Anguillidae - WikiHQ

The Anguillidae are a family of ray-finned fish that contains the freshwater eels. All the extant species and six subspecies in this family are in the genus Anguilla, and are elongated fish of snake-like bodies, with long dorsal, caudal and anal fins forming a continuous fringe. They are catadromous, spending their adult lives in freshwater, but migrating to the ocean to spawn.

Eels are an important food fish and some species are now farm-raised, but not bred in captivity. Many populations in the wild are now threatened, and Seafood Watch recommends consumers avoid eating anguillid eels.

Description

Adult freshwater eels are elongated with tubelike, snake-shaped bodies. They have large, pointed heads and their dorsal fins are usually continuous with their caudal and anal fins, to form a fringe lining the posterior end of their bodies. They have relatively well developed eyes and pectoral fins compared to saltwater eels that they use to navigate and maneuver through river bottoms and shallow water. Unlike most eels, freshwater eels have not lost their scales, and instead have soft, thin, scales that are embedded in the epidermis. Additionally, freshwater eels possess small, granular teeth arranged in bands on the jaws and vomer. Adult Anguillidae can vary in color, but normally are brown, olive or olive-yellow, and can be mottled. Coloration matches the floor of rivers and lakes which prevents the eels from being seen by predators while in clear or shallow water. Some species are known to burrow into the sea bed/sediment, including species that utilize head-first or tail-first burrowing techniques. This is related to both foraging and anti-predatory behavior. Conservation is difficult for this taxon because not much is known about their life history and behaviors. However, many Anguillid eels are of conservation concern, including the European eel (A. anguilla), the American eel (A. rostrata), the Japanese eel (A. japonica), the New Zealand longfin eel (A. dieffenbachii), and the Indonesian longfinned eel (A. borneensis). Threats to these species include: habitat loss/modification, migration barriers, pollution, parasitism, exploitation, and consumption, as eels are a popular food source especially in Asia and Europe. Fluctuating oceanic conditions associated with climate change also make these species vulnerable, with reduced water quality leading to biodiversity loss among the largest threats. In the Northern hemisphere, anguillid eels have had large declines in populations due to a number of reasons including overexploitation and migration inhibition via migration barriers. According to the IUCN Anguillid Eel Specialist Group, or the AESG, the need for conservation of this family is clear given recent declines. However, conservation efforts are being inhibited by a lack of knowledge of the biology of these species, especially in their social and spawning behavior, as well as a lack of long-term data sets. Some eel species have been observed consuming the eggs of predatory fish such as trout, aiding in population control in these systems.

Juvenile eels occupy small spaces in between rocks, in crevices or mud. Freshwater eels are widespread and are catadromous, meaning they spend most of their life in freshwater (rivers mainly) and migrate to the ocean to breed. Leptocephali (larval) migration can range from months to up to almost a year. Temperate eels migrate on average for approximately 6–10 months, while tropical eels undergo shorter migrations between approximately 3–5 months on average. The European eel (A. anguilla) has one of the longest migrations of all freshwater eels, migrating up to 6000 km (over 3700 miles) in a single migration loop. Migration loops may be flexible in some species, and this variability is still being investigated. However, some eels in this family have altered their migration loop to become completely marine, not returning to fresh waters to develop. Tsukamoto and associates found evidence of Japanese eels (A. japonica) may synchronize their breeding cycles during the spawning season with the new moon.

Members of this family spend their lives in freshwater rivers, lakes, or estuaries, and return to the ocean to spawn. All eels pass through several stages of development through their life cycle. Anguillid eels undergo morphological changes during these developmental stages that are associated with environmental conditions and aid in preparing them for further growth and finally reproduction. Anguillid eels begin their life as an egg in the ocean, and once hatched, enter a larval stage called leptocephali. The young eel larvae live only in the ocean and consume small particles called marine snow. Anguillid eels lay adhesive demersal eggs (eggs that are free-floating or attached to substrate), and most species have no parental care. Japanese eels (A. japonica) can lay between 2 million and 10 million eggs.

Seafood Watch, one of the better-known sustainable seafood advisory lists, recommends consumers avoid eating anguillid eels due to significant pressures on worldwide populations. Several species used as unagi have seen their population sizes greatly reduced in the past half century. Catches of the European eel, for example, have declined about 80% since the 1960s. Although about 90% of freshwater eels consumed in the US are farm-raised, they are not bred in captivity. Instead, young eels are collected from the wild and then raised in various enclosures. In addition to wild eel populations being reduced by this process, eels are often farmed in open-net pens, which allow parasites, waste products, and diseases to flow directly back into wild eel habitat, further threatening wild populations. Freshwater eels are carnivores so are fed other wild-caught fish, adding another element of unsustainability to current eel-farming practices.

Systematics

The exact placement of freshwater eels is still being debated, but there is a general consensus that Anguillidae are firmly nested within Anguilliformes. Traditionally, molecular studies have placed Anguillidae in the subclass "Anguilloidei" with two other families: Nemichthyidae (snipe eels) and Serrivomeridae (sawtooth eels). Until 2013, this subclass had been lumped together into a cohesive clade. However, recent molecular studies have suggested that Anguillidae are actually more closely related to the Saccopharyngiforms (Gulpers and relatives) than they are to the other Anguilloid families.

Based on genetic analysis, it is estimated that Anguillidae diverged from other eels about 52 million years ago, and extant Anguilla species began speciating about 20 million years ago.

Species

†Anguillidarum Schwarzhans 2003
†Anguillidarum semisphaeroides Schwarzhans 2003
Anguilla Garsault 1764
Anguilla anguilla (Linnaeus, 1758) (European eel)
†Anguilla annosa Stinton 1975
Anguilla australis J. Richardson, 1841
Anguilla australis australis J. Richardson, 1841 (short-finned eel)
Anguilla australis schmidti Phillipps, 1925
Anguilla bengalensis (J. E. Gray, 1831) (mottled eel)
Anguilla bengalensis bengalensis (J. E. Gray, 1831) (Indian mottled eel)
Anguilla bengalensis labiata (W. K. H. Peters, 1852) (African mottled eel)
Anguilla bicolor McClelland, 1844
Anguilla bicolor bicolor McClelland, 1844 (Indonesian shortfin eel)
Anguilla bicolor pacifica E. J. Schmidt, 1928 (Indian shortfin eel)
Anguilla borneensis Popta, 1924 (Borneo eel)
Anguilla breviceps Y. T. Chu & Y. T. Jin, 1984
†Anguilla brevicula Agassiz 1833–1845
Anguilla celebesensis Kaup, 1856 (Celebes longfin eel)
Anguilla dieffenbachii J. E. Gray, 1842 (New Zealand longfin eel)
Anguilla interioris Whitley, 1938 (Highlands longfin eel)
Anguilla japonica Temminck & Schlegel, 1847 (Japanese eel)
Anguilla luzonensis S. Watanabe, Aoyama & Tsukamoto, 2009 (Philippine mottled eel)
Anguilla marmorata Quoy & Gaimard, 1824 (giant mottled eel)
Anguilla megastoma Kaup, 1856 (Polynesian longfin eel)
Anguilla mossambica (W. K. H. Peters, 1852) (African longfin eel)
†Anguilla multiradiata Agassiz 1833–1845
Anguilla nebulosa McClelland, 1844 (mottled eel)
Anguilla obscura Günther, 1872 (Pacific shortfinned eel)
†Anguilla pachyura Agassiz 1833–1845
†Anguilla pfeili Schwarzhans 2012
†Anguilla rectangularis Stinton & Nolf 1970
Anguilla reinhardtii Steindachner, 1867 (speckled longfin eel)
Anguilla rostrata (Lesueur, 1817) (American eel)
†Anguilla rouxi Nolf 1974
†Eoanguilla Blot, 1978

Paleontology

Although Anguilla is the only modern representative of the family Anguillidae, a second marine genus, Eoanguilla, is known from the Early Eocene of Monte Bolca in Italy, and appears to be the oldest known representative of Anguillidae known from full body fossils (slightly younger than Anguilla ignota).

There are two important fossils used to date the origin of freshwater eels. The first is the fossil Nardoechelys robinsi which represents the ancestor to all extant eels, and marks the lower-boundary of the age of Anguillidae. The second is Anguilla ignota, which is the fossil that represents the ancestor to all extant freshwater eels and marks the upper boundary of the age of Anguillidae. Using these two fossil calibration points, freshwater eels are said to originate between 83 million years ago and 43.8 million years ago.

N. robinsi was found by Italian scientists in 2002 in the Santonian-Campanian Calcari di Melissano, which is a fossil bed located near the town of Nardò. Strontium-isotope stratigraphy concluded the age of N. robinsi to be 83 million years old. The fossil was discovered incomplete and lacked the skull and part of the anterior skeleton. Despite the morphological uncertainty, cranial and branchial features confirmed it was an eel. At first, it was classified the earliest member of the eel family, Ophichthidae (snake eels). Therefore, if the oldest eel is 83 million years old, it can be concluded that Anguillidae could not have originated any earlier than that.

The earliest known, unequivocal fossil of an anguillid eel is Anguilla ignota and was found in Messel, Germany. The Messel fossil deposit is dated to be 43.8 million years old during the mid-Eocene epoch. During this time period, Messel was undergoing intense volcanic activity which resulted in the formation of freshwater maar lakes. A. ignota was found in the geological remains of one of these lakes, which makes it the oldest eel to inhabit a freshwater environment. This fossil is commonly used as a calibration fossil to pinpoint the lower boundary of the age of freshwater eels.