True eels are teleost fish. They have long and narrow bodies like snakes. Adult eels can be as short as 10 cm  or as long as 3 m. It depends on their species. The large eels can weigh up to 65 kg.

Eels have fewer fins than other fish. They do not have all the belly and chest fins. The back and anal fins are long and usually connected to the tail fin. The fins do not have spines.

The shoulder bones are separate from the skull. The scales have smooth edges or are absent.

Eels hatch from eggs. Baby (larval) eels are flat and transparent (clear). They are called leptocephalus (Greek for "thin head"). A young eel is called an elver. For a long time people did not know where eels came from, because baby eels look very different from adults. They thought the babies were a different species.

Most eels prefer to live in the shallowest parts of the ocean.  They live at the bottom of the ocean, sometimes in holes. Eels in the Anguillidae family come to fresh water to live there. Eels in the Nemichthyidae family swim about 500 m below the ocean surface. Eels in the Synaphobranchidae family live as deep as 4000 m below the surface.

Hand netting (catching with nets) is the only legal way of catching eels in England. It has been done for thousands of years on the River Parrett and River Severn.

Most eels are predators. They hunt their prey.

Classification
True eels belong to the order (group) Anguilliformes (Latin for "eel-shaped").

This order has 4 suborders (smaller groups) called Anguilloidei, Nemichthyoidei, Congroidei, and Synaphobranchoidei.

The suborders are separated into 19 families. The families are divided into 110 genera. The genera are then divided into species.

There are 400 species of eels in all.

Uses 
Freshwater eels (unagi) and marine eels (Conger eel, anago) are used in Japanese food. Eels are used in Cantonese (Hong Kong) food and Shanghai food.

Some people eat the European eel and other eels that live in lakes all around the world. A traditional London food is "jellied eels." The Spanish meal, angulas, consists of deep-fried elvers.

Eel skins are used to make some wallets and purses.

Suborders and families

Taxonomy based on Neslon, Grande and Wilson 2016.

 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)

In some classifications, the family Cyematidae of bobtail snipe eels is included in the Anguilliformes, but in the FishBase system that family is included in the order Saccopharyngiformes.

The electric eel of South America is not a true eel, but is a South American knifefish more closely related to the carps and catfishes.

Phylogeny
Phylogeny based on Johnson et al. 2012.

References

Teleosts
Edible fish