Marine hatchetfishes or deep-sea hatchetfishes are small deep-sea mesopelagic ray-finned fish of the stomiiform

subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end subfamily botanical names with "-oideae", and zoologi ...

Sternoptychinae. They should not be confused with the

freshwater hatchetfish The freshwater hatchetfish are a family, Gasteropelecidae, of ray-finned fish from South and Central America. The common hatchetfish is the most popular member among fish keeping hobbyists. The family includes three genera: '' Carnegiella'' ...

es, which are not particularly closely related Teleostei in the characiform family Gasteropelecidae. (2006): ''

Fishes of the World ''Fishes of the World'' by the American ichthyologist Joseph S. Nelson (1937–2011) is a standard reference for fish systematics. Now in its fifth edition (2016), the work is a comprehensive overview of the diversity and classification of the ...

'' (4th ed.). John Wiley & Sons, Inc. p.209 The

scientific name In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

means "''Sternoptyx''-subfamily", from ''

Sternoptyx ''Sternoptyx'' is an oceanic ray-finned fish genus which belongs in the family Sternoptychidae. This is the type genus of the Sternoptychidae, as well as the marine hatchetfish subfamily Sternoptychinae. ''Sternoptyx'' have silvery, high ...

'' (the type genus) + the standard animal family

suffix In linguistics, a suffix is an affix which is placed after the stem of a word. Common examples are case endings, which indicate the grammatical case of nouns, adjectives, and verb endings, which form the conjugation of verbs. Suffixes can carry ...

"-inae". It ultimately derives from Ancient Greek ''stérnon'' (στέρνον, "breast") + ''ptýx'' (πτύξ, "a fold/crease") + Latin ''forma'' ("external form"), the Greek part in reference to the thorax shape of marine hatchetfishes. (2006)
Family Sternoptychidae
Version of 2006-OCT-10. Retrieved 2009-OCT-02.

Description and ecology

Found in tropical,

subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Geographical z ...

and temperate waters of the Atlantic, Pacific and Indian Oceans, marine hatchetfishes range in size from '' Polyipnus danae'' at to the c.-long giant hatchetfish (''Argyropelecus gigas''). They are small deep-sea fishes which have evolved a peculiar body shape and like their relatives have

bioluminescent Bioluminescence is the production and emission of light by living organisms. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some Fungus, fungi, microorganisms including ...

photophores. The latter allow them to use counter-illumination to escape predators that lurk in the depths: by matching the light intensity with the light penetrating the water from above, the fish does not appear darker if seen from below. They typically occur at a few hundred meters below the surface, but their entire depth range spans from 50 to 1,500 meters deep. (1998): Sternoptychidae. ''In:'' : ''Encyclopedia of Fishes'': 121. Academic Press, San Diego. The body is deep and laterally extremely compressed, somewhat resembling a hatchet (with the thorax being the "blade" and the caudal peduncle being the "handle"). The genus ''

Polyipnus ''Polyipnus'' is a genus of oceanic ray-finned fish in the family Sternoptychidae. This is the largest genus of the marine hatchetfishes subfamily Sternoptychinae and indeed of the entire Sternoptychidae. It is not quite as apomorphic as their re ...

'' is rounded, the other two – in particular ''

'' – decidedly angular if seen from the side. Their

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

is rotated to a vertical position. The mouth is located at the tip of the snout and directed almost straight downwards. Their

scale Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...

s are silvery, delicate and easily abraded. In some species, such as the highlight hatchetfish (''

Sternoptyx pseudobscura ''Sternoptyx'' is an oceanic ray-finned fish genus which belongs in the family Sternoptychidae. This is the type genus of the Sternoptychidae, as well as the marine hatchetfish subfamily Sternoptychinae. ''Sternoptyx'' have silvery, high, and ...

''), large sections of the body at the base of the

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

and/or caudal fin are transparent. They have perpendicular spines and blade-like

pterygiophore A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through ...

s in front of the dorsal fin. The

has 11-19 rays and in some species is divided in two parts; almost all have an adipose fin. Their large, sometimes tube-shaped eyes can collect the faintest of light and focus well on objects both close and far. They are directed somewhat upwards, most conspicuously in the genus ''

Argyropelecus ''Argyropelecus'' is an oceanic ray-finned fish genus in the deep sea hatchetfish family Sternoptychidae. A collective name is " silver hatchetfishes", but this can also refer to a species of the freshwater hatchetfishes which are not particula ...

''. This allows to discern the silhouettes of prey moving overhead against the slightly brighter upper waters.

Genera

There are three genera in this

, with some 40 species altogether: * ''

'' – silver hatchetfishes (7 species) * ''

'' (32 species) * ''

'' (4 species)

Footnotes

References

{{Taxonbar, from=Q6764289 Sternoptychidae Bioluminescent fish Deep sea fish