Leptocephalus
Encyclopedia
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
of the eel
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
, marine eels, and other members of the Superorder Elopomorpha
Elopomorpha
The superorder Elopomorpha contains a variety of types of fishes that range from typical silvery colored fish-like species such as the tarpons and ladyfishes of the Elopiformes and the bonefishes of the Albuliformes, to the long and slender, smooth bodied eels of the Anguilliformes...
. Fishes with a leptocephalus larva stage include the most familiar eels such as the conger
Conger
Conger is a genus of marine congrid eels. It includes some of the largest types of eels, ranging up to 3 m in length, in the case of the European conger...
, moray eel
Moray eel
Moray eels are cosmopolitan eels of the family Muraenidae. The approximately 200 species in 15 genera are almost exclusively marine, but several species are regularly seen in brackish water and a few, for example the freshwater moray can sometimes be found in freshwater...
, and garden eel
Garden eel
Heteroconger cobra, sometimes known as the cobra garden eel, is a species of garden eel of the family Congridae, found in the western Central Pacific from Honiara, the Solomon Islands to Port Moresby, Papua New Guinea....
, and the freshwater eels of the family Anguillidae
Anguillidae
Anguillidae is a family of fishes that contains the freshwater eels. There are 19 species/subspecies in this family, all in genus Anguilla. They are catadromous, meaning they spend their lives in freshwater rivers, lakes, or estuaries and return to the ocean to spawn...
, 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
Tarpon
Tarpons are large fish of the genus Megalops. There are two species of Megalops, one native to the Atlantic, and the other to the Indo-Pacific oceans.They are the only members of the family Megalopidae.- Species and habitats :...
, bonefish
Bonefish
The bonefish is the type species of the Albulidae family, or bonefishes in order Albuliformes. It is amphidromous, living in inshore tropical waters, moving onto shallow mudflats to feed with the incoming tide, and retreating to deeper water as the tide ebbs...
, spiny eel
Spiny eel
The name spiny eel is used to describe members of two different families of fish: the freshwater Mastacembelidae of Asia and Africa, and the marine Notacanthidae. Both are so-named because of their eel-like shape and sturdy fin spines....
, and pelican eel
Pelican eel
The pelican eel, or Eurypharynx pelecanoides, is a deep-sea fish rarely seen by humans, though the creatures are occasionally caught in fishing nets. It is an eel-like fish, the only known member of the genus Eurypharynx and the family Eurypharyngidae. It belongs to the order Saccopharyngiformes...
.
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 cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
s 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
Marine snow
In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below. The term was first coined by the explorer William Beebe as he...
. 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:http://www.australianmuseum.net.au/movie/Ophichthid-leptocephalus-Kona-Hawaii/
Some progress has been made in recent years to grow the leptocephali of the Japanese eel
Japanese eel
The Japanese eel, Anguilla japonica, is a species of eel found in Japan, Korea, Vietnam the East China Sea and the northern Philippines. Like the all the eels of its family, it is catadromous, meaning it lives parts of its life in both freshwater and saltwater. The specific spawning grounds have...
in the laboratory however. The goal of that effort is to produce glass eels
Glass eels
Glass eels are the intermediary stage of eels between the leptocephalus stage and the juvenile stage. The term typically refers to a tranparent glass eel of the family Anguillidae. These are the freshwater eels that spawn in the ocean, and then enter estuaries as glass eels and swim upstream to...
through artificial spawning and larval rearing, to be used for aquaculture to produce unagi
Unagi
Unagi is the Japanese word for freshwater eels, especially the Japanese eel, Anguilla japonica. Saltwater eels are known as anago in Japanese. Unagi are a common ingredient in Japanese cooking...
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
Conger myriaster
Whitespotted conger, Conger myriaster, is in the family of conger eel. Widespread in the Northwest Pacific near the coasts of Japan, Korean Peninsula, also in the East China Sea.Conger myriaster inhabit shallow sea bottom sand and mud. Conger myriaster is also consumed as food. Maximum total...
, are called Noresore·のれそれ in Kochi Prefecture
Kochi Prefecture
is a prefecture of Japan located on the south coast of Shikoku. The capital is the city of Kōchi.- History :Prior to the Meiji Restoration, Kōchi was known as Tosa Province and was controlled by the Chosokabe clan in the Sengoku period and the Yamauchi family during the Edo period.- Geography...
, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
, and are often served un-cooked to the table, and are eaten after dipping in Tosazu mixed vinegar
Vinegar
Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...
. It is a spring seasonal specialty.