Leptocephalus

This group is thought to have arisen in the Cretaceous period over 140 million years ago.

Leptocephali of eight species of eels from the South Atlantic Ocean were described by Meyer-Rochow [3] The fishes of the other four traditional orders of elopomorph fishes that have this type of larvae are more diverse in their body forms and include the tarpon, bonefish, spiny eel, pelican eel and deep sea species like Cyema atrum[3] and notacanthidae species, the latter with giant Leptocephalus-like larvae.

[6] Leptocephali differ from most fish larvae because they grow to much larger sizes[5] and have long larval periods of about three months to more than a year.

[7] Leptocephali appear to feed on tiny particles floating freely in the ocean, which are often referred to as marine snow.

Leptocephali larvae live primarily in the upper 100 metres (330 ft) of the ocean at night,[5] and often a little deeper during the day.

Leptocephali are present worldwide in the oceans from southern temperate to tropical latitudes, where adult eels and their close relatives live.

[5] The leptocephalus stage of the larvae begins after the nutrients from the yolk have been absorbed and the eyes and teeth are formed.

After the leptocephali have reached their maximum size, they enter their glass eel stage.

In other teleosts (those without leptocephali), the egg hatches and then the larvae get nutrients from a yolk sac.

In those fish with a leptocephalus stage; however, after hatching and obtaining nutrients from the yolk, the larvae do not begin external feeding.

[10] Leptocephali are poorly understood, partly because they are very fragile and eat particulate material instead of zooplankton, and their good swimming ability lets them avoid most standard-sized plankton nets used by marine biologists.