Hexatrematobatis longirostris Chu & Meng, 1981
Hexatrygon brevirostra Shen, 1986
Hexatrygon taiwanensis Shen, 1986
Hexatrygon yangi Sheng & Liu, 1984
The sixgill stingray (Hexatrygon bickelli) is a species of stingray and the only extant member of the family Hexatrygonidae. Although several species of sixgill stingrays have been described historically, they may represent variations in a single, widespread species. This flabby, heavy-bodied fish, described only in 1980, is unique among rays in having six pairs of gill slits rather than five. Growing up to 1.7 m (5.6 ft) long, it has a rounded pectoral fin disc and a long, triangular, and flexible snout filled with a gelatinous substance. It is brownish above and white below, and lacks dermal denticles.
Benthic in nature, the sixgill stingray is usually found over upper continental slopes and seamounts at depths of 500–1,120 m (1,640–3,670 ft). It has been recorded from scattered locations in the Indo-Pacific from South Africa to Hawaii. This species probably uses its snout to probe for food in the bottom sediment. Its jaws are greatly protrusible, allowing it to capture buried prey. The sixgill stingray gives live birth, with litters of two to five pups. The IUCN has assessed this ray as Least Concern, because it faces minimal fishing pressure across most of its range.
The first known sixgill stingray, an intact female 64 cm (25 in) across, was found on a beach near Port Elizabeth, South Africa. It was described as a new species and placed in its own family by Phillip Heemstra and Margaret Smith, in a 1980 article for the Ichthyological Bulletin of the J. L. B. Smith Institute of Ichthyology. The generic name Hexatrygon is derived from the Greek hexa ("six") and trygon ("stingray"), referring to the number of gill slits. The specific name bickelli honors Dave Bickell, a journalist who discovered the original specimen.[3][4]
Following the description of H. bickelli, four additional species of sixgill stingray were described on the basis of morphological differences. However, their validity was brought into question after comparative studies revealed that traits such as snout shape, body proportions, and tooth number vary greatly with age and among individuals. Taxonomists therefore concluded tentatively that there is only a single species of sixgill stingray,[4] though genetic analysis is needed to determine whether this is truly the case.[1] Phylogenetic studies using morphological and genetic data have generally concurred that the sixgill stingray is the most basal member of the stingray lineage.[5][6][7][8] An extinct relative, H. senegasi, lived during the Middle Eocene (49–37 million years ago).[9]
The sixgill stingray has a bulky, flabby body with a rounded pectoral fin disc that is longer than wide. The triangular snout is much longer in adults than in juveniles (making up almost two-fifths of the disc length), and is filled with a clear gelatinous material; because of this, the snout of a dead specimen can shrink significantly when exposed to air or preservatives. The tiny eyes are placed far apart and well ahead of the larger spiracles. Between the widely spaced nostrils are a pair of short and fleshy flaps that are joined in the middle to form a curtain of skin. The mouth is wide and nearly straight. In either jaw are 44–102 rows of small, blunt teeth arranged in a quincunx pattern; the teeth are more numerous in adults. Six pairs of small gill slits occur on the underside of the disc; all other rays have five pairs (a few sharks also have six or more pairs of gill slits, for example in the genus Hexanchus).[2][4][10] One recorded specimen had six gill slits on the left side and seven on the right.[11] Their pelvic fins are rather large and rounded.[10]