A leptocephalus (meaning “slim head”) is the flat and transparent larva of the eel, marine eels, and other members of the Superorder Elopomorpha. This is the most diverse group of teleosts, containing 801 species over the span of 24 orders, 24 families, and 156 genera. Fishes with a leptocephalus larva stage include the most familiar eels such as the conger, moray eel, garden eel, and the freshwater eels of the family Anguillidae. Leptocephali (singular leptocephalus) all have laterally compressed bodies that contain transparent jelly-like substances on the inside of the body and a thin layer of muscle with visible myomeres on the outside.
Their body organs are small and they possess only a simple tube for a gut. 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. [Source: Wikipedia]
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