The eel is a long, thin bony fish of the order Anguilliformes. Because fishermen never caught anything they recognised as young eels, the life cycle of the eel was a mystery for a very long period of scientific history. Although more than 6500 publications mention eels, much of their life history remains an enigma.
The European eel (Anguilla anguilla) was historically the one most familiar to Western scientists, beginning with Aristotle, who wrote the earliest known inquiry into the natural history of eels. He speculated that they were born of "earth worms", which he believed were formed of mud, growing from the "guts of wet soil" rather than through sexual reproduction. Many centuries passed before scientists were able to demonstrate that such spontaneous generation does not occur in nature.
In 1777, the Italian Carlo Mondini located an eel's gonads and demonstrated that eels are a kind of fish. In 1876, as a young student in Austria, Sigmund Freud dissected hundreds of eels in search of the male sex organs. He had to concede failure in his first major published research paper, and turned to other issues in frustration.
Larval eels — transparent, leaflike two-inch (five-cm) creatures of the open ocean — were not generally recognized as such until 1893; instead, they were thought to be a separate species, Leptocephalus brevirostris (from the Greek leptocephalus meaning "thin- or flat-head"). In 1886, however, the French zoologist Yves Delage discovered the truth when he kept leptocephali alive in a laboratory tank in Roscoff until they matured into eels, and in 1896 Italian zoologist Giovanni Battista Grassi confirmed the finding when he observed the transformation of a Leptocephalus into a round glass eel in the Mediterranean Sea. (He also observed that salt water was necessary to support the maturation process.) Although the connection between larval eels and adult eels is now well understood, the name leptocephalus is still used for larval eel.
The Danish professor Johannes Schmidt, beginning in 1904, led a series of expeditions into the Mediterranean Sea and the North Atlantic to investigate eels. The expeditions were largely financed by the Carlsberg Foundation. He noted that all the leptocephali he found were very similar, and hypothesized that they all must have descended from a common ancestor species. He also observed that the farther out to sea in the Atlantic Ocean he went, the smaller the leptocephali he caught were. In a 1922 expedition, he sailed as far as the Sargasso Sea, south of Bermuda, where he caught the smallest eel-larvae that had ever been seen.
Although Schmidt did not directly observe eel spawning, or even find ready-to-spawn adult eels, he was able to deduce the following about the life history of the eel, based on the size distribution of the leptocephali he collected:
The larvae of European eels travel with the Gulf Stream across the Atlantic Ocean, and grow to 75–90 mm within one to three years, before they reach the coasts of Europe. Eels in this so-called "recruitment" developmental stage are known as glass eels because of the transparency of their bodies. Glass eels typically refers to an intermediary stage in the eel's complex life history between the leptocephalus stage and the juvenile (elver) stage. Glass eels are defined as "all developmental stages from completion of leptocephalus metamorphosis until full pigmentation". The term typically refers to a transparent glass eel of the family Anguillidae.
One well-known place where glass eels are collected large-scale (for deli food and stocking) is Epney, on the Severn, in England. (Glass eels are a food item in Spain.) Once they recruit to coastal areas, they migrate up rivers and streams, overcoming various natural challenges — sometimes by piling up their bodies by the tens of thousands to climb over obstacles — and they reach even the smallest of creeks.
The eels can propel themselves over wet grass and dig through wet sand to reach upstream headwaters and ponds, thus colonising the continent. In fresh water they develop pigmentation, turn into elvers (young eels), and feed on creatures such as small crustaceans, worms, and insects. For 10 to 14 years they mature, growing to a length of 60 to 80 cm. During this stage they are called yellow eels because of their golden pigmentation. Marine eels of the order Anguilliformes also have a leptocephalus stage, and likely pass through a stage similar to the anguillid glass eels, but they are rarely seen in the ocean.
In July, some mature individuals migrate back towards the sea, crossing wet grasslands at night to reach rivers that lead to the sea. Eel migration out of their freshwater growth habitats from various parts of Europe, or through the Baltic Sea in the Danish straits, have been the basis of traditional fisheries with characteristic trapnets.
How the adults make the 6,000 km (3,700 mi) open ocean journey back to their spawning grounds north of the Antilles, Haiti, and Puerto Rico remains unknown. By the time they leave the continent, their gut dissolves, making feeding impossible, so they have to rely on stored energy alone. The external features undergo other dramatic changes, as well: the eyes start to enlarge, the eye pigments change for optimal vision in dim blue clear ocean light, and the sides of their bodies turn silvery, to create a countershading pattern which makes them difficult to see by predators during their long open-ocean migration. These migrating eels are typically called "silver eels" or "big eyes".
German fisheries biologist Friedrich Wilhelm Tesch, an eel expert and author, conducted many expeditions with high-tech instrumentation to follow eel migration, first down the Baltic, then along the coasts of Norway and England, but finally the transmitter signals were lost at the continental shelf when the batteries ran out. According to Schmidt, a travel speed in the ocean of 15 km per day can be assumed, so a silver eel would need 140 to 150 days to reach the Sargasso Sea from around Scotland and in about 165 to 175 when leaving from the English Channel.
Tesch — like Schmidt — kept trying to persuade sponsors to provide more funding for expeditions. His proposal was to release 50 silver eels from Danish waters, with transmitters that would detach from the eels each second day, float up toward the surface, and broadcast their position, depth, and temperature to satellite receivers. He also suggested that countries on the western side of the Atlantic could perform a similar release experiment at the same time. However, even today, only preliminary experiments along these lines have ever been performed. Migration was mapped in 2016.
Knowledge of what happens to individual silver eels after they leave the continental shelf is based solely on the study of three eels found in the stomachs of deepsea fishes and whales — caught off the coasts of Ireland and the Azores — and on laboratory research into the physiology of eels.
Another Atlantic eel species is known: the American eel, Anguilla rostrata. First it was believed European and American eels were the same species due to their similar appearance and behavior, but they differ in chromosome count and various molecular genetic markers, and in the number of vertebrae, A. anguilla counting 110 to 119 and A. rostrata 103 to 110.
The spawning grounds for the two species are in an overlapping area of the southern Sargasso Sea, with A. rostrata apparently being more westward than A. anguilla, and with some spawning by the American eel possibly even occurring off the Yucatán Peninsula off the Gulf of Mexico, but this has not been confirmed. After spawning in the Sargasso Sea and moving to the west, the leptocephali of the American eel exit the Gulf Stream earlier than the European eel and begin migrating into the estuaries along the east coast of North America between February and late April at an age around one year and a length around 60 mm.
The spawning area of the Japanese eel, Anguilla japonica, has also been found. Their breeding site is to the west of the Suruga seamount (14–17°N, 142–143°E), near the Mariana Islands. and their leptocephali are then transported to the west to East Asia by the North Equatorial Current.
In June and August 2008, Japanese scientists discovered and caught matured adult eels of A. japonica and A. marmorata in the West Mariana Ridge.
Southern Africa's four species of freshwater eels (A. mossambica, A. bicolor bicolor, A. bengalensis labiata, and A. marmorata) have an interesting migratory pattern: It takes them on a long journey from their spawning grounds in the Indian Ocean north of Madagascar to high up in some of the Southern African river systems and then back again to the ocean off Madagascar.
No one yet knows the reasons, but beginning in the mid-1980s, glass eel arrival in the spring dropped drastically — in Germany to 10% and in France to 14% of their previous levels — from even conservative estimates. Data from Maine and other North American coasts showed similar declines, although not as drastic.
In 1997, European demand for eels could not be met for the first time ever, and dealers from Asia bought all they could. The traditional European stocking programs could not compete any longer: each week, the price for a kilogram of glass eel went up another US$30. Even before the 1997 generation hit the coasts of Europe, dealers from China alone placed advance orders for more than 250,000 kg, some bidding more than $1,100 per kg. Asian elvers have sold in Hong Kong for as much as $5,000 to $6,000 a kilogram at times when $1,000 would buy the same amount of American glass eels at their catching sites. Such a kilogram, consisting of 5000 glass eels, may bring at least $60,000 and as much as $150,000 after they leave an Asian fish farm. In New Jersey, over 2000 licences for glass eel catch were issued and reports of 38 kg per night and fisherman have been made, although the average catch is closer to 1 kg.
The demand for adult eels has continued to grow, as of 2003. Germany imported more than $50 million worth of eels in 2002. In Europe, 25 million kg are consumed each year, but in Japan alone, more than 100 million kg were consumed in 1996. As the European eels become less available, worldwide interest in American eels has increased dramatically.
New high-tech eel aquaculture plants are appearing in Asia, with detrimental effects on the native Japanese eel, A. japonica. Traditional eel aquaculture operations rely on wild-caught elvers, but experimental hormone treatments in Japan have led to artificially spawned eels. Eggs from these treated eels have a diameter of about 1 mm, and each female can produce 2 to 10 million eggs.
Strong concerns exist that the European eel population might be devastated by a new threat: Anguillicola crassus, a foreign parasitic nematode. This parasite from East Asia (the original host is A. japonica) appeared in European eel populations in the early 1980s. Since 1995, it also appeared in the United States (Texas and South Carolina), most likely due to uncontrolled aquaculture eel shipments. In Europe, eel populations are already from 30% to 100% infected with the nematode. Recently, this parasite was shown to inhibit the function of the swimbladder as a hydrostatic organ. As open ocean voyagers, eels need the carrying capacity of the swimbladder (which makes up 3–6% of the eel's body weight) to cross the ocean on stored energy alone.
Because the eels are catadromous (living in fresh water but spawning in the sea), dams and other river obstructions can block their ability to reach inland feeding grounds. Since the 1970s, an increasing number of eel ladders have been constructed in North America and Europe to help the fish bypass obstructions.
In New Jersey, an ongoing project monitors the glass eel migration with an online in situ microscope. As soon as more funding becomes available, it will be possible to log into the system via a Longterm Ecological Observatory (LEO) site.
Anago (穴子, or アナゴ) is the Japanese word for salt-water eels, normally referring to ma-anago (Conger myriaster). Ma-anago are used for a seafood dish in Japan. They are often simmered (sushi) or deep-fried (tempura), compared to unagi (freshwater eels) which are usually barbecued with a sauce (kabayaki). Anago is also slightly less rich and oily than unagi. Anago has a very soft texture and sweet taste.Biological life cycle
In biology, a biological life cycle (or just life cycle when the biological context is clear) is a series of changes in form that an organism undergoes, returning to the starting state. "The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, or sexual reproduction.
In some organisms, different "generations" of the species succeed each other during the life cycle. For plants and many algae, there are two multicellular stages, and the life cycle is referred to as alternation of generations. The term life history is often used, particularly for organisms such as the red algae which have three multicellular stages (or more), rather than two.Life cycles that include sexual reproduction involve alternating haploid (n) and diploid (2n) stages, i.e., a change of ploidy is involved. To return from a diploid stage to a haploid stage, meiosis must occur. In regard to changes of ploidy, there are 3 types of cycles:
haplontic life cycle — the haploid stage is multicellular and the diploid stage is a single cell, meiosis is "zygotic".
diplontic life cycle — the diploid stage is multicellular and haploid gametes are formed, meiosis is "gametic".
haplodiplontic life cycle (also referred to as diplohaplontic, diplobiontic, or dibiontic life cycle) — multicellular diploid and haploid stages occur, meiosis is "sporic".The cycles differ in when mitosis (growth) occurs. Zygotic meiosis and gametic meiosis have one mitotic stage: mitosis occurs during the n phase in zygotic meiosis and during the 2n phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively termed haplobiontic (single mitotic phase, not to be confused with haplontic). Sporic meiosis, on the other hand, has mitosis in two stages, both the diploid and haploid stages, termed diplobiontic (not to be confused with diplontic).Carl Friedrich Wilhelm Claus
Carl Friedrich Wilhelm Claus (2 January 1835 – 18 January 1899) was a German zoologist. He was an opponent of the ideas of Ernst Haeckel.EcoSCOPE
The ecoSCOPE is an optical sensor system, deployed from a small remotely operated vehicle (ROV) or fibre optic cable, to investigate behavior and microdistribution of small organisms in the ocean.Eel
An eel is any ray-finned fish belonging to the order Anguilliformes (), which consists of four suborders, 20 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage, and most are predators. The term “eel” originally referred to the European eel, and the name of the order means “European eel-shaped.”
The term “eel” is also used for some other eel-shaped fish, such as electric eels (genus Electrophorus), spiny eels (family Mastacembelidae), and deep-sea spiny eels (family Notacanthidae). These other clades, however, evolved their eel-like shapes independently from the true eels.Eel as food
Eels are elongated fish, ranging in length from 5 centimetres (2.0 in) to 4 metres (13 ft). Adults range in weight from 30 grams to over 25 kilograms. They possess no pelvic fins, and many species also lack pectoral fins. The dorsal and anal fins are fused with the caudal or tail fin, forming a single ribbon running along much of the length of the animal.
Most eels live in the shallow waters of the ocean and burrow into sand, mud, or amongst rocks. A majority of eel species are nocturnal and thus are rarely seen. Sometimes, they are seen living together in holes, or "eel pits". Some species of eels live in deeper water on the continental shelves and over the slopes deep as 4,000 metres (13,000 ft). Only members of the Anguillidae family regularly inhabit fresh water, but they too return to the sea to breed.Eel blood is poisonous to humans and other mammals, but both cooking and the digestive process destroy the toxic protein. The toxin derived from eel blood serum was used by Charles Richet in his Nobel winning research which discovered anaphylaxis (by injecting it into dogs and observing the effect).
The Jewish laws of Kashrut forbid the eating of eels. Similarly, according to the King James version of the Old Testament, it is acceptable to eat fin fish, but fish like eels, which were erroneously believed to lack fins or scales, are an abomination and should not be eaten.Japan consumes more than 70 percent of the global eel catch.Eel ladder
An eel ladder is type of fish ladder designed to help eels swim past barriers, such as dams and weirs or even natural barriers, to reach upriver feeding grounds. (Many eels are catadromous, living in fresh water but spawning at sea.) The basic design of an eel ladder has the eel swim over the barrier using an eel ascending ramp, which provides the eels a climbing substrate to "push against" while slithering upstream. For some higher barriers, elevator-style systems are also used.
An eel ladder typically consists of four parts: an eel ascending ramp, a supporting structure, a water-feeding system, and a side gutter. The eel ascending ramp can be a fairly simple construction, such as a hollowed out tree filled with recycled fishing net, or a more complex structure designed to accommodate specific species or ages of eels. The supporting structure mounts the ladder to the barrier. The rampside gutter provides an attraction flow to draw eels toward the ladder while the water-feeding system ensures the proper flow of water to the gutter.Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres.
Fish usually migrate to feed or to reproduce, but in other cases the reasons are unclear.
Migrations involve the fish moving from one part of a water body to another on a regular basis. Some particular types of migration are anadromous, in which adult fish live in the sea and migrate into fresh water to spawn, and catadromous, in which adult fish live in fresh water and migrate into salt water to spawn.
Marine forage fish often make large migrations between their spawning, feeding and nursery grounds. Movements are associated with ocean currents and with the availability of food in different areas at different times of year. The migratory movements may partly be linked to the fact that the fish cannot identify their own offspring and moving in this way prevents cannibalism. Some species have been described by the United Nations Convention on the Law of the Sea as highly migratory species. These are large pelagic fish that move in and out of the exclusive economic zones of different nations, and these are covered differently in the treaty from other fish.
Salmon and striped bass are well-known anadromous fish, and freshwater eels are catadromous fish that make large migrations. The bull shark is a euryhaline species that moves at will from fresh to salt water, and many marine fish make a diel vertical migration, rising to the surface to feed at night and sinking to lower layers of the ocean by day. Some fish such as tuna move to the north and south at different times of year following temperature gradients. The patterns of migration are of great interest to the fishing industry. Movements of fish in fresh water also occur; often the fish swim upriver to spawn, and these traditional movements are increasingly being disrupted by the building of dams.Glossary of marine sciences terms
This is a glossary of terms used in fisheries, fisheries management and fisheries science.Index of fishing articles
This page is a list of fishing topics.Jellied eels
Jellied eels are a traditional English dish that originated in the 18th century, primarily in the East End of London. The dish consists of chopped eels boiled in a spiced stock that is allowed to cool and set, forming a jelly. It is eaten cold.Kabayaki
Kabayaki (蒲焼) is a preparation of fish, especially unagi eel, where the fish is split down the back (or belly), gutted and boned, butterflied, cut into square fillets, skewered, and dipped in a sweet soy sauce-based sauce before being broiled on a grill.
Besides unagi, the same preparation is made of other long scaleless fish such as hamo (pike conger), dojō (loach), catfish, anago (conger eel), and gimpo (ギンポ) (gunnels). One can also find canned products labeled as kabayaki-style sanma (Pacific saury).
Kabayaki eel is very popular and a rich source of vitamins A and E, and omega-3 fatty acids. A popular custom from the Edo period calls for eating kabayaki during the summer to gain stamina, especially on a particular mid-summer day called doyō-no ushi-no-hi (土用の丑の日), which can fall anywhere between July 18-August 8 each year.The eel kabayaki is often served on top of a bowl (donburi) of rice, and called unadon, the fancier form of which is the unajū, placed inside a tiered lacquered boxes called jūbako. It is also torn up and mixed up evenly with rice to make hitsumabushi (櫃まぶし), which is enjoyed especially in the Nagoya area.Principle of Priority
Priority is a fundamental principle of modern botanical nomenclature and zoological nomenclature. Essentially, it is the principle of recognising the first valid application of a name to a plant or animal. There are two aspects to this:
The first formal scientific name given to a plant or animal taxon shall be the name that is to be used, called the valid name in zoology and correct name in botany.
Once a name has been used, no subsequent publication of that name for another taxon shall be valid (zoology) or validly published (botany).There are formal provisions for making exceptions to this principle. If an archaic or obscure prior name is discovered for an established taxon, the current name can be declared a nomen conservandum (botany) or conserved name (zoology), and so conserved against the prior name. Similarly, if the current name for a taxon is found to have an archaic or obscure prior homonym, the current name can be declared a nomen protectum (zoology) or the older name suppressed (nomen rejiciendum, botany).Short-finned eel
The short-finned eel (Anguilla australis), also known as the shortfin eel, is one of the 15 species of eel in the family Anguillidae. It is native to the lakes, dams and coastal rivers of south-eastern Australia, New Zealand, and much of the South Pacific, including New Caledonia, Norfolk Island, Lord Howe Island, Tahiti, and Fiji.Unadon
Unadon (鰻丼, an abbreviation for unagi donburi, "eel bowl") is a dish originating in Japan. It consists of a donburi type large bowl filled with steamed white rice, and topped with fillets of eel (unagi) grilled in a style known as kabayaki, similar to teriyaki. The fillets are glazed with a sweetened soy-based sauce, called tare and caramelized, preferably over charcoal fire. The fillets are not flayed, and the grayish skin side is placed faced down. Sufficient tare sauce is poured over so that some of it seeps through the rice underneath. By convention, pulverized dried berries of sanshō (called Japanese pepper, although botanically unrelated) are sprinkled on top as seasoning.