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.Coad, B. W., "Review of the Freshwater Eels of Iran (Family Anguillidae). Additionally, freshwater eels possess small, granular teeth arranged in bands on the jaws and vomer. Anguillidae do exhibit size-dependent sexual dimorphism. Male anguillids invest more energy into mating with as many females as they can, than they do into growth. Therefore, female anguillids are usually larger, ranging from , while male anguillids rarely get larger than long. 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. Freshwater eels go through physical changes in their bodies when going to and from the ocean for different stages of life.
Aside from their reduced pectoral fins, eels lack paired appendages. They use axial-based lateral undulation as means of locomotion, similar to snakes. High maneuverability of the trunk is an adaptation for hunting in structurally-complex habitats such as reefs. 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.
Sensory
Anguillidae, unlike their other relatives, have a fully developed lateral line along their trunk. The lateral line provides the ability for Anguillidae to sense their surrounding environment through water displacement, which aids in predation and hunting, especially because they are predominantly nocturnal generalists.
Olfactory senses in this family are heightened for various reasons. Within the nasal sac are olfactory cells which have the capability of detecting extremely diluted chemicals as low as three to four molecules. This is extremely helpful in their nocturnal endeavors as well as for migratory purposes. They use terrestrial odors as cues in migration as well as low salinity and colder temperatures to direct themselves.
Geomagnetic sensing has been identified as one of the most important specialized senses in this family. Unlike the other relatives, the Anguillidae are catadromous, meaning they must migrate for an extended period and, depending on what life stage they are in, they may be in the open ocean. The Anguillidae were placed in a "magnetic displacement" experiment where the geomagnetic north could be altered, and their actions could be monitored. The results showed that at different stages of life, the Anguillidae, are capable of responding to the geomagnetic field and will alter their direction of interest accordingly. They depend on the intensity and inclination of the magnetic field to migrate. Within this experiment researchers also looked at how the glass eel may rely on the circatidal rhythm in the ocean to work its way back to the coast and into the freshwater systems, but this problem is not yet fully understood.
Physiology
Paired frontal bones of skull make for a stronger cranium which aids in their variable burrowing tendencies with mud as well as maneuvering through terrestrial obstacles when hiding under rocks and logs that they encounter at the water's bottom where they spend most of their time during the day.
Ventral lateral gill slits make up eighty-five percent of gas exchange and are highly efficient in converting between salt and freshwater. This feature separates the Anguillidae, freshwater eels, from other eels that have an internal gill chamber.
Many species have variegated skin, which means they will vary in color depending on their environment. This adaptation helps them camouflage, which they utilize for ambushing prey and hiding from predators.
The dorsal, anal and caudal fins are fused and pelvic fins are lacking. Dorsal fin begins at the mid body, creating a long continuous fin, where in other species it begins more posterior and is not as prominent in length. Their body movement depends highly on undulation originating near the anterior axial end. Due to the fins being fused we see a highly skilled swimmer amongst the Anguillidae which aids in migration and hunting/predation.
It was mistakenly reported that Anguillidae lack a scapular bone but after further research and more advanced staining techniques they have a scapula and coracoid which make up their pectoral girdle. The presence of a scapula is important in muscle attachment and allows for the upper head to move in various directions as well as increase strength of body undulation, thus increasing swimming capabilities. The presence of a scapula allows for stronger movements of pectoral fins which aid in movement across terrestrial obstacles.
Other features
Cutaneous respiration accounts for approximately fifteen percent of their oxygen intake but when they are out of water, they are capable of receiving approximately fifty percent of their oxygen through gas exchange via the outer integument. This is an important feature since Anguillidae at many times need to move between bodies of water to maintain an aquatic environment. It is also known that they will burrow down into mud so having the capability to exchange gas outside of water is highly beneficial to this family.
It is known that when bodies of water start to dry up the Anguillidae burrow down into the mud and wait for rain while undergoing torpor. Since rain is not predictable, torpor allows for the organism to lower its metabolic rate as well as its body temperature increasing its survivability.
Anguillidae are great swimmers due to their axial muscle attachment and W-shaped myomeres giving them the capability of swimming backwards just as well as they can swim forward. Not many other fish can do so. Since their mouths are not very large, they use their swimming capabilities to aid in feeding where they will bite onto their food and twist/spin rapidly tearing off a piece perfectly sized for them.
Population density sex determination is a feature where the Anguillidae regulate their population's gender depending on the abundance of eggs present. High egg concentrations will result in more male than female ratios as well as vice versa. It does not mean that the eggs will all become one sex but rather have a higher ratio in one of the two sexes present.
Mucous cells within the epidermis are found in both the non-sexually mature and sexually mature adult stages. The mucous cells are made of glycoproteins which are found in higher concentrations on the dorsal and ventral sides of the body. It is believed that the family Anguillidae has higher concentrations of slime than other families. This aids in predation as well as helping keep themselves moist outside of water, increasing the efficiency of cutaneous respiration.
Metamorphosis is a big part in the lives of Anguillidae and many changes happen in preparation for migration amongst the adults going from the yellow eel stage to the silver eel stage. The gas bladder adapts for higher pressures which it will be exposed to in the ocean where it will dive much deeper in search of food and avoiding strong currents. Fat reserves increase in preparation for less abundant food sources in the ocean. Females will experience a higher increase than males for the reason of egg production. The eyes also change increasing in size by two times and retinal pigments which are sensitive to red light in shallow waters change to pigments that are sensitive to blue light which is better adapted for the deep ocean that the silver eel will be experiencing.
One experiment talked about the driving force that is seen in Anguillidae. When held in captivity it was reported that they would hit their heads against the glass or make fast for an escape route most likely looking for the fresh or saltwater they seek. This was a key indicator that they are constantly migrating.
Distribution and conservation
Anguillid eels have a global distribution, and inhabit the waters of more than 150 countries. They are mainly found in tropical and temperate waters, except in the Eastern Pacific and South Atlantic. 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.
Biology
Freshwater eels are aquatic and live in various habitats, including freshwater, estuaries, and saltwater/marine habitats. They occupy the roles of both predator and prey, and evidence has been found of nematode parasitism in some species.
Eels in the family Anguillidae are known to be primarily solitary in nature; they are not known to communicate socially or actively school, however large masses of elvers can be found as a result of synchronicity in response to environmental conditions. These eels are known as generalists and opportunistic feeders; most will consume whatever acceptable prey they happen upon, including things like crustaceans, fishes, and other aquatic fauna. Some eel species have been observed consuming the eggs of predatory fish such as trout, aiding in population control in these systems.