A leptocephalus (meaning “slim head”) is the flat and transparent larva of the eel, marine eels, and other members of the Superorder Elopomorpha. These fishes with a leptocephalus larva stage include the most familiar eels such as the conger, moray eel, and garden eel, and the freshwater eels of the family Anguillidae, plus more than 10 other families of lesser-known types of marine eels. These are all true eels of the order Anguilliformes. The fishes of the other four traditional orders of elopomorph fishes that have this type of larva are more diverse in their body forms and include the tarpon, bonefish, spiny eel, and pelican eel.
Leptocephali (more than one leptocephalus) all have laterally compressed bodies that contain transparent jelly-like substances on the inside of the body and a thin layer of muscle on the outside. Their body organs are small, and this combination of features results in them being very transparent when they are alive. They also lack red blood cells until they begin to metamorphose into the juvenile glass eel stage when they start to look like eels.
Leptocephali differ from most fish larvae because they grow to much larger sizes (about 60–300 mm and sometimes larger) and have long larval periods of about 3 months to more than a year. They move with typical anguilliform swimming motions and can swim both forwards and backwards. Their food source was difficult to determine because no zooplankton, which are the typical food of fish larvae, were ever seen in their guts. It was recently found though, that they appear to feed on tiny particles floating free in the ocean, which are often referred to as marine snow. Leptocephalus larvae live primarily in the upper 100 meters of the ocean at night, and often a little deeper during the day. Leptocephali are present worldwide in the ocean from southern temperate to tropical latitudes, where adult eels and their close relatives live.
This particular type of fish larva is poorly understood, partly because they are very fragile and eat particulate material instead of zooplankton, plus their good swimming ability enables them to avoid most standard sized plankton nets used by marine biologists. A video recording of a naturally swimming leptocephalus filmed at night off the Island of Hawaii shows an example of their swimming behavior:
Some progress has been made in recent years to grow the leptocephali of the Japanese eel in the laboratory however. The goal of that effort is to produce glass eels through artificial spawning and larval rearing, to be used for aquaculture to produce unagi for food. Unagi is a popular food in Japan and East Asia.
Leptocephali themselves are rarely used as food, except in some parts of Japan. The leptocephali of the common Japanese conger, Conger myriaster, are called Noresore·のれそれ in Kochi Prefecture, Japan, and are often served un-cooked to the table, and are eaten after dipping in Tosazu mixed vinegar. It is a spring seasonal specialty.