do Eels Reproduce

Do scientists know how eels reproduce?

Anguilliformes or eels are fascinating creatures with long bodies, which make them resemble snakes, Over 800 species of it exist, with some found in freshwater and the sea. Eels exist around the world, and unsurprisingly people often eat them as food.

For example, they are delicacies in some parts of Japan, Now despite their prevalence, there are still so many things we do not know about eels. In fact, we do not even know exactly how do eels reproduce. That’s right: eel reproduction is quite mysterious! This is despite centuries of studies and observations.

How is this possible? How can we not know about the reproductive habits of one of the world’s most common kinds of fish? That is what this article will explain, and more.

Has anyone seen an eel reproduce?

Until recently, nobody knew how eels reproduce. On August 14, 2021, TikTok user ColeTheScienceDude published a video named We Don’t Know Where Eels Come From. The user shares information about how little is known about the eel species Anguilla anguilla, also referred to as freshwater eels or the European eel, reproductive cycle.

He says: “We have been to the moon, and yet we still do not know how eels sexually reproduce.” The user then describes more qualities of eels that experts find puzzling, including: “if you dissect one and you look inside, you will not find sexual organs.” He added. “We have never observed eels mating in captivity or the wild.” Most Anguilla anguilla eels lack reproductive organs; however, scientists can bring them to sexual maturity by treating them with hormones.

With hormone treatment, eels can be brought together in captivity to reproduce. In the wild, however, during the last phase of their complex life cycle, they do develop reproductive organs. This final stage is known as the silver stage, and during this stage, the eels’ stomach dissolves before their sexual organs develop.

Why didn’t we know how eels reproduce?

4. Sigmund Freud was an eel fan, too – Speaking of gonads, Sigmund Freud (yes, that Freud) spent the early years of his research career trying to understand the sexual anatomy of eels, Unfortunately for Freud, and the eels, the only way to tell if an eel is male or female is to dissect it to observe it’s internal reproductive organs.

Why can’t eels breed in captivity?

Wild-caught broodstock is used to benchmark results obtained from farm-raised eels. Source: Sune Riis Sørensen, DTU Aqua Steady progress towards understanding the eel This article was featured in EUROFISH Magazine 4 / 2020, An ongoing project to further knowledge about the European eel and to close the breeding cycle brings together researchers from DTU Aqua and companies interested in farming eels.

The work in the project builds on the results from two others also coordinated by DTU Aqua. Significant progress has been made, but commercial production is probably still a decade away. The European eel ( Anguilla anguilla ) is in such dire straits that it is included in CITES’ Appendix II, a list of species not necessarily threatened with extinction, but in which trade must be controlled in order to avoid utilization incompatible with their survival.

Under EU legislation it is an Appendix B species, trade in which is permitted but under even stricter conditions than those in Appendix II. Following scientific advice from the International Council for the Exploration of the Sea (ICES) that warned that the stock of the European eel was outside safe biological limits and that urgently recommended the development of a recovery plan for the entire European eel stock, the EU enacted legislation in 2007 that required Member States to develop and implement eel management plans.

  • Research into eel has a long history DTU Aqua, the National Institute of Aquatic Resources, is an institute at the Technical University of Denmark that educates and researches at university level and off ers advice on the sustainable exploitation of aquatic resources.
  • A team comprising researchers from the institute in partnership with several representatives from the private sector is currently working on a project to breed the European eel in captivity.

The project, ITS-EEL, builds on the work done in two previous projects, the first of which, PRO-EEL, started a decade ago. But the history of research into eel goes back at least to 2004 when, what was at the time the Danish Eel Producers’ Organisation approached the Danish Fisheries Research Institute, the precursor to DTU Aqua, to initiate research into a closed production cycle to breed eel.

  1. The work today is carried out in Hirtshals on the Danish west coast, where DTU Aqua manages a 650 sq.
  2. M facility.
  3. Denmark has a long history of eel farming and in those days production was significantly higher than it is currently.
  4. But already then eel farmers had foreseen they would face problems sourcing glass eels in the desired quantities and at affordable prices.

They were not wrong—farmed production of eel in Denmark has been declining since 2009 when it was 1,700 tonnes to 450 tonnes in 2018, according to Statistics Denmark. The farmers were also highly interested in closing the breeding cycle as it removes dependence on wild catches of glass eels enabling a far more predictable production.

  1. The experience of farmers cultivating other species, whether trout, salmon, or seabass and seabream also suggested that independence from wild catches made eminent commercial sense.
  2. The challenge was reproducing the eel’s highly complex life cycle in captivity, when there were vast gaps in the knowledge about the species.

Breeding eel was therefore unlike farming any of the other species widely cultivated in Europe. To address these issues an international consortium formulated the project PRO-EEL that aimed to breed eel in captivity with a view to contributing to the development of a self-sustaining farmed production.

Among the challenges the project wanted to address was the improvement of methods to induce and finalise gamete development and to understand the nutritional requirements of the female broodstock necessary for the production of healthy eggs and larvae. Among the outcomes of the project was the stable production of viable larvae.

Commercially viable production of eel is the goal PRO-EEL was followed by another project, EEL-HATCH, that also included partners from the private sector and that focused on developing the larvae to the stage where they begin to feed prior to their transformation into glass eels.

The ambition was to develop larval feeding protocols and hatchery technology and to test them at a commercial scale. Dr Jonna Tomkiewicz, Senior Scientist at DTU Aqua, who coordinated both projects, points out that the industry does not benefi t from the production of glass eels that are too expensive to be commercially viable.

It is critical therefore to carry out large-scale trials during the project itself rather than subsequently. EEL-HATCH concluded in December 2017 and the results are being followed up on in ITS-EEL, where the goal is to establish larval culture technology for a hatchery production of glass eels for closedcycle European eel aquaculture.

  • The efforts that are going into studying the eel are intended both to throw light on a fish, parts of whose lifecycle remain an enigma, as well as to enable a captive breeding programme for aquaculture and possibly for restocking purposes.
  • Restocking presently assists the wild catch sector whose capture of eel decreased significantly from 423 tonnes in 2010 to 182 tonnes in 2019, reports the Danish Fisheries Agency.

For eel is a valuable species—medium-sized eel, smoked, wholesales for EUR47/ kg in France, according to FAO Globefish’s European Price Report. Denmark’s exports of eel (in different product forms) has also shrunk with the decline in farmed and capture production.

  • Closing the cycle and breeding the fish in containment thus has potential economic, social, and environmental benefits.
  • Th is however is more easily said than done.
  • The European eel is catadromous (migrates from freshwater to the sea to spawn) and semelparous (spawns only once in its lifetime).
  • It has a complex life history, parts of which are still shrouded in mystery, but it is fairly well established that eels spawn in the southwestern part of the Sargasso Sea.

Th is is body of water within the Atlantic Ocean and the only sea in the world without a land boundary. Instead, its borders are defined by currents, the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south.

When the fertilised eggs hatch, the larvae feed on the yolk sac initially and then graduate to feeding, though precisely what they feed on is yet to be established. Theories abound, says Dr Tomkiewicz, gelatinous plankton and marine snow (organic detritus falling from the upper layers of the ocean to the bottom) are possible candidates, but neither what they eat nor where in the water column they feed is known for certain.

Th e lack of this kind of knowledge means that the researchers in the ITS-EEL project are often operating in the dark, all the information has to be created by the researchers themselves based on their observations and trials—of feeds, rearing conditions, and microbiology of the water.

Parts of the eel’s life cycle is still a mystery In the wild, the European eel larvae are carried by currents from the Sargasso Sea to the shores of western Europe. It is a long and hazardous journey. As larvae they are feed for predators, such as cod, herring and mackerel, and when they develop into glass eels, the next stage of their lifecycle, they are hunted by fishers in particular from Portugal, Spain, the UK, and France as the fish leave the sea and head upstream.

They now enter the growth phase of their lives, when they are called yellow eels, which can last from two to 25 years and culmi nates when they transform into silver eels. At this stage they begin their long migration back to the Sargasso Sea, whence they came as larvae, to spawn and then die, an act that has so far not been witnessed.

  1. Researchers have established that the development of the ovaries and the testes is controlled by a complex hormonal mechanism.
  2. At the start of the migration to the Sargasso Sea, an inhibitory mechanism kicks in to prevent the development of these organs.
  3. As the fish approach their destination this mechanism is deactivated allowing the development of these organs to continue and the gametes (eggs and sperm) to form.

In captivity (or in European waters), eels do not breed naturally because of this inhibition of the development of their reproductive organs. Assisted Reproductive Technologies used to breed eels Dr Tomkiewicz explains how she and her colleagues overcome this hurdle.

  • For about two years, until they reach an appropriate size for broodstock, female fi sh are fed a specially formulated diet which combines essential fats, proteins, and vitamins in the proportions necessary for the eggs to have the correct ratio of these nutrients.
  • The fish are then transferred to a system where they are subjected to assisted reproduction procedures.

This involves treating them with hormones to induce them to produce gametes and then further with a steroid to provoke maturation and ovulation of the eggs. The eggs are then stripped and mixed with sperm removed from male fish. These male fish are usually from a local farmer.

In a paper (*) in which she and her colleagues explore four issues: broodstock establishment and dietary requirements; assisted reproduction procedures; fertilisation techniques and incubation technology; larvae culture techniques and dietary requirements, Dr Tomkiewicz notes that comparisons between wild and farmed broodstock reproductive success, such as fertilisation and hatching success and larval deformities, show that while the provenance of females matters, the male fish’s origin does not have a significant influence on these factors.

Therefore, studies focus on female diets and assisted reproduction protocols in order to enhance egg quality and the production of healthy off spring, thereby reducing proportions of off spring with deformities— an issue to be considered when breeding eels.

The fertilised eggs are incubated and hatch after two days. Th e larvae feed on the yolk sac for the first 10 to 14 days (within the limits, the lower the water temperature the longer the yolk sac lasts). During this phase, the researchers studied the influence of temperature, light, salinity, and microbial activity with a view to improving rearing conditions and thereby survival rates.

They established that temperature influenced several traits including time to hatch, hatching success, and incidence of deformities, and that 18-20 degrees C was the optimal level for higher growth, fewer deformities, and lower stress. Mapping the factors that affect successful embryo/larva production Light affected both embryos and yolksac larvae, and after studying the reactions the researchers concluded that both did best under a 12 hour:12 hour light/dark photoperiod and low-intensity light regime, and, in addition, larval survival was better under red light than under green or white light.

  • Although eels can tolerate a wide range of salinities (euryhaline) and, being catadromous, their eggs and larvae develop in salt water, the researchers found that gradually reducing salinity improved growth and led to a four-fold increase in survival.
  • This they attributed to the creation of an energy surplus due to the reduction in metabolic demands from osmoregulation.

This excess energy may be used for somatic development resulting in improved survival and growth efficiency. Lowering salinity also reduced deformities such as spinal curvature and emaciation. These results enabled the researchers to improve incubation and larval culture technologies and produce large numbers of healthy larvae.

  1. In spring 2019 over a period of five months they produced 5.5m larvae, while this year they have more than 200,000 going into feeding experiments.
  2. Identifying the first feeding diet starts with studying mouth parts Scientists have analysed the stomach contents of eel larvae living in their natural environment, but so far have been unable to fully identify what they feed on.

In ITS-EEL, efforts are aimed at improving larval culture technologies with a focus on rearing conditions and on feeds suitable for on-growing into the leptocephalus larval stage—the migratory stage that transforms into glass eels on arrival at continental waters.

Dr Tomkiewicz and her colleagues are therefore studying the morphology of the larval feeding apparatus for clues as to what European eel larvae might eat. By estimating the biting force and the size of the particles they can ingest, they concluded that larvae have a preference for very soft and/or small food organisms and/or particles.

They also tested potential diets, studied swimming behaviour, looked at the effect of light on feeding, and investigated physiological mechanisms at the molecular level. For instance, a diet of enriched rotifers, concentrated and ground into a paste was consumed by up to 50% of the larvae in the experiment, throwing light on larval feeding biology.

In addition, the researchers noted that larvae were able to execute complex swimming behaviours to capture their food. At higher light intensities ingestion improved, suggesting that larvae use light to detect their food. Other tests showed that they also used other stimuli (taste and smell) to capture prey.

At the molecular level, larvae on the enriched rotifer diet, showed higher levels of protein digesting enzymes compared with enzymes to digest carbohydrates or fats indicating a nutritional predisposition for proteins. Th e results explain some of the physiological changes the larvae undergo as they develop from newly hatched animals feeding on the yolk sac to creatures that must actively catch their own food.

Research into eel has made significant progress over the last decade or so. But more needs to be done before the goal of a completely closed culture of eel at a commercial scale is achieved. Dr Tomkiewicz is confident though, that within 10 years eel farmers will be able to base their production on glass eels produced from broodstock in a hatchery.

(*) Tomkiewicz, J., Politis, S.N., Sørensen, S.R., Butts, I.A.E., & Kottmann, J.S. (2019). European eel – an integrated approach to establish eel hatchery technology in Denmark. In A. Don, & P. Coulson (Eds.), Eels – Biology, Monitoring, Management, Culture and Exploitation: Proceedings of the First International Eel Science Symposium (pp.340-374).5M Publishing.

Do marine biologists know how eels reproduce?

And we’re losing the chance to ever find out – A family of Moray eels. At a time when mankind has touched the moon and explored the deepest points of our harrowing ocean, not many would think that one of the mysteries still plaguing our world has to do with the reproduction of eels. And yet this unassuming creature has eluded and beguiled us with its mystery of life.

Not only have eels never been seen mating in the wild, their eggs have never been found nor have their reproductive organs been easy to find despite hundreds of dissections over thousands of years. Many of these dissections were carried out by none other than Sigmund Freud himself. He sliced the long, slimy creatures open in the hopes of being the first to find their reproductive organs.

He was so possessed by this objective that at one point he wrote, “My hands are stained by the white and red blood of the sea creatures. All I see when I close my eyes is the shimmering dead tissue, which haunts my dreams, and all I can think about are the big questions, the ones that go hand in hand with testicles and ovaries — the universal, pivotal questions.” Despite this, Freud would never find what he was looking for.

  1. Theories during this time included everything from spontaneous conception from mud, to eels coming from beetles and horse hair and the meaty, red gills of fish.
  2. The Egyptians believed eels formed when the sun’s warm rays fell on the Nile.
  3. Little blame can be placed on Freud or his companions.
  4. Even with our modern technology we are not much closer to discovering this enigma, though our theories are more scientifically sound.

The enigma has stood because the story of the eel’s life is one of profound transformation and intrepid journeys, voyages few other creatures could ever undertake. The electric eel, shown above, is not an actual eel. True eels travel from freshwater to saltwater to lay their eggs but electric eels lay theirs in freshwater. Electric eels also have to surface for air and are missing the lengthy dorsal fin that should combine with their caudal and anal fin.

How do eels reproduce 2023?

They spawn, according to researchers, through external fertilization. Millions of eggs are released into the water by the females, where the male’s sperm fertilizes them. In a dark turn of events, the eels then die.

Do eels change gender?

Home Ocean Life Fish Blue Ribbon Eel

photo All blue ribbon eels undergo an immense transformation within their lifetime. At birth, they begin life as males and as they mature they make the switch and become females. Although seemingly remarkable, in the fish world, this occurs more than you might think.

Why are eels such a mystery?

A European eel swims in the Baltic Sea. Reinhard Dirscherl / ullstein bild via Getty Images Every three years, Reinhold Hanel boards a research ship and voyages to the only sea in the world that’s located in the middle of an ocean. The Sargasso, bounded by currents instead of land, is an egg-shaped expanse that takes up about two-thirds of the North Atlantic, looping around Bermuda and stretching east more than 1,000 kilometers.

Dubbed the “golden floating rainforest” thanks to the thick tangles of ocher-colored seaweed that blanket the water’s surface, the Sargasso is a slowly swirling sanctuary for over 270 marine species. And each year, the eels arrive. The European eel and the American eel—both considered endangered by the International Union for Conservation of Nature—make this extraordinary migration.

The Sargasso is the only place on Earth where they breed. The slithery creatures, some as long as 1.5 meters, arrive from Europe, North America, including parts of the Caribbean, and North Africa, including the Mediterranean Sea. Hanel, a fish biologist and director of the Thünen Institute of Fisheries Ecology in Bremerhaven, Germany, makes his own month-long migration here alongside a rotating cast of researchers, some of whom hope to solve mysteries that have long flummoxed marine biologists, anatomists, philosophers, and conservationists: What happens when these eels spawn in the wild? And what can be done to help the species recover from the impacts of habitat loss, pollution, overfishing, and hydropower? Scientists say that the answers could improve conservation.

  1. But, thus far, eels have kept most of their secrets to themselves.
  2. The idea that eels have sex at all is a fairly modern notion.
  3. Ancient Egyptians associated eels with the sun god Atum and believed they sprang to life when the sun warmed the Nile.
  4. In the fourth century BCE, Aristotle proclaimed that eels spontaneously generated within “the entrails of the earth” and that they didn’t have genitals.
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The no-genital theory held for generations. Roman naturalist Pliny the Elder asserted that eels rubbed against rocks and their dead skin “scrapings come to life.” Others credited eel provenance to everything from horses’ tails to dew drops on riverbanks.

  • In medieval Europe, this presumed asexuality had real economic consequences and helped make the European eel a culturally important species, according to John Wyatt Greenlee, a medieval cartographic historian who wrote part of his dissertation on the subject.
  • Frequent Christian holidays at the time required followers to adhere to church-sanctioned diets for much of the year.

These prohibited adherents from eating “unclean” animals or meat that came from carnal acts, which could incite, as Thomas Aquinas put it, “an incentive to lust.” Fish were the exception, Greenlee says, and eels, given their abundance and “the fact that they just sort of appear and that nobody can find their reproductive organs at all,” appealed to anyone trying to avoid a sexy meal. Young eels swim in a transport box from which they are released by commercial fishermen into the Havel River and its tributaries. Jens Kalaene / picture alliance via Getty Images Eels could be practically anything to anyone: dinner or dessert; a cure for hangovers, drunkenness, or ear infections; material for wedding bands or magical jackets.

  • They were even used as informal currency.
  • Since yearly rent and taxes in medieval Europe were often due during Lent—the roughly 40-day period preceding Easter—and monasteries owned land people lived on, tenants sometimes paid with dried eels.
  • Entire villages could pay 60,000 eels or more at once.
  • Eventually, spontaneous generation theories died.

But eel genitals landed in the spotlight again after an Italian surgeon found ovaries in an eel from Comacchio, Italy, and the findings were published in the 18th century. The legitimacy of the so-called Comacchio eel remained in question for decades until an anatomist published a description of ovaries from a different Comacchio eel, launching a race to find testicles.

  • Even the granddaddy of psychosexual development theory got involved: near the beginning of his career, in 1876, Sigmund Freud dissected at least 400 eels in search of gonads.
  • It would be about another two decades before someone discovered a mature male eel near Sicily.
  • It’s no surprise that it took so long to find eel sex organs.

There are more than 800 species, about 15 of which are freshwater varieties, and their bodies change so dramatically with age that scientists long thought the larvae were a different species than adult eels. Eels transform from eggs to transparent willow-leaflike larvae, to wormy see-through babies called glass eels, and onward until full size.

Like most eel species, American and European eels don’t fully develop gonads until their last life stage, usually between 7 and 25 years in. Around that time, they leave inland fresh and brackish waters, where people can easily observe them, and migrate up to about 6,000 kilometers—roughly the distance from Canada’s easternmost tip to its westernmost—to the Sargasso.

By now, researchers have seen eels mate in lab settings, but they don’t know how this act plays out in the wild. The mechanisms that guide migration also remain somewhat enigmatic, as do the exact social, physical, and chemical conditions under which eels reproduce.

  1. Mature eels die after spawning, and larvae move to freshwater habitat, but when that happens and how each species finds its home continent are also unknown.
  2. We think that the European eel reproduces in the Sargasso Sea because this is the place where we have found the smaller larvae, but we have never found a European eel egg or the eels spawning,” says Estibaliz Díaz, a biologist at AZTI marine research center in Spain, who studies European eel population dynamics and management.

“It’s still a theory that has not been proven.” The same applies to the American eel, and yet more questions remain about how many eels survive migration, what makes the Sargasso so singular, and how factors like climate change might affect it. Both species have dropped in number, but researchers debate which threat is the biggest.

  • Habitat loss is huge—humans have drained wetlands, polluted waters with urban and agricultural runoff, and built hydropower turbines that kill eels and dams that block the animals from migrating in or out of inland waters.
  • Fishing further reduces eel numbers.
  • Commercial fisheries for adult eels exist, but most eels consumed globally come from the aquaculture industry, which pulls young glass eels from the wild and raises them in farms.

American and European eels are among the top three most commercially valuable species alongside the Japanese eel, which is also endangered. While it’s legal to fish for all three, regulations on when, where, and how many eels can be sold vary between countries.

The European Union requires member nations to close their marine fisheries for three consecutive months around the winter migration season each year—countries themselves determine exact dates—and prohibits trade outside of member countries, but these management efforts are undermined by black-market traders who illegally export more than 90 tonnes of European eels to Asia every year.

The International Union for Conservation of Nature (IUCN) lists European eels as critically endangered—populations have plummeted more than 90 percent compared with historical levels, and it’s “rather unclear,” as one report notes, whether the decline continues today.

By counting glass eels in estuaries and inland waters, researchers found that eel numbers dropped precipitously between the 1980s and 2011, but plateaued afterward without clear cause. American eels are thought to be faring better—they’re considered endangered only by IUCN standards, not by other conservation and research groups—though their numbers have also decreased since the 1970s.

Captive breeding might one day reduce the aquaculture industry’s dependence on wild catches, but isn’t yet viable. Scientists must induce eel gonad development with synthetic hormones. It’s also hard to keep larvae alive. Many researchers believe that, in their natural habitat, larvae eat marine snow—a mélange of decaying organic matter suspended in the water that is impractical to reproduce at commercial scales.

Illuminating what happens in the Sargasso could help guide better conservation measures. That’s why Reinhold Hanel heads to sea. After three years of COVID-19-related delay, in 2023, Hanel will send a research vessel on a 14-day trip from Germany to Bermuda. He’ll fly there and meet up with 11 other eel researchers, then he’ll spend about a month slowly traversing the southern Sargasso, recording ocean conditions, trawling for eel larvae with mesh plankton nets, and sampling for environmental DNA—genetic material shed from skin, mucus, and poop—to track eels by what they leave behind.

Hanel has led voyages like these since 2011. His main goal is to document the abundance of larvae and young eels and, secondarily, to identify possible locations for spawning. By sampling estuaries and inland waters, researchers can identify trends over time to figure out if glass eels in continental waters are increasing or not, but without comparing those trends with similar ones in the Sargasso, it’s impossible to judge whether either American or European eels are bouncing back.

Meanwhile, protective regulations aren’t enough, Hanel contends. In 2007, the European Union mandated that member countries develop European eel recovery plans, but several prominent fishery and marine science organizations have criticized the particulars, In tandem with other measures aimed at reducing eel mortality, provisions like closing fisheries make sense, Hanel says—last year, an international consortium of researchers, of which Hanel is a member, recommended closing fisheries until glass eel stocks recover.

But other requirements aren’t rooted in research, including one to ensure 40 percent of adult eels survive to migrate from inland waters to the sea each year. “Scientists cannot say if 40 percent is sufficient to recover the stock,” Hanel says. That’s why Hanel’s work is so important, says Martin Castonguay, a marine biologist and scientist emeritus at Fisheries and Oceans Canada, who has collaborated with Hanel.

Financial obstacles often prevent eel scientists from conducting research outside of inland waters. Research vessels can cost anywhere from CAN $30,000 to $50,000 per day, or just under $1-million for a month-long trip, Castonguay says, requiring scientists to have hefty grants or government support to venture all the way to the Sargasso.

Despite the barriers, scientists keep trying to find answers to how to help eels recover. They have planted hydroacoustic devices in hopes of tracking migrating eels by sound, pored over satellite photos, and injected eels with hormones to induce gonad development before releasing them into the Sargasso to try to study how deep beneath the surface they spawn.

  • Back at home in the lab, they’ve developed algorithms to scan for and spot eels in sonar images of inland waters and built hyperbaric swimming tubes to observe how eels respond to changes in pressure and current strength.
  • They’ve even tried to follow them with satellite transmitters.
  • In the mid-2010s, Castonguay and four other researchers sewed buoyant trackers to 38 American eels and released them off the coast of Nova Scotia.

Every 15 minutes, the trackers recorded the depth at which the eels were swimming, the water temperature, and light levels. The sensors were designed to detach several months later and transmit the data along with the eels’ final location. Unfortunately, they detached before the eels reached any specific spawning locations, though one eel got as close as 100 to 150 kilometers from the spawning region.

  1. Still, “it was the first time that an eel was documented in the Sargasso,” says Castonguay.
  2. Previously, only larvae had been found there.
  3. We were extremely excited.” If more governments and research institutions were willing to spend the resources, Castonguay adds, these eels wouldn’t be so mysterious.

Research on a similar species in Japan offers a case study for how that could work. On the other side of the globe from the Sargasso, the Japanese eel makes a 3,000-kilometer annual migration from Japan and surrounding countries to the West Mariana Ridge in the western Pacific Ocean.

With support from the Japanese government and other scientific institutions, researchers there have identified a spawning location, collected fertilized eggs, and tracked tagged eels swimming to their spawning area—all feats never attained in the Sargasso. They’ve found that Japanese eels spawn over a period of a few days before the new moon, at depths of 150 to 200 meters, and that spawning is triggered in part by temperature shifts that happen as eels move from deep to shallower water.

Some eels, they learned, might spawn more than once during a spawning season. Public outreach efforts have also been important, says University of Tokyo eel biologist Michael Miller. The researcher who led most of the eel work, Katsumi Tsukamoto—a University of Tokyo scientist emeritus known as Unagi Sensei, or Dr.

Eel—has worked hard to raise the eels’ public profile. His findings have helped build the case that eels are “something other than just a meal,” Miller says. “It’s something part of the Japanese culture and it’s worth conserving,” which has helped boost efforts to protect them. Hanel is trying to do the same for the eels of the Sargasso and for other species.

He speaks to the press and the public as often as he can. He believes, as many others do, that successfully conserving these creatures hinges on whether there’s a unified international effort to do so. But so long as data snapshots come only every few years, answers to questions about spawning and species well-being will stay hidden somewhere in the watery depths, just like the eels themselves.

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Why did eels become electric?

How electric fish evolved their electric organs Electric organs help fish such as the electric eel do all sorts of amazing things. They send and receive signals that are akin to bird songs, helping them recognize other electric fish by species and sex. Now a -funded study published in explains how small genetic changes enabled electric fish to evolve their electric organs.

  • The finding might also help scientists pinpoint the genetic mutations behind some human diseases.
  • Evolution took advantage of a quirk of fish genetics to develop electric organs.
  • All fish have duplicate versions of the sodium channel gene that produces tiny muscle switches.
  • To evolve electric organs, electric fish turned off one duplicate of the gene in muscles and turned it on in other cells.

The tiny switches that typically make muscles contract were repurposed to generate electric signals, producing a new organ with astonishing capabilities. “We can see how a small change in a gene can completely change where it’s expressed,” said Harold Zakon, a neuroscientist and integrative biologist at and corresponding author of the study.

  1. Researchers from UT Austin and Michigan State University describe discovering a short section of this sodium channel gene – about 20 letters long – that controls whether the gene is expressed in a cell.
  2. They confirmed that in electric fish, the control region is altered or entirely missing.
  3. That’s why one of the two sodium channel genes is turned off in the muscles of these fish.

But the implications go far beyond the evolution of electric fish. “This control region is in most vertebrates, including humans,” Zakon said. “The next step in terms of human health would be to examine this region in databases of human genes to see how much variation there is in normal people, and whether some deletions or mutations in this region could lead to a lowered expression of sodium channels, which might result in disease.” : How electric fish evolved their electric organs

Do eels have a purpose?

Eel remains a mystery, except in its importance to ecosystem James Prosek has spent much of his professional life writing about and painting pictures of glamour fish — the trout, the salmon — that are appealing to the eye and the palette. Born the same week and year I graduated from college (sigh), Prosek has 10 fishing books to his credit (double sigh).

  1. Galleries have had shows featuring his fish paintings and he won a Peabody — broadcasting’s equivalent of the Pulitzer — for his ESPN documentary on Izaak Walton, the 17th-century author and trout angler.
  2. He helped create the Yale Anglers Journal while a student at the university, a periodical I recommend to anyone who enjoys reading lyrical pieces about fish and fishing.

But instead of writing another pretty book about another pretty fish, Prosek has turned his artist’s eye to eels. You know, creepy, slithery, evil-looking critters that Chesapeake Bay anglers use for bait with little thought about their origin. Tough sell, right? As we talk on the phone, he laughs and concedes, “The eel is not an easy fish to like.

  • People feel uneasy about eels and I think it’s because they don’t fit in any clear category.
  • Is it a snake? Is it a fish? People want to pin everything down and they can’t with the eel.” By taking us with him on a globe-trotting adventure that spanned more than a decade, Prosek explores how the eel has been part of history, religion and culture for centuries.

He follows scientists on the Sargasso Sea, believed to be the spawning ground for all North American and European eels; watches Japanese merchants at Tsukiji, the world’s largest seafood market; and listens to fisherman and folklorists in Micronesia.

  • I love the fact that for all we know, this fish has been able to retain its mystery,” Prosek said.
  • I’m not tired of eels, even after 11 years.” The eel admiration society also includes Rachel Carson, the Sun’s outdoors writer in the 1930s (writing under the name R.L. Carson).
  • In an Oct.9, 1938 feature, Carson wrote: “,

the most remarkable of all Chesapeake Bay fishes is born in alien waters. Before it is half as long or as thick as a man’s thumb, it makes a journey across 1,000 miles of strange, wild waters without benefit of chart or compass, finding the shores from which its parents came a year and a half before.

In bays, rivers and streams, it feeds and grows for 10 years, perhaps 15 or 20. At last, obeying an instinct as old as the tribe of eels, it sets out on the return journey to the Sargasso to produce its young and itself to die. Thus the life cycle of the eel is completed.” Eels are catadromous, the only fish to spawn in the ocean but live their adult lives in freshwater rivers and streams.

But man has done his damndest — literally — to break the life cycle by building blockages across waterways, a practice that has ended and, thankfully, is being reversed. While in the 1890s, Maryland lawmakers tried to eradicate eels (and appropriated $3,400 for the job), state officials now are working with American Rivers and the National Oceanic and Atmospheric Administration to remove dams and restore habitat.

More than $1 million was spent to take out Union Dam on the Patapsco River and Simkins Dam will be gone by the end of the year, another job that will cost nearly $1 million. Why such a fuss over American eels? Keith Whiteford, the eel man for the Department of Natural Resources, says the fish are important to Maryland economically — it is one of the state’s most productive fisheries — and as part of the food chain.

“Ecologically eels are very important,” he said. “They are a top-order predator in freshwater streams, which help regulate the population of other animals. They themselves are a significant source of food for fish, mammals, turtles and birds. They can be used as a bio-indicator for pollution since they are long lived — up to 15 years in Maryland — and often spend their time in the same system.” Prosek also points out the ecological importance of the American eel by contrasting two nearby watersheds: the Susquehanna River and the Delaware River.

  • Surveys show the Delaware has about two million freshwater mussels living in each mile of the waterway that filter water and feed other critters.
  • The Susquehanna, on the other hand, is a no man’s land for mussels.
  • The difference is that while the Susquehanna has large hydro dams spanning its lower main stem, the Delaware has few blockages, making it a superhighway for migrating eels and the mussel larvae that hitchhike on their skins.

As a result, Prosek notes, each day mussels are filtering two billion gallons of water per mile of the Delaware. Despite all of his research — and the research of the scientists he profiles — the eel remains a shadowy character. We really don’t know how they find their spawning ground and how the baby eels find their way here.

Are eels intelligent?

Moray eels: The most cosmopolitan of reef fish, but why? Joshua Reece became interested in moray eels in 2005 when he was applying to the PhD program at the University of Hawai’i. Instead of taking him on a campus tour, his host, Brian Bowen, PhD, a biologist at the university, took him on a dive.

Along the southwest coast of Oahu, Reece looked under a rock ledge and was startled to see five different species of moray eels looking back. When he later captured the eels, he found the same fish species in all of their bellies. Reece immediately recognized the five eels as a dissertation project on a platter.

“How can you have seven species of the same fish eating the same thing and, quite literally, living under the same rock?” he asks. “Species don’t do that; if they exploit the same niche they don’t diversify, and if they diversify they don’t exploit the same niche.” What was up with the eels? Reece, who is currently a graduate student in biology in Arts & Sciences at Washington University in St.

  • Louis, still doesn’t know the answer but having just completed the first phylogeographic survey of the Muraenidae, the family to which the eels belong, he now has a better sense of the problem.
  • The study, published in the online edition of the Journal of Heredity, was co-authored by Bowen, Allan Larson, PhD, professor of biology at Washington University and Reece’s advisor, and two biology undergraduate students at Washington University: Kavita Joshi, now a student at the Washington University School of Medicine, and Vadim Goz, now at Mount Sinai Medical School.

Reece and his colleagues collected two species of eels, the undulated moray ( Gymnothorax undulatus ) and the yellow-edged moray ( G. flavimarginatus ) at a dozen different locations across the Indo-Pacific ocean. They were looking for genetic differences that might indicate interruptions in gene flow among populations of the eels either now or in the past.

  • Their results qualify the morays as the most cosmopolitan of reef fish but only deepens the mystery of how they – and the other 150 species of moray eel in the Indo-Pacific – formed separate species in the first place.
  • Nasty Customers
  • One advantage of studying moray eels is that you’re unlikely to be scooped by another doctoral student – not if it means diving on reefs to collect eels.
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Eels have many traits that suggest a lively intelligence. They are known, for example, for going on hunger strikes in captivity, and they hunt cooperatively with groupers in the wild. On the other hand, they’re also famous for sudden and egregious acts of aggression.

Taped to Reece’s office door is a meat-red photograph of a spectacular eel injury, a flayed forearm. He says he keeps it to remind himself to be careful around these fish. “Morays have two sets of jaws,” Reece explains: “Oral jaws and a set of jaws in their throat called pharyngeal jaws. Set into both pairs of jaws are big rear-facing fangs.

When the eels attack prey, they actually bite them twice. They skewer prey with their oral jaws and then they launch the pharyngeal jaws into their mouths to drag it down their throats. What’s more, says Reece, once they bite, they often won’t let go. You have to crush their skulls to get them off of you.

How do you collect an animal that served as the inspiration for the movie Alien ? Reece initially tried poking holes in a steel can, stuffing it full of fish and hanging it in a trap he threw into about four feet of water. “Within about 10 minutes,” he says, “a four-foot moray swam up, nosed the trap, and swam into its opening.

‘Oh, I thought, this is going to be easy.'” “But then the moray bit the can in half in one bite. ‘Whoa, I thought.’ And then the moray turned around and bit through the mesh trap and swam away, taking its DNA with it.” Not so easy after all. In the end the team used two collection methods.

Some eels were caught in hook-festooned funnel traps. “I’ve caught as many as 100 between four traps in a night.” Reece says. Others were darted underwater. The biopsy dart, mounted on a seven-foot spear, takes a piece of tissue about the size of a pencil eraser, Reece says, and as a bonus dissuades the moray from molesting the diver.

His team collected 289 eels with only two injuries: a bite on a hand and one on a foot (of a documentary filmmaker who was tagging along with the scientists). Ocean swimmers Less than one percent of the ocean consists of appropriate reef habitat, and adult reef fishes prefer to hang out at home, hiding from predators in the coral.

  1. The answer is that most reef fish have a juvenile stage called a pelagic (oceanic) larva that lets them disperse over open ocean.
  2. The larvae either swim or drift in surface currents, sometimes modifying their buoyancy to use both surface and deep currents, writes Reece in the Journal of Heredity.
  3. So important is this wandering phase that some biologists have proposed as a rule of thumb that the longer-lived the larvae (the longer the “pelagic duration”) the more genetically homogeneous the species is likely to be.

Moray eels provide an ideal test of this idea. They are poor swimmers as juveniles, and adults stick to a few square meters of reef. On the other hand, they have an extremely long-lived pelagic form. Whereas another fish’s pelagic larvae might live for a month or two, moray eel larvae can persist for several months or even years.

  • The slender, elongate larva, called a leptocephalus, is one of the simplest self-sustaining vertebrate forms, says Reece.
  • Its body wall is only one cell thick and it has no digestive tract to speak of.
  • Most reef-fish larvae feed on plankton in the water column,” Reece says.
  • Moray larvae are so simple, they can’t digest plankton, so they feed on the shed exoskeleton of plankton and on plankton excrement.” “It’s ironic these vicious predators start out too delicate to eat even plankton – they have to eat plankton poop and work their way up,” he says laughing.

Barriers in the ocean? In biology textbooks the go-to mechanism for speciation (or the splitting of a population into two groups that no longer interbreed) is geographic isolation. One group gets separated from the other, by for example, the formation of a mountain, and then over time natural selection and genetic drift gradually remodel the two similar groups into two dissimilar ones.

  1. The question is whether there are barriers in the ocean that play the same role in speciation as the mountain ranges do on land.
  2. Two suspects are the Eastern Pacific Barrier and the Sunda Shelf.
  3. The Eastern Pacific Barrier is a stretch of open ocean 3000 to 5000 kilometers wide that separates the Hawaiian archipelago and the Central Pacific Islands from the Americas.

“That’s just too much open ocean for larvae to cross,” says Reece.

  • The Sunda Shelf, on the other hand, is an area of relatively shallow water between the Indian Ocean and the Pacific Ocean that has been repeatedly exposed when sea levels fell during periods of extensive glaciation.
  • Were the undulated and yellow-edged morays on either side of these barriers distinguishable genetically?
  • To find out the biologists looked at selected mitochondrial and nuclear genes and asked whether there were unique alleles (variants) of these genes and whether the degree of variance was correlated with geographic separation.

They found lots of variation among the eel genes they examined, but virtually none of it had any geographic structure. In other words where the eel was living didn’t predict its genetic makeup. “Some of the sampling sites are about 22,000 kilometers apart, so essentially two-thirds of the globe,” says Reece.

“Yet you find the same alleles in South Africa as you do in Panama.” “The only way to get shared genetic material across these distances,” says Reece, “is through gene flow.” The leptocephali, in other words, are efficiently ferrying genes from one population to the next, keeping the gene pool well stirred.

Several other reef fishes are also known to have highly dispersive pelagic stages. However, even these species show genetic partitions at major biogeographic barriers. The moray eels are in a class unto themselves, the champion dispersers.

  1. Unsolved mystery
  2. Although the phylogeographic survey answered some questions, it raised others.
  3. If both species of eel are able to maintain genetic connectivity across the entire ocean basin, how did the species arise in the first place?

And not just these two species. Altogether there are 150 species of moray eel in the Indo-Pacific, including the giant, ribbon, snowflake, zebra, dragon, pink-lipped and freckled morays, the slenderjaw, hookjaw, viper, vagrant, lipspot and enigmatic eels,

and many more. When and how did they form separate species if their larvae make them nearly impervious to geographic isolation? To further complicate things, says Reece, many of eel species share habitat, share distributions and share prey items. In most groups, he says, when that happens one species outcompetes the other and the loser disappears.

The same rules don’t seem to apply in the morays and nobody knows why. In other words, he may have found his postdoctoral as well as his doctoral research lurking under that ledge. : Moray eels: The most cosmopolitan of reef fish, but why?

Do eels become male?

Illustration: Nicole Elmer

If someone asks you to imagine Sigmund Freud, what do you see? An older gent with a well-trimmed white beard, cigar in hand? Is he perhaps listening to a patient who talks freely about personal issues while laying on a couch? This is the Freud most know as the famous neurologist and founder of psychoanalysis.

  1. But well before Freud developed theories such as the “Oedipus complex” or “Penis envy,” he studied zoology, and had quite a time with European eel ( Anguilla anguilla ) sex specifically.
  2. Until somewhat recently, the reproductive life of the European eel was a great mystery.
  3. No one had seen them laying eggs, giving birth, or mating.

To add to the mystery, eels lack genitalia and determining their sex requires dissection and careful examination of their gonads. The surprise to early scientists was that males appeared to not exist. Without two sexes, how did these things reproduce? This was a question tossed around for millennia, quite literally.

Top: larva (photo: Sönke Johnsen) Bottom: glass eels

It wasn’t until the Renaissance that folks began to question spontaneous generation, and started to make connections that would explain a creature’s inception in ways we recognize today. The invention of the microscope and the new insights it gave to scientists would help put spontaneous generation to rest.

In the beginning of the 18 th century, an Italian doctor had managed to locate ovaries in a female. But without the discovery of male organs, reproduction was still elusive. Freud would change this. At the ripe age of 17 in 1873, Freud entered the University of Vienna. His intention was to study law, but he instead gravitated towards medicine.

His studies would include philosophy, physiology, and zoology, the latter subject under Darwinist professor, Carl Claus. Claus had the intention to prove that eels produce sexually, and set out to do this by locating a male eel. The year 1876 would find the young Freud assisting with this mission at Claus’ zoological research station in Trieste, a seaport city in northeastern Italy. But why did so many of these eels not have male parts? To understand this requires knowing two things: how their sex is determined and their life cycle. Both the American Eel ( Anguilla rostrata ) and the European Eel start out as eggs drifting about in the Sargasso Sea, in the middle of the Atlantic Ocean.

They then hatch into larva, or “leptocephalus.” In this stage, they look like transparent willow leaves. They are not strong swimmers at this point, so the ocean currents will carry them. As they get closer to freshwater near land, they transform into what is called a “glass eel.” At this stage, they look more like an eel but are still quite transparent.

They begin en masse to swim upstream into freshwater sources. Here, they transform into “elvers,” losing their transparency to look more like their adult form. They can travel very far upstream in this stage. It will take about 20 years for them to reach maturity.

  • They then synchronously return to the Sargasso Sea to mate and then die.
  • This cycle happens for both the American Eel as well as the European Eel.
  • For the latter, they have a slightly longer migration to reach Europe.
  • What Freud, his instructor, and the scientists before them did not know is that the sex of an eel is not determined by genetics as it is in human beings.

When an eel first hatches, it is neither male or female. The sex will later be determined by their environment, but the details around this are still unknown. Males do not become obvious until they are migrating or have reached the Sargasso Sea. This is the point in their life cycle when they start developing testes.

So Freud was dissecting many eels that would have become males later in their journey, had they not met the knife. The male that Freud found was possibly one developing early, which was something of a fluke, or one that came from the ocean. Either way, this was an incredible stroke of luck for Freud. Even if he was later missive about it, this discovery answered a question that had stumped so many before him.

Thanks to Adam Cohen, Icthyology Collection Manager, for his edits to this article.

Why is it hard to breed Japanese eels?

The future of eel farming – Eel farming has long been a worldwide industry. However, there is now concern over its sustainability, due the decreasing amount of elvers found in the wild. This is particularly significant for the European eel, whose numbers have dramatically declined in recent years. The reason for the decline is not yet known, although there are a few contributing factors. Eel farmers could face significant challenges in the futures as wild stocks decrease One reason could be from the over fishing of glass eels, meaning less eels reach maturity in rivers and streams, leading to the decline in the overall wild population.

  • Another factor could be the increasing amount of flood defence construction, that prevent the elvers from being able to progress up the rivers and streams as they could do with ease before.
  • Again this would affect the numbers of the wild population.
  • This means that in the future there may be regulations in place as to the quantities of elvers that can be caught for the purpose of restocking eel farms.

There are programmes now in place in parts of the UK whereby 60 percent of all caught elvers can be sold to farms, and the other 40 percent are taken and released further upstream, in an effort to boost the population numbers. Because of the complex lifecycle of the eel, breeding stock in captivity is something that is not possible as yet.

Why do eels stop eating?

Change in appetites: A young eel has no reproductive organs. Once it develops into an adult eel ready to mate, it stops eating.

Have we ever seen an eel egg?

From Ancient Greece to the 20th century, Aristotle, Freud, and numerous other scholars were all looking for the same thing: eel testicles. Freshwater eels could be found in rivers across Europe, but no one had ever seen them mate and no researcher could find eel eggs or identify their reproductive organs.

Are eels actually electric?

Have You Ever Wondered. –

Are eels really electric? Are electric eels really eels? Could an electric eel’s shock kill a human?

Should you be worried about getting shocked by an electric eel the next time you wade in a creek or go for a swim in a lake or river? Unless you live in South America, the answer is no! But electric eels are real. Known by the scientific name Electrophorus electricus, the electric eel is an electric fish able to generate powerful electric shocks.

  • Electric eels use their shocking abilities for hunting and self-defense.
  • In fact, electric eels are not actually eels at all.
  • They’re a specific kind of knifefish that lives mainly in bodies of fresh water in South America, such as the Amazon River.
  • Electric eels are more closely related to catfish than true eels.

True eels cannot produce electric shocks like electric eels can. Electric eels have long bodies (up to six feet long) shaped like a cylinder, Since they look a lot like true eels, they’ve been called electric eels since they were discovered. Electric eels can weigh up to 45 pounds.

  • Unlike many fish, electric eels breathe air,
  • They regularly rise to the surface every 10 minutes or so to take a breath before heading back underwater.
  • Electric eels have three sets of internal organs that produce electricity,
  • The organs are made up of special cells called “electrocytes.” Electric eels can create both low and high voltage charges with their electrocytes.

Electric eels generate their electric shocks much like a battery, Like the stacked plates of a battery, the stacked electric cells can generate an electrical shock of 500 volts and 1 ampere. Such a shock would be deadly for an adult human! Electric eels can vary the intensity of their shocks, using lower voltages for hunting and higher voltages for self-defense.

Most of the time, electric eels produce lower voltage shocks just strong enough to stun prey or deter a threatening animal. When threatened, electric eels can produce intermittent electric shocks for at least an hour without showing any signs of getting tired. Electric eels are usually only safe to be around when they’re in a zoo or an aquarium.

Some people have been able to keep them as pets, but they’re very difficult — and dangerous — to try to catch in the wild. Electric eels need at least a 200-gallon tank and usually must be kept by themselves since they will attack other fish,

Are some eels actually electric?

Overview – Electrophorus electricus—everything about this fish’s scientific name says high voltage! So, it’s no surprise that of the fishes able to generate an electrical discharge, electric eels are the champions, producing up to 600 volts. Electric eels live in muddy waters.

  1. Mostly blind, they rely on low-level electrical pulses to navigate and explore their surroundings.
  2. Higher levels of voltage are generated to stun or kill prey and to protect them from predators.
  3. Though commonly referred to as an eel, this fish is not considered a “true” eel.
  4. While true eels are classified in the order Anguilliformes, the electric eel is actually in the order Gymnotiformes, the knife fishes.

Knife fishes have no dorsal fin and a long, extended anal fin. Although not true eels, these nearly scaleless fish look the part with long, cylindrical bodies and a slightly flattened head. The electric eel’s anal fin extends from the tip of the tail nearly to the chin.

When was it discovered how eels reproduce?

The Origin of Eels Slithering, slimy, and brown, the European eel is not the most charismatic nor charming aquatic creature. It may not bring about ideas of mystery or intrigue, yet its existence has plagued scientists with questions for millennia. With its many life stages, its biology is truly vexing.

  1. But perhaps the biggest mystery has been tied to just where all eels set out for when they head toward the ocean via coastal streams and rivers at the end of their lives.
  2. Through the efforts of a team of scientists it is a mystery no more.
  3. In 2022, researchers finally tracked a group of migrating eels to the middle of the Atlantic Ocean in an area known as the Sargasso Sea.

The mystery of the eel can be traced back to ancient times when the Egyptians told of how the eels sprang up from the Nile at the warming of the sun. For thousands of years, it was the consensus that eels were the exception in the animal kingdom and could bypass the rules of procreation and spontaneously rise from the river mud.

There was no greater promoter of this theory than, who, after immense study of their anatomy, proclaimed that they came from within “the entrails of the earth” since they seemed not to have any sex genitalia and therefore could not procreate through sex. Pliny, the Elder of Rome, had his own thoughts and believed baby eels sprung from the particles shed by adult eels as they rubbed their bodies on riverbed rocks.

Generation after generation of scientists continued to be stumped, until finally, in the 1890s scientists observed the last metamorphosis of an eel into its sexually mature state—with sex organs glaringly present. The life cycle of the eel includes several metamorphoses from a larva to a mature eel and is the reason for much of the confusion in the scientific community.

  • Each life stage is so different from the next, they were once mistaken as different species, and today each stage is identified with its own name to specify which one is being discussed.
  • After hatching from an egg in the ocean, the leptocephalus larval eel is swept across the ocean in the Gulf Stream.

It is only millimeters long, with a tiny head and a see-through body— very different in appearance from its adult forms. The larvae’s journey takes several years, and during that time, it slowly grows by feeding on tiny bits of marine debris and phytoplankton.

  • By the time it gets to the shores of Europe it makes its first metamorphosis into a glass eel, which then travels up rivers, seamlessly transitioning to a freshwater life.
  • This is when it makes the next transition to a young eel, or elver, and then two years later to a yellow eel, a muscular fish with the ability to go dormant in cold seasons and slither across land to find new bodies of water to call home.

The eel will spend most of its life as a yellow eel, and this is the most familiar life stage for people living in coastal and riverine areas. Then as if on some hidden cue, a yellow eel will transform into the sexually mature silver eel and make its final migration back out to sea.

Though scientists had seemingly solved the mystery of the eel and its reproduction by the early 1900s through the discovery of its sex organs, no one knew where they went to procreate. The eels disappeared into the ocean, and aged larval eels appeared near the shores. The question remained, where did they go? The first inkling that the eels might be traveling to the Sargasso Sea came from the herculean efforts of Danish zoologist Johannes Schmidt.

From 1904 to 1921 (with a five-year break for World War I) Schmidt tirelessly trawled the ocean to find the larvae and by extension the birthplace of eels. Starting from the shores of Europe he discovered that only the largest larvae, those around two to three inches in length that were closest to turning into glass eels, inhabited the coastal waters.

  1. He needed to expand his search.
  2. As he moved west, the larvae continued to get smaller, and it was through this trial and error that he found himself in the middle of the Atlantic with larvae so tiny “that there can be no questionwhere the eggs were spawned.” It would be 100 more years until confirmation of Schmidt’s discovery that adult eels migrated to the Sargasso Sea to lay eggs.

Between 1921 and 2022 the understanding that eels travel to the Sargasso Sea was based on knowledge about the larvae—that’s where the smallest larvae are, therefore, it must be where they hatch. No spawning eels nor eggs had ever been found there. Then, in 2022, came a breakthrough.

  • Satellite tagging, a technology that allows scientists to track animals via satellite through the placement of a tag, finally became advanced enough in the mid-2000s to be applied to the mystery of the eel.
  • In 2018 and 2019, 26 female eels were tagged off the coast of the Azores right before they were to embark on their migration.
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A year later, five arrived in the Sargasso Sea, which led to a in 2022 announcing the discovery. The eels did indeed make a 3,000 – 6,200 mile (5,000 – 10,000 km) migration across the Atlantic Ocean, the second longest recorded migration of a bony fish.

Scientists have yet to observe a mating in the wild, which will likely be the next great eel mystery to solve, but the discovery of adult eels in the Sargasso Sea is a tremendous milestone. Since the 1980s the European eel has experienced a 95 percent decline in its population, putting it on the critically endangered list.

The decline is attributed to multiple issues, including dam building, disease, illegal trade, and climate change, and the latter likely influences life stages that live in the ocean rather than during its freshwater life stages. Frustratingly, this is the stage scientists know about the least.

Do all eels spawn in the ocean?

An American eel (Anguilla rostrata) captured for tagging in Union County, PA. (Photo by Steve Droter/Chesapeake Bay Program). The American Eel ( Anguilla rostrata ) is a fish that has fascinated me since childhood. It began at an early age while fishing at my family’s cabin.

  • We were night-fishing in a small stream for bullhead catfish when I hooked into something that was obviously too large to be a bullhead.
  • By the time I hauled the fish to shore, we were shocked to discover that it was (a very large) American eel.
  • My family had been fishing this creek for generations, and no one was aware that such a large, mysterious creature inhabited this small waterway.

I remember being in awe of a fish that looked so different (and almost foreign) compared to the bass and pickerel I was accustomed to catching. At the time, we had no understanding of this creature, and quickly released it in an effort to avoid its jaws and the profuse slime that covered its body.

  • This chance encounter would lead me to pursue a better understanding of the eel’s life history and ecology.
  • As a result, I would come to realize that my small creek (and the eels that inhabit it) are part of a much larger, interconnected global ecosystem.
  • Life History The life history of freshwater eels had long been a mystery to science.

For centuries, philosophers and scientists in Europe could not explain the occurrence of eels within their freshwater habitats. Eels were found in many European waterways, including ephemeral (temporary) waters and landlocked impoundments. But these eels were never observed spawning, and the presence of reproductive structures could not be identified upon ‘closer inspection.’ Notable figures like Aristotle and Sigmund Freud pondered the ‘eel question,’ and others developed some interesting theories as to how eels reproduce. An American eel (Anguilla rostrata) captured for tagging in Union County, PA (Photo by Steve Droter/Chesapeake Bay Program). Thanks in part to Danish researcher Johannes Schmidt, we now know that all European eels ( Anguilla anguilla ) and American eels ( Anguilla rostrata ) originate in a region of the northwest Atlantic Ocean known as the Sargasso Sea.

  • In the early 20 th century, Schmidt set off to search the ocean for juvenile eels, mapping locations of smaller and smaller larvae, until after almost twenty years he was able to trace them back to their point of origin in the Sargasso Sea.
  • The Sargasso ‘Sea’ is a region of the Atlantic that is not bordered by land masses.

It lies between ocean currents that form what is known as a ‘gyre.’ Gyres are circular ocean currents that are influenced by wind patterns and the rotation of the earth. The name ‘Sargasso’ is derived from descriptions by early explorers of the brown Sargassum seaweed commonly found throughout the region.

It is within the Sargasso Sea that all adult American (and European) eels congregate to spawn. Eels are known as a catadromous fish, which means they live their adult lives in freshwater habitats before returning to a marine environment to reproduce. The eels then begin a complex life cycle that includes multiple physical metamorphoses and a round-trip migration of thousands of miles.

After hatching in the Sargasso, eels emerge as a tiny, laterally compressed, transparent larvae. This type of larvae is known as Leptocephalus (meaning ‘slim-headed’) a common condition in juvenile marine eels. These larvae then begin a long journey following ocean currents toward freshwater habitats.

(Both American and European eels are born in the Sargasso Sea, but it is still unclear how their respective larvae know which way to go). As eels begin to reach the shores of their destination, they morph into a form known as a ‘glass eel.’ The eels are still transparent, just slightly larger. As they gain pigmentation, they are sometimes called ‘elvers.’ The eels then enter freshwater habitats along the coast, morphing again into a more familiar form, known as a ‘yellow eel.’ During this phase, their bodies become serpentine and muscular.

They migrate hundreds or thousands of miles upstream until they find a home. During this journey the eels are capable of moving over land and persisting for extended periods of time within dry or drying habitats. It was this ability that led the eels I caught at my family cabin to arrive at their destination. Photo of waterfall between author’s eel catch and larger downstream river (Photo by Jim Kauffman). Adult eels then live out their lives as freshwater predators, feeding at night on a variety of prey items (in my experience, it was a dead minnow on a hook).

Eels are capable of living within their freshwater homes for fifty or more years, but at some point in their lives they begin to migrate back downstream toward their place of birth in the Sargasso Sea. (What triggers them to reproduce is still not fully understood). The eel’s body transforms again, developing physical traits that are more conducive to a marine environment.

It is only now that eels finally develop reproductive structures (which explains why early European scientists were unable to observe them). Eels in this final reproductive form are known as ‘silver eels.’ Once they reach the Sargasso Sea, the eels spawn and die, completing a compelling and mysterious journey that likely spanned decades.

Conservation Status American eels face numerous threats that have impacted population numbers throughout the Chesapeake Bay watershed and beyond. Since the 1970s, eel harvest numbers have declined globally, an indication that populations are likely decreasing. This can be attributed to a number of factors, including human development, commercial harvest, climate change, pollution, impediments to migration, and invasive species.

Dams have altered the waterways that eels use to migrate between freshwater habitats and their spawning grounds in the Sargasso Sea, preventing passage and impacting reproduction. Invasive species like the flathead catfish (recently introduced into the Chesapeake Bay Watershed) may be impacting American eels through predation.

Changes in climate that impact ocean temperatures and currents may impact migration and reproduction. Pollutants and effluent entering freshwater systems through runoff are degrading habitats used by adult eels. Microplastics have been identified in their breeding grounds within the Sargasso Sea, in addition to accumulations of trash and macroplastics (see North Atlantic Garbage Patch).

With the species facing so many threats, it isn’t surprising that the International Union for Conservation of Nature (IUCN) has listed the American eel as ‘Endangered’ due to decreasing population trends and the decline of mature individuals. Eels in the Chesapeake Bay Historically, the American eel was found throughout the Chesapeake Bay Watershed.

Eels would have moved freely through the bay, migrating between their birthplace in the Sargasso Sea and the many freshwater tributaries throughout the bay watershed. Eels were an important food source for Native Americans who constructed weirs within freshwater river systems to capture eels during migration.

Some of their stone weirs are still visible today. The Swatara Creek in Pennsylvania, a tributary of the Susquehanna (and ultimately the Chesapeake) was named for a Susquehannock word, meaning ‘the place where we feed on eels.’ After European settlement, commercial eel fisheries developed in many areas, and millions of pounds of eels were taken each year.

Eventually, dams were constructed on many river systems within the bay watershed that blocked the migration routes of eels. Eels still exist in many of these water systems, but their numbers have plummeted due to historic commercial fishing and the presence of dams. Fisheries managers are now working to restore these migration routes through the installation of fish passages.

American eels, European eels, and Japanese eels are still used for table fare, particularly in traditional sushi dishes. And American eels are commonly used as bait by anglers in the Chesapeake, especially those fishing for ‘stripers’ (or if you’re south of Pennsylvania) ‘rockfish. Staff from the Alliance’s PA office and Lancaster County Conservation District gather data on specimens caught while electrofishing for monitoring purposes. An American eel was captured here on the Octoraro Creek, which meets the Susquehanna River below the Conowingo Dam (Photo by Jenna Mackley).

An Interesting Relationship The eastern elliptio ( Elliptio complanata ) is a common freshwater mussel found throughout the Chesapeake Bay Watershed. Mussels are bivalve mollusks, and are known as ‘filter feeders.’ To feed, the mussel siphons organic particles from the water through its gills, collecting bacteria, algae, and other microscopic organisms.

Mussels generally require clean, cool waterways that are low in nutrients without excessive sedimentation. Eels also require unpolluted freshwater habitats with an abundance of prey, in addition to unobstructed waterways for migration. Historically, these habitats were found throughout the bay watershed, and both mussels and eels were common. Eastern elliptio mussel tagged during a survey in West Virginia’s Cacapon River (Photo by Andy Meyer). Eastern elliptios are still widespread, but were historically more abundant throughout the bay watershed. Recent mussel surveys in some Susquehanna River tributaries have revealed a lack of juvenile eastern elliptios, indicating a decline in reproductive success.

Many adults continue to persist, with some being estimated to be 100 or more years old. So why the dramatic decline in juvenile mussels? Most mussels in North America rely on fish hosts to complete their reproductive cycle. Eastern elliptios are no exception. Male elliptios release sperm into the water, which are later taken up by females as they take in water.

Eggs are fertilized and begin to develop inside the female mussel. The young mussels develop into larvae, called ‘glochidia,’ and are then released into the water. It is here that the glochidia attach to the gills of a host fish where they develop into miniature mussels.

  • The mussels then release from the fish and find a home in the stream sediment, where they can live for decades.
  • Eastern elliptios rely on only a handful of fish species to reproduce, with American eels as their primary host.
  • While they can reproduce using other fish species, reproduction through eels as their host has been shown to be the most productive.

With the presence of so many dams along the Susquehanna blocking their migration route, eel numbers in many tributaries have declined, leading to a corresponding decline in elliptio reproduction. To mitigate this decline, biologists and fisheries managers have begun releasing eels in waterways above these dams.

  1. This strategy, coupled with the development of fish passages appears to be having a somewhat positive effect on eel migration and elliptio reproduction.
  2. The Future of Eels and Mussels in the Chesapeake There is a very clear connection between our work to restore the Chesapeake Bay Watershed and the future of eels and mussels.

The Alliance strives to improve waterways through the implementation of conservation practices that promote clean water and healthy ecosystems throughout the bay watershed. Our riparian buffer and reforestation initiatives protect these waterways through streambank stabilization, decreased sedimentation, trapping of nutrients, and filtering runoff.

Tree canopies provide shade and create cooler water temperatures. Our agricultural team also works with farmers and producers to install Best Management Practices that prevent livestock from entering waterways and better manage manure. These practices combine to create a healthier ecosystem and brighter future for eels and mussels.

There was a time when every American eel living in the bay watershed traveled freely through a pristine Chesapeake Bay at some point in their lives. Although the journey is now more perilous, there’s hope for their future. A better understanding of their life history, coupled with dam removal/fish passage structures and habitat initiatives can only improve the future of this species.

  1. My chance encounter with an eel so many years ago and the subsequent understanding of its complex life history would be just one of the many factors that influenced my decision to enter the field of natural resource management.
  2. As the Alliance’s new Pennsylvania Forest Projects Coordinator I now have the opportunity to promote and implement habitat initiatives that will benefit eels, mussels, and many other organisms throughout the bay watershed.

It’s my hope to protect these resources so others can enjoy them the way I did, when I was just a kid fishing at my family’s cabin and collecting mussels in the creek.

How long do eels live?

Reproduction and Life Cycle – Eels are catadromous, meaning they live in freshwater rivers and spawn in the ocean. In October, sexually mature eels swim out of the Bay to the Sargasso Sea, an area of the Atlantic Ocean east of the Bahamas. In January, the eels spawn there, then die.

Tiny eel larvae drift in the ocean for 9 to 12 months. During this time, larvae transform to the “glass eel” stage. Ocean currents carry the transparent glass eels thousands of miles to the U.S. coast. Before entering the Bay, the glass eels become pigmented. These brown eels, called elvers, are only about 2.4 inches long.

Some elvers stay in the Bay, but most continue to swim many miles up the Bay’s rivers to fresh water. After a few months, the elvers transform into the adult “yellow eel” stage. Adults remain in freshwater rivers and streams for the majority of their lives.

Have we ever seen an eel egg?

From Ancient Greece to the 20th century, Aristotle, Freud, and numerous other scholars were all looking for the same thing: eel testicles. Freshwater eels could be found in rivers across Europe, but no one had ever seen them mate and no researcher could find eel eggs or identify their reproductive organs.

When was it discovered how eels reproduce?

The Origin of Eels Slithering, slimy, and brown, the European eel is not the most charismatic nor charming aquatic creature. It may not bring about ideas of mystery or intrigue, yet its existence has plagued scientists with questions for millennia. With its many life stages, its biology is truly vexing.

  1. But perhaps the biggest mystery has been tied to just where all eels set out for when they head toward the ocean via coastal streams and rivers at the end of their lives.
  2. Through the efforts of a team of scientists it is a mystery no more.
  3. In 2022, researchers finally tracked a group of migrating eels to the middle of the Atlantic Ocean in an area known as the Sargasso Sea.

The mystery of the eel can be traced back to ancient times when the Egyptians told of how the eels sprang up from the Nile at the warming of the sun. For thousands of years, it was the consensus that eels were the exception in the animal kingdom and could bypass the rules of procreation and spontaneously rise from the river mud.

There was no greater promoter of this theory than, who, after immense study of their anatomy, proclaimed that they came from within “the entrails of the earth” since they seemed not to have any sex genitalia and therefore could not procreate through sex. Pliny, the Elder of Rome, had his own thoughts and believed baby eels sprung from the particles shed by adult eels as they rubbed their bodies on riverbed rocks.

Generation after generation of scientists continued to be stumped, until finally, in the 1890s scientists observed the last metamorphosis of an eel into its sexually mature state—with sex organs glaringly present. The life cycle of the eel includes several metamorphoses from a larva to a mature eel and is the reason for much of the confusion in the scientific community.

Each life stage is so different from the next, they were once mistaken as different species, and today each stage is identified with its own name to specify which one is being discussed. After hatching from an egg in the ocean, the leptocephalus larval eel is swept across the ocean in the Gulf Stream.

It is only millimeters long, with a tiny head and a see-through body— very different in appearance from its adult forms. The larvae’s journey takes several years, and during that time, it slowly grows by feeding on tiny bits of marine debris and phytoplankton.

  • By the time it gets to the shores of Europe it makes its first metamorphosis into a glass eel, which then travels up rivers, seamlessly transitioning to a freshwater life.
  • This is when it makes the next transition to a young eel, or elver, and then two years later to a yellow eel, a muscular fish with the ability to go dormant in cold seasons and slither across land to find new bodies of water to call home.

The eel will spend most of its life as a yellow eel, and this is the most familiar life stage for people living in coastal and riverine areas. Then as if on some hidden cue, a yellow eel will transform into the sexually mature silver eel and make its final migration back out to sea.

Though scientists had seemingly solved the mystery of the eel and its reproduction by the early 1900s through the discovery of its sex organs, no one knew where they went to procreate. The eels disappeared into the ocean, and aged larval eels appeared near the shores. The question remained, where did they go? The first inkling that the eels might be traveling to the Sargasso Sea came from the herculean efforts of Danish zoologist Johannes Schmidt.

From 1904 to 1921 (with a five-year break for World War I) Schmidt tirelessly trawled the ocean to find the larvae and by extension the birthplace of eels. Starting from the shores of Europe he discovered that only the largest larvae, those around two to three inches in length that were closest to turning into glass eels, inhabited the coastal waters.

  • He needed to expand his search.
  • As he moved west, the larvae continued to get smaller, and it was through this trial and error that he found himself in the middle of the Atlantic with larvae so tiny “that there can be no questionwhere the eggs were spawned.” It would be 100 more years until confirmation of Schmidt’s discovery that adult eels migrated to the Sargasso Sea to lay eggs.

Between 1921 and 2022 the understanding that eels travel to the Sargasso Sea was based on knowledge about the larvae—that’s where the smallest larvae are, therefore, it must be where they hatch. No spawning eels nor eggs had ever been found there. Then, in 2022, came a breakthrough.

  1. Satellite tagging, a technology that allows scientists to track animals via satellite through the placement of a tag, finally became advanced enough in the mid-2000s to be applied to the mystery of the eel.
  2. In 2018 and 2019, 26 female eels were tagged off the coast of the Azores right before they were to embark on their migration.

A year later, five arrived in the Sargasso Sea, which led to a in 2022 announcing the discovery. The eels did indeed make a 3,000 – 6,200 mile (5,000 – 10,000 km) migration across the Atlantic Ocean, the second longest recorded migration of a bony fish.

Scientists have yet to observe a mating in the wild, which will likely be the next great eel mystery to solve, but the discovery of adult eels in the Sargasso Sea is a tremendous milestone. Since the 1980s the European eel has experienced a 95 percent decline in its population, putting it on the critically endangered list.

The decline is attributed to multiple issues, including dam building, disease, illegal trade, and climate change, and the latter likely influences life stages that live in the ocean rather than during its freshwater life stages. Frustratingly, this is the stage scientists know about the least.

Do eels reproduce in Bermuda Triangle?

All American and European eels are born in the Sargasso Sea — inside the Bermuda Triangle. The mystery of how eels reproduce fascinated thinkers from Aristotle to Sigmund Freud. At last, very firm evidence from tracking data links eels to their spawning site.

Can eels change gender?

Home Ocean Life Fish Blue Ribbon Eel

photo All blue ribbon eels undergo an immense transformation within their lifetime. At birth, they begin life as males and as they mature they make the switch and become females. Although seemingly remarkable, in the fish world, this occurs more than you might think.