The eelpouts are the ray-finned fish family Zoarcidae. As the common name suggests, they are somewhat eel-like in appearance. All of the 300 species are marine and mostly bottom-dwelling, some at great depths. Eelpouts are predominantly found in the Northern Hemisphere. The Arctic, north Pacific and north Atlantic oceans have the highest concentration of species; however, species are found around the globe.
They are conventionally placed in the "perciform" assemblage; in fact, the Zoarcoidei seem to be specialized members of the Gasterosteiformes-Scorpaeniformes group of Acanthopterygii.[1]
The largest member of the family is Zoarces americanus, which may reach 1.1 m in length. Other notable genera include Lycodapus and Gymnelus.
The eelpout family was first proposed as the family Zoarchidae in 1839 by the English naturalist William Swainson but the spelling was changed to Zoarcidae after the spelling of the genus Zoarces was corrected by Theodore Gill in 1861. The 5th edition of Fishes of the World classifies this family within the suborder Zoarcoidei, within the order Scorpaeniformes.[2] Other authorities classify this family in the infraorder Zoarcales within the suborder Cottoidei of the Perciformes because removing the Scorpaeniformes from the Perciformes renders that taxon non monophyletic.[3]
Fishes of the World mentions four subfamilies but does not assign genera to the subfamilies[2] but these were set out in Anderson and Federov's Annotated Checklist[4] and this has been followed by FishBase and Catalog of Fishes.
Eelpout species have evolved to efficiently give birth to future generations. They utilize demersal eggs, which are eggs that are deposited on the seafloor, and can be either free or connected to the substrate. These egg clusters can range from 9.2 mm, to 9.8 mm, which are the largest compared to any other marine egg cluster. It has been found that eelpouts grow larger and heavier in areas where the water is relatively shallow. In these areas, this species consumes molluscs, invertebrates, and small fish. The difference of biodiversity at varying depths has led to the evolution of distinct populations, connecting to the study that temperature might have a significant effect on them.[5] Studies have shown that there are three large families of eelpout species; Zoarcidae, Stichaeidae, and Pholidae. These species have been thought to have evolved in northern, colder seas, each diverging off of each other at different points in time, millions of years ago.[6] The notched-fin eelpout, which is commonly found in the Sea of Okhotsk, have shown researchers what the average length of an adult eelpout is, usually sitting between 21 and 26 cm long (females typically larger than males).[7] Their size has been found to increase as the depth of water in which they have been studied lowers. They feed commonly on Gammarids (small, shrimp like organisms), Polychaetes (marine worms), and Bivalves (clams and muscles) on the seafloor.
The eelpouts are classified into four subfamilies and 61 genera with around 300 species:
The body of eelpouts is relatively elongated and laterally compressed.[8] Their heads are relatively small and ovoid. Juveniles have a more rounded snout and relatively larger eye than adults.[8] Their scales are absent or very small.[9] The dorsal and anal fins are continuous down their bodies up to their caudal fin. They produce the pigment biliverdin, which turns their bones green. This feature has no apparent evolutionary function and is harmless.[10] Overall, there is no sexual dimorphism.[11]
Little is known about eelpout populations because they often slip through nets in sampling studies, and because some species live in inaccessibly deep habitats. Species for which trophic ecology has been documented are typically, if not always, benthic scavengers or predators.[10] [12] At least one species has also adapted the ability to breathe air when out of water.[10]
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Species of eelpouts have adapted in order to grow and thrive in the extreme low temperatures of their habitats. The metabolic responses of Antarctic and temperate eelpout species during exercise and subsequent recovery at 0 °C[13] is a point of emphasis when understanding this species. Contrary to the hypothesis of reduced glycolytic capacity in Antarctic fish as an adaptation to low temperatures, findings revealed similar increases in white muscle lactate, intracellular pH drop, and phosphocreatine depletion during strenuous exercise in both species. Notably, Antarctic eelpout exhibited faster recovery kinetics, including lactate clearance. This suggests a superior metabolic cold compensation mechanism compared to temperate eelpout. The study also proposed a correlation between reduced ATP energy content and muscular fatigue, highlighting the intricate metabolic adjustments crucial for sustaining activity in extreme cold conditions.[14] These environmental factors surrounding this species show how it has adapted and survived over time.
As global temperatures continue to rise, understanding how aquatic species adapt to thermal stress becomes increasingly crucial. The physiological responses of temperate eelpout (Zoarces viviparus) from the North Sea and Antarctic eelpout (Pachycara brachycephalum) to gradually increasing water temperatures were examined. The study explored parameters such as standard metabolic rate (SMR), intracellular pH regulation, and the upper critical temperature limit (TcII), to explain the species' thermal tolerance.[15] Results revealed distinct differences in metabolic responses between the two species, indicating varied thermal sensitivities and adaptation strategies. The habitat of an eelpout can vary greatly throughout the year, as seasonal temperatures can change drastically between 3 and 12 degrees C. With increasing temperatures of the water in these regions, the eelpouts struggle to cope. Certain signs of this struggle are apparent when being studied in a lab, as they raise their pectoral fins, swim around more vigorously, and attempt to jump out of their holding aquariums, leading to the conclusion that higher temperatures lead to higher levels of agitation. For short periods of time, however, this species is able to cope. These findings have implications for understanding the physiological constraints faced by eelpout fish under thermal stress and offer insights into potential shifts in species distribution patterns driven by global warming.