Sternoptychidae

The marine hatchetfishes or deep-sea hatchetfishes as well as the related bottlelights, pearlsides and constellationfishes are small deep-sea ray-finned fish of the stomiiform family Sternoptychidae. They are not closely related to and should not be confused with the freshwater hatchetfishes, which are teleosts in the characiform family Gasteropelecidae. The Sternoptychidae have 10 genera and about 70 species altogether. The scientific name means 'Sternoptyx-family', from Sternoptyx (the type genus) + the standard animal family suffix '-idae'. 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. Found most often at depths of 200–600 meters in tropical, subtropical and temperate waters of the Atlantic, Pacific and Indian Oceans, marine hatchetfishes range in size from Polyipnus danae at 2.8 cm (1.1 in) to the c.12 cm (4.7 in)-long Giant Hatchetfish (Argyropelecus gigas). The members of the disputed subfamily Maurolicinae have a more conventional fish shape. The mouth is located at the tip of the snout and directed downwards, more so in the Sternoptychinae. Unlike other Stomiiformes, they still have fully developed pseudobranchs. Their branchiostegal rays are 6-10, three of them attach to the posterior ceratohyal (epihyal). Their bodies are covered in delicate silvery scales which abrade easily. In some species, such as the Highlight Hatchetfish (Sternoptyx pseudobscura), large sections of the body at the base of the anal fin and/or caudal fin are transparent. The anal fin has 11-38 rays and may be divided in two parts. An adipose fin is usually present. The Sternoptychinae have preopercular spines and blade-like pterygiophores in front of the dorsal fin. Their large, sometimes tube-shaped eyes can collect the faintest of light and focus well on objects both close and far. In many genera, the eyes are fixed gazing permanently upwards, enabling them to discern the silhouettes of prey moving overhead against the slightly brighter upper waters. Sternoptychidae undertake nightly mass migrations from depths of 3,600 metres to the upper 50–100 metres of the starlit water column. There they feed throughout the night, returning to the depths by daybreak. Their prey consists primarily of tiny crustaceans, such as amphipods, copepods, euphausiids (krill) and ostracods (seed shrimp), and of fish smaller than themselves. What little is known of their life cycle suggests that at least some members of this family are short-lived, dying after no more than a year. They spawn in the open water, and do not guard or otherwise care for their offspring; species with a short lifespan are presumably semelparous The fry – even of Sternoptychinae – look like tiny pearlsides (Maurolicus). Marine hatchetfishes are not the only animals that seek out prey by watching for silhouettes from below. Indeed, many fishes that consider Sternoptychidae prey do so, and to foil their predaceous attempts, the Sternoptychidae have evolved an astounding ability: bioluminescent counter-illumination. Counterillumination (or counter-lighting) involves the production of light by the fish for the purpose of camouflaging its silhouette from observers lurking below. Sternoptychidae produce this light with organs called photophores, of which they have between 3 and 7 – usually 6 – on the branchiostegal membrane along the lower edge of the chest and belly. The intensity of the light produced is controlled by the fish, an appropriate brightness chosen according to how much light reaches the eyes from above. The patterns of light created by the photophores are also unique to each species, probably playing a role in courtship.