Marine hatchetfishes or deep-sea hatchetfishes are small deep-sea mesopelagic ray-finned fish of the stomiiform subfamily Sternoptychinae. They should not be confused with the freshwater hatchetfishes, which are not particularly closely related Teleostei in the characiform family Gasteropelecidae.
The scientific name means "Sternoptyx-subfamily", from Sternoptyx (the type genus) + the standard animal family suffix "-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.
Description and ecology
thumb|[[Half-naked hatchetfish, Argyropelecus hemigymnus eating a shrimp]]
Found in tropical, subtropical 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 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.
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 is rounded, the other two – in particular Sternoptyx – decidedly angular if seen from the side. Their pelvis is rotated to a vertical position. The mouth is located at the tip of the snout and directed almost straight downwards.
- Eosternoptyx (middle-late Eocene of Iran)
- Polyipnoides (middle Eocene of Georgia)
- Horbatshia (Oligocene of the Carpathians)
- Discosternon (Miocene of Italy)