Leptocephalus

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Leptocephalus larva LeptocephalusConger.jpg
Leptocephalus larva

A leptocephalus (meaning "slim head" [1] ) 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 in 4 orders, 24 families, and 156 genera. This group is thought to have arisen in the Cretaceous period over 140 million years ago. [2]

Contents

Fishes 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. These are all true eels of the order Anguilliformes. Leptocephali of eight species of eels from the South Atlantic Ocean were described by Meyer-Rochow [3]

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 [3] and notacanthidae species, the latter with giant Leptocephalus-like larvae. [4]

Description

Leptocephali (singular leptocephalus) all have laterally compressed bodies [5] 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 have only a simple tube for a gut. This combination of features results in them being very transparent when they are alive. Leptocephali have dorsal and anal fins confluent with caudal fins, but 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. [6]

Leptocephali differ from most fish larvae because they grow to much larger sizes [5] and have long larval periods of about three months to more than a year. Another distinguishing feature of these organisms is their mucinous pouch. [2] They move with typical anguilliform swimming motions and can swim forwards and backwards. [5] Their food source was difficult to determine because zooplankton, which are the typical food of fish larvae, were never seen in their guts. [7]

Leptocephali appear to feed on tiny particles floating freely in the ocean, which are often referred to as marine snow. Leptocephali larvae live primarily in the upper 100 metres (330 ft) of the ocean at night, [5] and often a little deeper during the day. Leptocephali are present worldwide in the oceans from southern temperate to tropical latitudes, where adult eels and their close relatives live. American eels, European eels, conger eels, and some oceanic species spawn and are found in the Sargasso Sea. [8]

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. [9]

Development

The eggs of eels in the order Anguilliformes are quite large compared to those of many other fishes. They are about 1–4 millimetres (1,000–4,000 μm; 39–157 thou) 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 preleptocephalus stage is the period immediately after the larvae hatch. During this stage, the larvae do not yet feed externally. These larvae typically have poorly developed eyes and few or no teeth. [5]

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 teeth and will increase in number. 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. After the leptocephali 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. [5]

Leptocephali differ from other fish larvae in their development. In other teleosts (those without leptocephali), the egg hatches and then the larvae get nutrients from a yolk sac. Following this, external feeding begins once the yolk sac has depleted. The larvae begin to increase in size and develop into a juvenile fish once external feeding begins. In those fish with a leptocephalus stage; however, after hatching and obtaining nutrients from the yolk, the larvae do not begin external feeding. This is peculiar because 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. [10]

Leptocephali are poorly understood, partly because they are very fragile and eat particulate material instead of zooplankton, and their good swimming ability lets them 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. [11]

Visual system

Two facts are known regarding the visual system of the leptocephali. The first is that they have a rod-dominated visual system. [12] The second is that the Synaphobranchidae (a specific family of leptocephali) have telescopic eyes, meaning a tubular eye with a sphere-shaped lens on the top occurs. [5]

Aquaculture

Some progress has been made to grow the leptocephali of the Japanese eel in the laboratory. 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. [13] Due to the strange nature of the leptocephali, this has not been easy. Trying to artificially recreate marine snow has proven difficult for scientists. [14]

Leptocephali 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 Kōchi Prefecture, Japan, and are often served uncooked to the table, and are eaten after dipping in tosazu mixed vinegar. It is a spring seasonal specialty. [15]

See also

Related Research Articles

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<span class="mw-page-title-main">European eel</span> Species of fish

The European eel is a species of eel. They are critically endangered due to overfishing by fisheries on coasts for human consumption and parasites.

<span class="mw-page-title-main">American eel</span> Species of fish

The American eel is a facultative catadromous fish found on the eastern coast of North America. Freshwater eels are fish belonging to the elopomorph superorder, a group of phylogenetically ancient teleosts. The American eel has a slender, supple, snake-like body that is covered with a mucus layer, which makes the eel appear to be naked and slimy despite the presence of minute scales. A long dorsal fin runs from the middle of the back and is continuous with a similar ventral fin. Pelvic fins are absent, and relatively small pectoral fins can be found near the midline, followed by the head and gill covers. Variations exist in coloration, from olive green, brown shading to greenish-yellow and light gray or white on the belly. Eels from clear water are often lighter than those from dark, tannic acid streams.

<span class="mw-page-title-main">Eel life history</span> Eel life cycle

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<span class="mw-page-title-main">Frilled shark</span> Species of shark

The frilled shark, also known as the lizard shark, is one of the two extant species of shark in the family Chlamydoselachidae. The frilled shark is considered a living fossil, because of its primitive, anguilliform (eel-like) physical traits, such as a dark-brown color, amphistyly, and a 2.0 m (6.6 ft)–long body, which has dorsal, pelvic, and anal fins located towards the tail. The common name, frilled shark, derives from the fringed appearance of the six pairs of gill slits at the shark's throat.

<span class="mw-page-title-main">Teleost</span> Infraclass of fishes

Teleostei, members of which are known as teleosts, is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, and contains 96% of all extant species of fish. Teleosts are arranged into about 40 orders and 448 families. Over 26,000 species have been described. Teleosts range from giant oarfish measuring 7.6 m (25 ft) or more, and ocean sunfish weighing over 2 t, to the minute male anglerfish Photocorynus spiniceps, just 6.2 mm (0.24 in) long. Including not only torpedo-shaped fish built for speed, teleosts can be flattened vertically or horizontally, be elongated cylinders or take specialised shapes as in anglerfish and seahorses.

<span class="mw-page-title-main">Fish locomotion</span> Ways that fish move around

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<span class="mw-page-title-main">Pelican eel</span> Species of fish

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<span class="mw-page-title-main">Elopomorpha</span> Superorder of fishes

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<span class="mw-page-title-main">Snipe eel</span> Family of fishes

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The Japanese eel is a species of anguillid eel found in Japan, Korea, Taiwan, China, and Vietnam, as well as the northern Philippines. Like all the eels of the genus Anguilla and the family Anguillidae, it is catadromous, meaning it spawns in the sea but lives parts of its life in freshwater. Raised in aquaculture ponds in most countries, the Japanese eel makes up 95% of the commercially sold eel in Japan, the other 5% is shipped over by air to the country from Europe. This food in Japan is called unagi; they are an essential part of the food culture, with many restaurants serving grilled eel called kabayaki. However, presumably due to a combination of overfishing and habitat loss or changing water conditions in the ocean interfering with spawning and the transport of their leptocephali this species is endangered.

<i>Leptocephalus</i> (genus) Genus of fishes

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<span class="mw-page-title-main">Juvenile fish</span> Young fish

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Congriscus megastomus is an eel in the family Congridae. It was described by Albert Günther in 1877, originally under the genus Congromuraena. It is a marine, temperate-water dwelling eel which is known from Japan and the Kyushu–Palau Ridge, in the northwestern Pacific Ocean. It inhabits reefs. Males can reach a maximum total length of 40 centimeters. This species' remarkably large and distinct leptocephalus larvae were previously known under the name Thalassenchelys coheni. In 2016, scientists used genetic techniques to link the larvae to the adult C. megastomus. These larvae reach a maximum size of 30 cm (12 in) and have a number of unusual characteristics, including two forward-facing front teeth that may be used for feeding on different prey than other eel larvae.

References

  1. ↑ "Leptocephalus | Definition of leptocephalus by Merriam-Webster". www.merriam-webster.com. Merriam-Webster Online Dictionary. Retrieved 2015-11-27.
  2. 1 2 Inuoe, Jun, M. Miya, et al. “Mitogenomic evidence for the monophyly of elopomorph fishes (Teleostei) and the evolutionary origin of the leptocephalus larva.” Molecular Phylogenetics and Evolution 32 (2004): 274-286. Web. 2 Nov. 2012.
  3. 1 2 Meyer-Rochow, Victor Benno (1974). "Leptocephali and other transparent fish larvae from the South-Eastern Atlantic Ocean". Zoologischer Anzeiger. 192: 240–251.
  4. ↑ Castle, Peter H.J. (1973). "A giant notacanthiform Leptocephalus from the Chatham Islands, New Zealand". Records of the Dominion Museum of New Zealand. 8: 121–124.
  5. 1 2 3 4 5 6 7 Miller, Michael J. “Ecology of Anguilliform Leptocephali: Remarkable Transparent Fish Larvae of the Ocean Surface Layer.” Aqua-BioScience Monographs 2 (2009): 1-94. Web. 2 Nov. 2012.
  6. ↑ Fahay, Michael (1983-07-01). "Guide to the Early Stages of Marine Fishes occurring in the Western North Atlantic Ocean, Cape Hatteras to the Southern Scotian Shelf." Journal of Northwest Atlantic Fishery Science 4 (1983): 46-61, 1 July 1983. Retrieved on 2012-11-02 from http://journal.nafo.int/Volumes/Articles/ID/14/Guide-to-the-Early-Stages-of-Marine-Fishes-occurring-in-the-Western-North-Atlantic-Ocean-Cape-Hatteras-to-the-Southern-Scotian-Shelf.
  7. ↑ Mochioka, N.; Iwamizu, M. (1996-05-01). "Diet of anguilloid larvae: leptocephali feed selectively on larvacean houses and fecal pellets". Marine Biology. 125 (3): 447–452. doi:10.1007/BF00353257. ISSN   1432-1793.
  8. ↑ McCleave, J. D. "Physical and behavioural controls on the oceanic distribution and migration of leptocephali". Journal of Fish Biology. 43 (sA): 243–273. doi:10.1111/j.1095-8649.1993.tb01191.x. ISSN   0022-1112.
  9. ↑ "WoRMS - World Register of Marine Species - Leptocephalus brevirostris Kaup, 1856". marinespecies.org. Retrieved 2015-11-27.
  10. ↑ Pfeiler, Edward (1986). "Towards and Explanation of the Developmental Strategy in Leptocephalus Larvae of Marine Teleost Fishes." Environmental Biology of Fishes 15 (1986): 3-13. Retrieved on 2012-11-02 from Web.
  11. ↑ "Ophichthid leptocephalus off Kona, Hawaii - Australian Museum". australianmuseum.net.au. Retrieved 2015-11-27.
  12. ↑ Taylor, Scott Michael, Ellis Loew, and Michael Grace (2011). “A rod-dominated visual system in leptocephalus larvae of elopomorph fishes (Elopomorpha: Teleostei).” Environmental Biology of Fishes 92 (2011): 513-523. Retrieved on 2012-11-02 from Web.
  13. ↑ Shoji, Kaori (2018-10-06). "Expensive and unsustainable, 'unagi' remains as popular as ever". The Japan Times Online. ISSN   0447-5763 . Retrieved 2019-01-27.
  14. ↑ Bird, Winifred. "In Japan, Captive Breeding May Help Save the Wild Eel". Yale Environment 360. Yale School of the Environment. Retrieved 28 April 2021.
  15. ↑ Says, Gigibeyon (2009-01-15). "Japanese Seasonal Fish: Noresore/Conger Eel Whitebait". SHIZUOKA GOURMET. Archived from the original on 3 July 2018. Retrieved 2019-01-27.
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