Pacific Hatchetfish, Argyropelecus affinis: The Pacific Hatchetfish has a disc shaped body that tapers toward the end. The body is silvery in color with a black band at the top and a prominent row of photophores along the base. They have transparent anal, caudal and dorsal fins and a very long and narrow tail base. The anal and dorsal fins are without spines and contain 8 to 9 and 12 to 14 soft rays respectively.
They can be easily confused with the Tropical Hatchetfish, Argyropelecus lychnus (oval body, shorter tail base).
The Pacific Hatchetfish is an oceanic and mesopelagic species found in all tropical and subtropical waters of the Atlantic, Pacific and Indian Oceans including all Mexican oceanic waters. They reside primarily between 900 and 2,000 feet in the water column and reach a maximum length of 8.4 cm (3.3 inches).
The Pacific 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.