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. It is supposed that this group arose in the Cretaceous period over 140 million years ago. 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 (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. While leptocephali have dorsal and anal fins that are confluent with caudal fins, they lack pelvic fins. 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 are also characterized by their fang-like teeth that are present until metamorphosis, when they are lost.
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. Another distinguishing feature of these organisms is their mucinous pouch.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[by whom?] 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. Leptocephali 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. American eels, European eels, conger eels, as well as some oceanic species spawn and are found in the Sargasso Sea.
Indian glassy fish
The Indian glassy fish, Parambassis ranga, is a species of freshwater fish in the Asiatic glassfish family (family Ambassidae) of order Perciformes. It is native to an area of south Asia from Pakistan to Malaysia. The Indian glassy fish has a striking transparent body revealing its bones and internal organs; the male develops a dark edge to the dorsal fin. The fish grows to a maximum overall length of 80 millimetres (3.1 in).
The glass catfish (Kryptopterus bicirrhis) is an Asian glass catfish of the genus Kryptopterus. Until 1989, it included its smaller relative the ghost catfish, now known as K. minor. Its scientific name and common name are often still used in the aquarium fish trade to refer to the ghost catfish; as it seems, the larger and more aggressive K. bicirrhis was only ever exported in insignificant numbers, if at all.