Leptocephalus is the larval developmental stage of angvilloid fishes. The leptocephalus larva has a small head, a tall, laterally compressed transparent body.

From the larvae of other fish leptocephalus differ completely transparent, strongly compressed from the sides of the body, the absence of red blood cells in the blood and other anatomical features, as well as a long period of life in this stage.

Leptocephals tend to be found in the water column in oceanic pelagics and are a typical component of oceanic plankton down to depths of 500-600 m, although adult fish may be found just offshore or near the bottom. It has been observed that eels spawning above the deepest depths have leptocephalus with a long digestive tract, while those gravitating towards the coastal shallow zone have a short digestive tract. The presence of leptocephals of a particular species does not mean that the adults stay in the same places. To a large extent, this is caused by the transport of leptocephals by sea currents, so, knowing the habitats of adults, we can generally know the area of distribution of their leptocephals.

Leptocephals usually feed on whatever they can catch. In small forms, the main food is plankton (including phytoplankton), but large ones can feed on fry, shrimp and other small crustaceans, etc., as well. A characteristic feature of most leptocephals - long teeth, often strongly protruding outward. The leptocephals themselves, in turn, serve as food for many sea creatures.

The larvae of elopsoids, hatching from eggs, initially have the appearance characteristic of larvae of most fishes, but almost immediately, after a short metamorphosis, turn into leptocephals. A notable feature of leptocephals - a long period of development. Leptocephalus of the common river eel is at the larval stage of development up to 3 years. In leptocephals of other elopsis (except eels) this period is shorter - 60-120 days, but it, nevertheless, exceeds the period of larval stage of most perch, except for some gubanids.

Unlike the larvae of most fishes, leptocephals usually have a much larger relative size compared to the size of the adult fish before transforming into the adult form. In the small Pacific monkey eel, which reaches 60 cm in length (and is usually smaller, about 30 cm), the leptocephalus is 10-11 cm long before metamorphosis begins. Tarpon leptocephals are the smallest (25-50 mm before the onset of metamorphosis), while the adult Atlantic tarpon reaches 2 m in length and 100 kg in weight; in addition, their tail, as in adult tarpon, is bifurcated. At the same time, the leptocephalus is up to 45 cm long in the one-and-a-half meter-long marine nitetail eel.

The transformation of leptocephalus into glass eels takes place in the open ocean. During metamorphosis, the leptocephalus loses a lot of weight (up to 90% of the body weight), the body shape changes from leaf-shaped to cylindrical. The long larval teeth are lost (in eel-like leptocephals the pelvic fins also disappear), the dorsal fin is strongly displaced backwards.