''Argyropelecus affinis'' is a species of

ray-finned fish Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species. The ray-finned fishes are so called because their fins are webs of skin supported by bony or hor ...

in the family Sternoptychidae, found in the tropical and subtropical Atlantic, Indian and Pacific Oceans. Common names for this fish include Pacific hatchetfish, deepsea hatchetfish and slender hatchetfish. It inhabits the

mesopelagic zone The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins ...

and is either non-migratory or performs short daily vertical migrations.

Description

''Argyropelecus affinis'' is a small laterally compressed fish with a

standard length Fish measurement is the measuring of individual fish and various parts of their anatomies. These data are used in many areas of ichthyology, including taxonomy and fisheries biology. Overall length * Standard length (SL) is the length of a fish m ...

of up to . It has a vertical mouth and tubular eyes that are directed upwards. There is a short spine in front of the operculum but no spine behind the eye. The dorsal fin has 9 short soft rays and there is also a dorsal adipose fin. The pectoral fin has 11 or 12 soft rays, the pelvic fin 6 and the anal fin 13. There are several short post-abdominal spines. The

swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled Organ (anatomy), organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their curren ...

is gas-filled and well-developed. This fish has a dark-coloured back and silvery sides, with darker pigment along the lateral line. The scales have a tendency to detach and adult fish have rows of photophores on the underside.

Distribution and habitat

This species is circum-global and is found in the warmer parts of the Atlantic, Indian and Pacific Oceans. It is plentiful off the coast of West Africa from about 10°S northwards, and is present as far south as 15°S near Madagascar. It is usually found in the

. During the day, trawls at depths between produce the highest catches and at night the greatest abundance of fish is in the depth range ; this indicates that some, but not necessarily all, fish make short daily vertical migrations.

Ecology

Adult ''Argyropelecus affinis'' feed on

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

ic organisms,

salp A salp (plural salps, also known colloquially as “sea grape”) or salpa (plural salpae or salpas) is a barrel-shaped, planktic tunicate. It moves by contracting, thereby pumping water through its gelatinous body, one of the most efficient ...

s, krill,

arrow worms The Chaetognatha or chaetognaths (meaning ''bristle-jaws'') are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and can ...

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...

s and

ostracod Ostracods, or ostracodes, are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typic ...

s. Smaller fish consume mainly copepods and ostracods, and their adult arrangement of photophores develop when they are about long. The lenses of the eyes of ''A. affinis'' contains yellow pigments that absorb short-wave radiation; the proportions of the two carotenoid-like pigments present change as the fish grows older. It is not clear what precise function is served by the pigments, but with some of the incident illumination being absorbed, the fish's absolute sensitivity to light must be reduced.

Status

''Argyropelecus affinis'' is a common fish and is abundant in many parts of its wide range. No specific threats have been identified and the

International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...

has rated its conservation status as being of "

least concern A least-concern species is a species that has been categorized by the International Union for Conservation of Nature (IUCN) as evaluated as not being a focus of species conservation because the specific species is still plentiful in the wild. T ...

References

{{Taxonbar, from=Q145738 Sternoptychidae Fish described in 1899 Bioluminescent fish Taxa named by Samuel Garman