Longspine Hatchetfish, Sternoptyx diaphana: The Longspine Hatchetfish has a rectangularity shaped body that tapers toward the end. The body is silvery black in color with a black band at the top and a small row of photophores along the base.
The snout is pointed with concave upper and lower profiles. They have transparent anal, caudal and dorsal fins and a distinguishing transparent membrane above the anal fin base. The anal and dorsal fins are without spines and contain 9 to 11 and 12 to 16 soft rays respectively.
They are an oceanic and mesopelagic species found in all tropical and subtropical waters of the Atlantic, Pacific and Indiana Oceans including all Mexican oceanic waters. They reside primarily between 1,500 and 2,400 feet in the water column and reach a maximum length of 5.5 cm (2.3 inches).
The Longspine Hatchetfish is a member of the Sternoptychidae or Silver Hatchetfish Family. They are a marine, deep-water species found in the bathypelagic zone, i.e. between 600 and 1,800 feet, where it is dark, cold and without plant life with high water pressures.
Globally there are 70 known members of the family found in 10 genera. All family members have narrow, laterally compressed, deeply keeled disc-shaped bodies that somewhat resemble a hatchet. The bodies are covered with silvery scales; the mouths are located at the tip of the snout and directed upwards; and they have large eyes.
At night they undergo mass migrations from depths of up to 12,000 feet to as shallow as 150 feet in search of prey returning to the depths at daybreak. They consume small crustaceans, such as amphipods, copepods, krill, seed shrimp, and fishes smaller than themselves. They are believed to be short lived with many fishes living less than a year. They spawn in open water and do not guard or otherwise care for their offspring.
They utilize counterillumination (with light being generated from a series of photophores) that produces light that camouflages their silhouettes from observers lurking below. Fossil records indicate that these fish have existed from the mid-late Eocene period, about 40 million years ago.