These unusual deep-sea fish found in tropical temperate waters of the oceans got their name for their bizarre characteristic appearance, resembling an ax in shape - a wide body and a narrow tail.
The ax fish, the description of which is given in the presented article, is most often found at depths of 200-600 meters, but was also seen at depths of about 2000 meters.
External features of the family
Deep-sea hatchet fish or Hatchet fish (Sternoptychidae) is a family belonging to the order Stomiform, which includes 2 subfamilies consisting of 10 genera and 73 species. Distributed in subtropical and tropical waters of three oceans: Indian, Pacific, Atlantic. They mainly inhabit the middle layers of deep-sea areas.
The body is in length from 2 to 14 centimeters. The ax fish (photo is presented in the article) is distinguished by a very high body, strongly flattened from the sides, and also a caudal peduncle, sharply tapering towards the caudal fin.
Most varieties of this family have a bright silver color with a metallic bluish sheen and a darker, and sometimes almost black, back. Their eyes are large, and in species of the genus Argyropelecus, they are also telescopic, looking up.
Description
A photo of an ax fish clearly demonstrates the originality of its shape. She has one more name - wedge-belly. The body of the fish, covered with silvery, easily bouncing scales, is strongly compressed from the sides. Some species have an extension of the trunk in the anal fin area. The frontal part of the dorsal fin has the shape of a blade of bones protruding at the ax over the back muscles, and the abdominal part of the body has a pointed keel. Large jaws in relation to the center line of the body are at an acute angle. There is also a forked spike located at the beginning of the ventral fin. The adipose fin is small.
Like many other deep-sea creatures, hatchet fish have photophores that emit light. Unlike other fish, they use the possibility of bioluminescence (emission of greenish light) in order to disguise, and not to attract prey. Photophores are located only on the belly of the fish, so their glow makes the fish invisible from below (the silhouette seems to dissolve against the background of sun rays breaking through the depths of the sea). In addition, hatchets are able to regulate the intensity of the glow, controlling with their eyes the brightness of the upper layers of water.
Lifestyle
Reliably little is known about the life cycle of an ax fish, because these representatives live in inaccessible places. According to many researchers, their life expectancy is not more than one year. At night, the fish is in shallow water (at a depth of about 200-300 meters), hunting small fish and plankton. Usually she catches prey, which itself floats over her. In the daytime, they again return to a depth of 2000 meters.
Some species can gather in large dense flocks, creating serious problems for vessels using sonar to determine depth. With such a "double bottom" sailors first encountered in the middle of the XX century.
Such a massive accumulation of hatchet fish attracts some species of large fish of ocean waters to these places. Among them there are also commercially valuable species, for example, tuna. In addition, hatchets form a large part of the diet and some other, larger inhabitants of the oceans, for example, deep-sea anglers.
This species of fish propagates either by laying off larvae that mix with plankton, and when it grows deeper, or by throwing eggs.
