The sixgill stingray (''Hexatrygon bickelli'') is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of stingray and the only extant member of the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

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 Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gra ...

in having six pairs of

gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In con ...

s rather than five. Growing up to long, it has a rounded

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

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 slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...

s and seamounts at depths of . It has been recorded from scattered locations in the Indo-Pacific from South Africa to

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...

. This species probably uses its snout to probe for food in the bottom

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

. 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 A least-concern species is a species that has been categorized by the International Union for Conservation of Nature (IUCN) as evaluated as not being a focus of species conservation because the specific species is still plentiful in the wild. T ...

, 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 Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''hexa'' ("six") and ''trygon'' ("stingray"), referring to the number of

s. 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 In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

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 The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', "da ...

(49–37 million years ago).

Description

The sixgill stingray has a bulky, flabby body with a rounded

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 A quincunx () is a geometric pattern consisting of five points arranged in a cross, with four of them forming a square or rectangle and a fifth at its center. The same pattern has other names, including "in saltire" or "in cross" in heraldry (d ...

pattern; the teeth are more numerous in adults. Six pairs of small

s occur on the underside of the disc; all other rays have five pairs (a few

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s also have six or more pairs of gill slits, for example in the genus ''

Hexanchus The sixgill sharks are a genus, ''Hexanchus'', of deepwater sharks in the family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purp ...

''). One recorded specimen had six gill slits on the left side and seven on the right. Their pelvic fins are rather large and rounded. The tail is moderately thick and measures about 0.5–0.7 times as long as the disc. One or two serrated stinging spines are present on its dorsal surface, well back from the base. The end of the tail bears a long, low leaf-shaped

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as ...

that is nearly symmetrical above and below. The skin is delicate and entirely lacking dermal denticles. The disc is purplish to pinkish brown above, darkening slightly at the fin margins; the skin is easily abraded, leaving white patches. The underside of the disc is white with dark margins on the pectoral and pelvic fins. The snout is translucent, and the tail and caudal fin are almost black. The largest known specimen is a female long.

Distribution and habitat

The sixgill stingray has been recorded at widely scattered locations in the Indo-Pacific. In the Indian Ocean, it has been reported from South Africa off Port Elizabeth and Port Alfred, southwestern India, several islands of Indonesia, and

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...

from

Exmouth Plateau The Exmouth Plateau is an elongate northeast striking extensional passive margin located in the Indian Ocean roughly 3,000 meters offshore from western and northwestern Western Australia. The plateau makes up the westernmost structural unit of t ...

to Shark Bay. In the Pacific Ocean, it has been found from Japan to

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the nort ...

and the Philippines, as well as off Flinders Reef in

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , establishe ...

, New Caledonia, and

. This bottom-dwelling species typically inhabits upper

s and seamounts at depths of . However, it occasionally ventures into shallower water, with one ray observed feeding at a depth of off Japan. It can be found over sandy, muddy, or rocky bottom substrates.

Biology and ecology

The long snout of the sixgill stingray is very flexible both vertically and horizontally, suggesting that the ray uses it to probe for food in the bottom sediment. The underside of the snout is covered by well-developed ampullae of Lorenzini arranged in longitudinal rows, which are capable of detecting the minute electric fields produced by other

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...

s. The mouth can be protruded downward farther than the length of the head, likely allowing the ray to extract buried prey. The jaws are poorly mineralized, suggesting that it does not feed on hard-shelled animals. There is a record of a specimen with a wound from a

cookiecutter shark The cookiecutter shark (''Isistius brasiliensis''), also called the cigar shark, is a species of small squaliform shark in the family Dalatiidae. This shark occurs in warm, oceanic waters worldwide, particularly near islands, and has been rec ...

(''Isistius brasiliensis''). Reproduction in the sixgill stingray is viviparous, with documented litter sizes of between two and five pups. Newly born rays measure around long. Both males and females mature sexually at approximately long.

Human interactions

For the most part, little fishing activity occurs at the depths occupied by the sixgill stingray, thus the IUCN has listed it as Least Concern. In the waters around Taiwan, it is caught in small numbers as bycatch in

bottom trawl Bottom trawling is trawling (towing a trawl, which is a fishing net) along the seafloor. It is also referred to as "dragging". The scientific community divides bottom trawling into benthic trawling and demersal trawling. Benthic trawling is tow ...

s. The catch rate seems to have decreased in recent years, leading to concerns that it may be locally overfished, though quantitative data are lacking.

References

{{Authority control Hexatrygonidae Monotypic fish genera Fish of the Pacific Ocean Fish of the Indian Ocean Marine fish of Asia Marine fish of Africa Marine fauna of Oceania Marine fauna of Southeast Asia Fish of Southeast Asia Fish described in 1980