Eels are ray-finned fish belonging to the order Anguilliformes (/æŋˈɡwɪlɪfɔːrmiːz/), which consists of eight suborders, 19 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” 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. Eels live both in salt water and fresh water and some species are catadromous.

Classification

A number of classifications of eels exist; some, such as FishBase, divide eels into 20 families, whereas other classifications such as ITIS and Systema Naturae 2000 include additional eel families, which are noted below the family with which they are synonymized in the FishBase system.

Identifying the origin of the freshwater species is considered to be problematic; however, genomic studies indicate they are a monophyletic group that originated among the deep-sea eels.^[rx]

Types of True Eels

Examples of true eels include the following:

Garden eels are a type of eel you may recognize! They look like tiny flowy pencils coming out of the ground, and they burrow if they get scared.

Moray eels, which are known for having some pretty neat color and camouflage techniques. A fun fact about moray eels is that the word “moray” translates to “eel” in Greek, which means that moray eels are actually “eel eels!”

Gulper eels were recently categorized as a true eel. They have huge mouths and can expand to multiple times their size in order to ward off predators and threats.

Ribbon eels are very pretty eels that, yes, look like ribbons! Their color changes throughout their lifespan, and they turn blue and yellow as they mature. Some say that this eel resembles a dragon!

Snake eels are another kind of true eel that have funky nostrils and fun colors to discourage predators from approaching them.

Suborders and families

Taxonomy based on Nelson, Grande and Wilson 2016.^[rx]

• Suborder Protanguilloidei
  • Family Protanguillidae
• Suborder Synaphobranchoidei
  • Family Synaphobranchidae (cutthroat eels) [incl. Dysommidae, Nettodaridae, and Simenchelyidae]
• Suborder Muraenoidei
  • Family Heterenchelyidae (mud eels)
  • Family Myrocongridae (thin eels)
  • Family Muraenidae (moray eels)
• Suborder Chlopsoidei
  • Family Chlopsidae (false morays)
• Suborder Congroidei
  • Family Congridae (congers) [incl. Macrocephenchelyidae; Colocongridae]
  • Family Derichthyidae (longneck eels) [incl. Nessorhamphidae]
  • Family Muraenesocidae (pike congers)
  • Family Nettastomatidae (duckbill eels)
  • Family Ophichthidae (snake eels)
• Suborder Moringuoidei
  • Family Moringuidae (spaghetti eels)
• Suborder Saccopharyngoidei
  • Family Eurypharyngidae (pelican eels, umbrellamouth gulpers)
  • Family Saccopharyngidae
  • Family Monognathidae (onejaw gulpers)
  • Family Cyematidae (bobtail snipe eels)
• Suborder Anguilloidei
  • Family Anguillidae (freshwater eels)
  • Family Nemichthyidae (snipe eels)
  • Family Serrivomeridae (sawtooth eels)

Natural history

Eels have a remarkable life cycle. Broadly, it consists of development and early growth in the open ocean: the planktonic (free-floating) dispersal of eggs and larvae, metamorphosis, juvenile and adult growth, and the migration of maturing adults to an oceanic spawning area. Eels share the leptocephalus phase with several other orders (Elopiformes [tarpons and relatives], Saccopharyngiformes [gulpers and relatives], and Albuliformes [bonefishes]). A prolarva, hatching from a relatively large egg (up to 2.5 mm [about 0.1 inch] in diameter), rapidly becomes a leaflike leptocephalus, which floats in the surface layers of the open ocean for as long as two and a half years before metamorphosing.

Although the leptocephali were once thought to have been fishes of a distinct group, their relationship with the Anguilliformes was soon recognized from transitional specimens that showed larval and adult characters. They proved so difficult to identify, however, that new larval types were named as species of the genus Leptocephalus (though they cannot actually be considered different species from the adults that produced them), accounting for the several hundred forms known.

Leptocephali are not uncommon in the upper 500 metres (roughly 1,600 feet) of the ocean, a distribution that may be associated with the availability of food (diatoms and minute crustaceans). Their predators include various pelagic fishes. In tropical eels, larval life is possibly four to six months, but temperate species may spend upward of a year as larvae. During this time leptocephali, in the presence of suitable currents, may disperse widely from the adult spawning area. Working on massive collections of larvae from 1905 to 1930, a Danish biologist, Johannes Schmidt, established the early life history of the European (Anguilla anguilla) and American freshwater eels (A. rostrata) and claimed that both species reproduced within the Sargasso Sea before using ocean currents to disperse to their respective freshwater habitats in Europe and North America. Although parts of his work have been questioned, especially the process by which these eels returned to their spawning areas, his description of a western Atlantic spawning and a trans-Atlantic dispersal of leptocephali of these eels is still largely accepted. Contemporary studies suggest, however, that adult American and European eels use a combination of chemical, geomagnetic, and geographic cues to return to separate spawning areas east of the Sargasso Sea near the Mid-Atlantic Ridge.

After reaching full growth, the larva begins a rapid metamorphosis in which the body undergoes several progressive changes. The body becomes cylindrical and greatly reduced in bulk, perhaps by as much as 90 percent by weight, and the anal vent advances from its subterminal position to about the midpoint. The larval teeth are lost, the snout becomes rounded, the dorsal fin originates farther forward, and the larval melanophores (black pigment cells) disappear. Other changes, such as the loss of the pectoral fins or a reduction of body length, may also occur.

Leptocephali are markedly unlike their adults, and the metamorphic changes are so great that a fundamental problem arises in the correlation of the great variety of known leptocephali with their adults. Metamorphosis has been observed in aquariums and deduced in the oceans from progressive growth series in plankton samples. Certain characters survive metamorphosis and are important in the recognition of eel species. These include the number of muscle segments (myomeres); the development of dorsal, anal, and caudal fin rays; and the relative positions of the renal vessels and the gallbladder. In many leptocephali, the larval melanophores also remain in the juvenile (or elver) stage.

Metamorphosis involves physiological and behavioral as well as structural changes, particularly those related to the assumption of a deep-sea, shallow-water, or freshwater mode of life. Metamorphosis is the mechanism by which the leptocephalus, after a period of growing, feeding, and competing with other similarly organized planktonic animals, can enter a markedly different habitat where body shape, differentiated feeding mechanisms, sense organs, and body coloration play an important role in survival. Metamorphosis in all eels is probably completed in the open ocean. The annual invasion of freshwaters by Anguilla elvers is a locally well-known process; it occurs during October–March in Europe and in spring in other temperate regions.

During several years’ growth to maturity, eels are essentially carnivores, feeding diversely on planktonic or benthic (bottom-living) animals. Maturity is reached after about 10 years in the European freshwater eel (A. anguilla) but possibly much earlier in tropical marine species. The process of growth and maturation has been most closely studied in the European freshwater eel. In this species, both sexes pass through successive phases of neutrality, precocious feminization, and juvenile hermaphroditism prior to becoming definitively male or female, the sex being determined mainly by environmental factors.

Spawning:

Fishing Tips

Fun Fact:

Precautions 

• The consumption of sea fish if one is trying to reduce inflammatory diseases such as heart disease.
• Farmed ones contain arsenic that leads to arsenic poisoning.
• Seafood should be cooked safely in order to prevent foodborne illness.
• Pregnant women, young children, older adults, people having lower stomach acid and compromised immune systems (HIV/AIDS, liver disease, cancer, diabetes, gastrointestinal disorders, people taking steroids, chemotherapy, or immune system) are prone to higher risk.
• It might be contaminated with bacteria such as Vibrio parahaemolyticus, Vibrio vulnificus, and other bacteria relate to land use, sewage discharges, runoff, etc. These microorganisms occur naturally in warm coastal waters which could cause even death or serious illness in individuals who are at higher risk.
• Listeria monocytogenes could cause a serious foodborne illness known as listeriosis.
• The virus Hepatitis A could survive in light cooking. So one should consume it after being properly cooked.
• Fish could have toxins that could cause illness such as ciguatoxin and scombrotoxin, or histamine poisoning.
• Sea fish is related to Scombrotoxin (histamine) which develops when fish is not kept cold enough. The symptoms develop quickly and also disappear completely within 24 hours.
• The flesh of tropical marine fishes might cause ciguatera poisoning experiencing gastrointestinal maladies that could last for several days, weakness in arms and legs, and reversal inability to differentiate between cold and hot. The symptoms could persist for weeks.
• Farmed seafood results in to increase in inflammation leading to weight gain, arthritis, and heart diseases.
• The imbalance ratio of Omega-6 and Omega-3 causes heart disease and hypertension.
• Seafood and fish in farms have polychlorinated biphenyls (PCBs), organophosphorus (OPs), organochlorine (OC), trifluralin pesticides, and hexachlorobenzene (HCB) causing diseases or even death.
• The seafood has a high content of mercury, which may lead to mercury toxicity.
• Consume it in moderate amounts. So consume it with caution or Avoid consuming it raw.
• Some people might get allergic reactions. So avoid it.
• It has a high chance of contamination.
• One should limit its intake.
• Consult the doctor by pregnant women and children before consuming it. Children and pregnant women should avoid it because the high content of mercury might cause a negative impact on the development of the nervous system of a fetus.
• Seafood or fish have purines in it which is harmful to people having purine-related problems. Excess purines result in an excess buildup of uric acid that could lead to the formation of kidney stones as well as gout.

References

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