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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 one of the most diverse groups 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 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.
The development of leptocephali can be summarized briefly. The eggs of eels in the Anguilliformes order are quite large in comparison to many other fishes. They are about 1-4mm in diameter. Once the larvae are newly hatched, the yolk extends posteriorly. Some larvae hatch with features of the head more developed than others. The pre-leptocephalus stage is the period immediately after the larvae hatch. During this stage, the larvae do not yet feed on externally. These larvae typically have poorly developed eyes as well as few or no teeth.
The leptocephalus stage of the larvae begins after the nutrients from the yolk have been absorbed and the eyes and teeth are formed. At this point, the larvae usually have long, forward-facing, pointy teeth. As the larvae grow larger, the teeth will be replaced by shorter ones and will also increase in number. The maximum size larvae reach varies from about 50-100 mm but can be as large as 300 mm or more depending on the species. After the leptocephali larvae have reached their maximum size, they enter their glass eel stage. Their laterally compressed bodies tend to become more rounded in this stage. The head also thickens, the olfactory organ enlarges, and their teeth are lost.
Leptocephali differ from other fish larvae is in their development. In other teleosts (those without leptocephali), the egg hatches and then the larvae gets nutrients from a yolk sac. Following this, external feeding begins once the yolk sac has depleted. The larvae begins to increase in size and develop into a juvenile fish once external feeding begins. In those fish with a leptocephali stage; however, after hatching and obtaining nutrients from the yolk, the larvae do NOT begin external feeding. This is peculiar due to the fact that the larvae still continue to grow in size. From this information, it is concluded that one of the biggest and most basic differences in the developmental cycles of teleosts without leptocephali and teleosts with leptocephali is the food source that the larvae use.
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:
There are two interesting facts regarding the visual system of the leptocephali. The first is that they have a rod-dominated visual system. The second is that the Synaphobranchidae (a specific family of leptocephalus) have telescopic eyes, meaning that there is a tubular eye with a sphere-shaped lens on the top.
Some progress has been made in recent years[when?] 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.
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- Taylor, Scott Michael, Ellis Loew, and Michael Grace. “A rod-dominated visual system in leptocephalus larvae of elopomorph fishes (Elopomorpha: Teleostei).” Environmental Biology of Fishes 92 (2011): 513-523. Web. 2 Nov. 2012.