![]() Leptocephali differ from other fish larvae in their development. The head also thickens, the olfactory organ enlarges, and their teeth are lost. Their laterally compressed bodies tend to become more rounded in this stage. After the leptocephali have reached their maximum size, they enter their glass eel stage. The maximum size larvae reach varies from about 5–10 centimetres (2–4 in) but can be as large as 30 centimetres (12 in) or more depending on the species. ![]() As the larvae grow larger, the teeth will be replaced by shorter teeth and will increase in number. At this point, the larvae usually have long forward-facing pointy teeth. The leptocephalus stage of the larvae begins after the nutrients from the yolk have been absorbed and the eyes and teeth are formed. These larvae typically have poorly developed eyes and few or no teeth. During this stage, the larvae do not yet feed externally. ![]() The preleptocephalus stage is the period immediately after the larvae hatch. Some larvae hatch with features of the head more developed than others. Once the larvae are newly hatched, the yolk extends posteriorly. The eggs of eels in the order Anguilliformes are quite large compared to those of many other fishes. Leptocephalus brevirostris was declared as a biological species in 1856, but was later found to be the larva of Anguilla anguilla, which was declared by Linnaeus in 1758 and thus has priority. American eels, European eels, conger eels, and some oceanic species spawn and are found in the Sargasso Sea. Leptocephali are present worldwide in the oceans from southern temperate to tropical latitudes, where adult eels and their close relatives live. Leptocephali larvae live primarily in the upper 100 metres (330 ft) of the ocean at night, and often a little deeper during the day. Leptocephali appear to feed on tiny particles floating freely in the ocean, which are often referred to as marine snow. Their food source was difficult to determine because zooplankton, which are the typical food of fish larvae, were never seen in their guts. They move with typical anguilliform swimming motions and can swim forwards and backwards. Another distinguishing feature of these organisms is their mucinous pouch. Leptocephali differ from most fish larvae because they grow to much larger sizes and have long larval periods of about three months to more than a year. Leptocephali are also characterized by their fang-like teeth that are present until metamorphosis, when they are lost. 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 have dorsal and anal fins confluent with caudal fins, but lack pelvic fins. This combination of features results in them being very transparent when they are alive. Their body organs are small and they have only a simple tube for a gut. 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. 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 and notacanthidae species, the latter with giant Leptocephalus-like larvae. ![]() Leptocephali of eight species of eels from the South Atlantic Ocean were described by Meyer-Rochow These are all true eels of the order Anguilliformes. įishes with a leptocephalus larval stage include the most familiar eels such as the conger, moray eel, and garden eel as well as members of the family Anguillidae, plus more than 10 other families of lesser-known types of marine eels. This group is thought to have arisen in the Cretaceous period over 140 million years ago. This is one of the most diverse groups of teleosts, containing 801 species in 4 orders, 24 families, and 156 genera. Leptocephalus (meaning "slim head" ) is the flat and transparent larva of the eel, marine eels, and other members of the superorder Elopomorpha.
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