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Sixgill stingray
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 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 . 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.
==Taxonomy and phylogeny==
The first known sixgill stingray, an intact female 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.〔〔
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,〔 though genetic analysis is needed to determine whether this is truly the case.〔 Phylogenetic studies using morphological and genetic data have generally concurred that the sixgill stingray is the most basal member of the stingray lineage.〔〔〔〔 An extinct relative, ''H. senegasi'', lived during the Middle Eocene (49–37 million years ago).〔
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