The beluga (), also known as the beluga sturgeon or great sturgeon (Huso huso), is a species of anadromous fish in the sturgeon family (Acipenseridae) of order Acipenseriformes. It is found primarily in the Caspian and Black Sea basins, and formerly in the Adriatic Sea. Based on maximum size, it is the third-most-massive living species of bony fish. Heavily fished for the female's valuable roe, known as beluga caviar, wild populations have been greatly reduced by overfishing and poaching, leading IUCN to classify the species as critically endangered. Etymology The common name for the sturgeon, as for the unrelated beluga whale, is derived from the Russian word белый (belyj), meaning "white," probably referring to the extensive pale colour on the flanks and belly in beluga compared to that of other sturgeons. Description Huso huso shows typical characteristics of other sturgeon, such as an elongated body, heterocercal tail, partially cartilaginous skeleton, naked skin and longitudinal series of scutes. The dorsal fin has 48 to 81 soft rays, and the anal fin, much shorter, has 22 to 41 soft rays. There are five in a series of longitudinal scutes: dorsal (one series, 9-17 scutes), lateral (two series, one per side, 28-60 scutes each) and ventral series (two series, one per side, 7-14 scutes each). The surface of the skin is covered by fine denticles. The rostrum is conical and contains numerous sensory pits on both ventral and dorsal surfaces. The mouth is large, crescent-shaped and protractile, with the upper lip continuous while the lower lip is interrupted by a large gap. The barbels are laterally compressed with foliate appendages, arranged in two pairs, originating midway or closer to the mouth than to the tip of the snout. However, during growth, the beluga sturgeon show evident morphologic changes: Juveniles are slender, and the head is quite narrow with a mouth ventrally placed but projecting upward. The snout is thin and pointed (almost half of the head), scutes are evident, back and flancs are dark grey or black and the belly is white. Adults are heavy-set, spindle-shaped, large and humpbacked. The head is massive with a very large protractile mouth that gradually moves in an almost frontal position during growth. The snout is quite short (one-third to one-quarter of the head), and scutes gradually undergo absorption and decrease in number with age. Colouring is blue-grey or dark brown, with silver or grey flancs and