Other attributes
Idiacanthus fishes are rather rare inhabitants of the ocean water column, mainly in the upper 1000-m layer; a number of findings refer to greater depths (up to 4000 m).
One genus Idiacanthus is known with four reliable and one doubtful species. Common Idiacanthus (I. fascio-1a) is the most widely distributed, occurring mainly in the tropical Pacific, Indian and Atlantic Oceans. Two species live only in the Pacific Ocean. The black iddiacanth (I. niger) is represented by only a dozen specimens, caught off the coast of Australia, New Zealand, South Africa.
Anatomical differences between female and male Idiacanthus atlanticus
The female and male idiacans differ sharply in structure. Females have a short, very long body, gradually thinning toward the tail, reaching a length of 40 cm. Mouth big - cut mouth reaches almost to the edge of the gill cover, jaw set with sharp, two-tipped teeth of different sizes, threatening kind of fangs do not give the jaws closed. Large teeth glow: inside their bases in triangular cavities are clusters of luminous substance. From the chin departs antennae, about 2-3 times the length of the head, a straight stem ends with a complex ellipse-shaped thickening. The dorsal and anal fins are low and long; the pectoral fins are absent in adult fish; the pelvic fins, consisting of 7 rays, are located either under the first rays of the dorsal fin or in front of it (I. niger). The caudal fin is emarginate. The skin is bare, usually brown or dark brown, and coal-black in the black Idiacanthus (I. niger). Numerous luminous organs and areas of luminous glandular tissue are scattered on the head, dorsal and lateral sides of the body, and on the fins. Distinguished among them clearly distinguished exorbital luminous organ and 4 longitudinal rows of photophores: two along the belly and one row along the bottom edge of the sides. In live fish photophores shimmer amazing shades of purple, golden and opalescent tones.
Males are much smaller than females, their length does not exceed 6 cm. Their structure is similar to that of larvae: both jaws without teeth and without a tendril. In contrast to females, the occipital lucent organ in males is very large, up to 1/3 of the head length. There are no pectoral or pelvic fins. Males painted lighter than females, their body is less small luminous organ, but the 4 longitudinal rows of photophores are clearly expressed.
In accordance with the different structure, the lifestyles of female and male idiacanths are also different. The first are predators, in search of food animals make vertical migrations, rising with the onset of darkness closer to the surface and descending at dawn into deeper layers of water. They are able to capture prey of relatively large size because their mouth is incredibly elongated. This occurs because the skull can be tilted up and back, thanks to the neokostenevayuschim first vertebrae and the reduction of very powerful muscles between the vertebral column and the crest of the skull. The lower jaw, in turn, with the help of several highly extensible ligaments, not only goes down, but also sharply extends forward. During these movements the heart, the aorta, and the gills are removed from the mouth cavity and are thus unaffected by the passage of a large fish. In such cases, in order to accommodate abundant food, the walls of the stomach and abdomen are monstrously stretched, so that the slender serpentine predator, the female idiacanthus, appears to be lying on a huge rounded tumor.
Unlike females, sexually mature males do not feed at all: not only are their teeth missing, but their intestines also degenerate. Undoubtedly, their lifespan is very short. Males do not make vertical migrations, they hover passively in the water column or are carried by currents, attracting females like small beacons by the light of their huge cheek (oropharyngeal) lights.
