Sixgill stingray
Encyclopedia
The sixgill stingray, Hexatrygon bickelli, is an unusual species
of deep-sea ray
. It is distinguished by its long, soft snout and six pairs of gill slit
s (all other rays have five). It is the sole member
of the genus
Hexatrygon and the family Hexatrygonidae. Although several species of Hexatrygon have been described historically, it is now believed that they may represent variations in a single, widespread species.
of the sixgill stingray was a nearly perfect, 105 cm-long female
specimen washed up on a beach
at Port Elizabeth in 1980. It was discovered by Dave Bickell, a former angling correspondent for the Eastern Province Herald. Since the original description of H. bickelli, four additional species of sixgill stingray were described based on morphological
differences: Hexatrematobatis longirostris (=Hexatrygon longirostra), Hexatrygon yangi, Hexatrygon taiwanensis, and Hexatrygon brevirostra. However, subsequent examination of newborns, juveniles, and adults from Hawaii
, South Africa
, and Taiwan
have shown that the snout shape, body width, and teeth counts vary considerably with growth and amongst individuals, and thus H. bickelli is now provisionally considered to be the only valid species.
and Pacific Ocean
s, from the east coast of South Africa
off Port Elizabeth and Port Alfred
, to the South China Sea
, Taiwan
, the Philippines
, and the Tokara Islands
and Okinawa Trough
off Japan
. It also ranges off western Australia
from the Exmouth Plateau to Shark Bay
, off Flinders Reef
, Queensland
, and off Hawaii
. A deep-sea species, it occurs mostly on the upper continental slopes at depths of 362-1120 m, although two specimens have stranded (one alive) on the beaches of South Africa, and another individual has been photograph
ed feeding in shallow water off Japan. It is found over soft bottom substrates.
-shaped pectoral fin disc and a moderately short, stout tail
0.5-0.7 times the disc length. The tail bears one or two prominent barbed stinging spines, well behind the pelvic fins. There are no dorsal fin
s, and the caudal fin is long and leaf-shaped, nearly symmetrical above and below the tail. The snout is highly distinctive; it is soft and filled with jelly, and is greatly elongated and broadly angular, measuring over six times the inter-orbit distance in length. The snout is much longer in adults than in young, and can shrink significantly when exposed to air or preservatives, likely contributing to taxonomic confusion.
The ampullae of Lorenzini
are well-developed, arranged in longitudinal rows underneath the snout. There are six pairs of small gill slit
s beneath the disc. The mouth is straight and moderately broad, containing small, bluntly rounded teeth numbering 44-102 rows in either jaw. The anterior nasal flaps are fused into a very broad, very short nasal curtain that reaches to just forward of the mouth. The coloration in life is purplish brown above and white below with dark disc and pelvic fin margins. The snout is lighter and translucent on the underside, and the tail is dark. The maximum known length is a 168 cm female.
of their skeleton likely precludes them from taking hard-shelled organisms. The long snout is prehensile and may be used to poke around the bottom searching for food. A specimen has been found with a wound from a cookie-cutter shark. Reproduction is ovoviviparous; one female examined contained three term fetus
es. The size at birth is 48 cm; males are estimated to mature at 110 cm long.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of deep-sea ray
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...
. It is distinguished by its long, soft snout and six pairs of gill slit
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, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s (all other rays have five). It is the sole member
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...
of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Hexatrygon and the family Hexatrygonidae. Although several species of Hexatrygon have been described historically, it is now believed that they may represent variations in a single, widespread species.
Taxonomy
The holotypeHolotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
of the sixgill stingray was a nearly perfect, 105 cm-long female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
specimen washed up on a beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...
at Port Elizabeth in 1980. It was discovered by Dave Bickell, a former angling correspondent for the Eastern Province Herald. Since the original description of H. bickelli, four additional species of sixgill stingray were described based on morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
differences: Hexatrematobatis longirostris (=Hexatrygon longirostra), Hexatrygon yangi, Hexatrygon taiwanensis, and Hexatrygon brevirostra. However, subsequent examination of newborns, juveniles, and adults from Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
, South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, and Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
have shown that the snout shape, body width, and teeth counts vary considerably with growth and amongst individuals, and thus H. bickelli is now provisionally considered to be the only valid species.
Distribution and habitat
The sixgill stingray is widely distributed in the IndianIndian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
and Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
s, from the east coast of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
off Port Elizabeth and Port Alfred
Port Alfred
Port Alfred is a small town with a population of just under 20,000 in the Eastern Cape province of South Africa. It is situated on the eastern seaboard of the country at the mouth of the Kowie River, almost exactly half-way between the larger cities of Port Elizabeth and East London and 30 km...
, to the South China Sea
South China Sea
The South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Singapore and Malacca Straits to the Strait of Taiwan of around...
, Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...
, the Philippines
Philippines
The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
, and the Tokara Islands
Tokara Islands
The Tokara Islands is a group of islands in part of the Satsunan Islands, which is part of the Nansei Islands. The whole island group belongs to Toshima Village, Kagoshima Prefecture, Japan....
and Okinawa Trough
Okinawa Trough
The is a seabed feature of the East China Sea. It is an active, initial back-arc rifting basin which has formed behind the Ryukyu arc-trench system in the West Pacific. It developed where the Philippine Sea Plate is subducting under the Eurasia Plate....
off Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. It also ranges off western Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
from the Exmouth Plateau to Shark Bay
Shark Bay
Shark Bay is a World Heritage listed bay in Western Australia. The term may also refer to:* the locality of Shark Bay, now known as Denham* Shark Bay Marine Park* Shark Bay , a shark exhibit at Sea World, Gold Coast, Australia* Shire of Shark Bay...
, off Flinders Reef
Flinders Reef
Flinders Reef is a small isolated reef near Moreton Island, 5 km north-west of Cape Moreton in South East Queensland. It has the highest number of coral species of any sub tropical reef system along Australia's east coast and is the nearest true coral reef to Brisbane. Flinders Reef is one of...
, Queensland
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...
, and off Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
. A deep-sea species, it occurs mostly on the upper continental slopes at depths of 362-1120 m, although two specimens have stranded (one alive) on the beaches of South Africa, and another individual has been photograph
Photograph
A photograph is an image created by light falling on a light-sensitive surface, usually photographic film or an electronic imager such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of...
ed feeding in shallow water off Japan. It is found over soft bottom substrates.
Description
The small eyes, flabby body, and thin skin of the sixgill stingray is consistent with their deepwater habits. Their bodies are large and heavy, with a broad, heartHeart (symbol)
The heart has long been used as a symbol to refer to the spiritual, emotional, moral, and in the past, also intellectual core of a human being...
-shaped pectoral fin disc and a moderately short, stout tail
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...
0.5-0.7 times the disc length. The tail bears one or two prominent barbed stinging spines, well behind the pelvic fins. There are no dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s, and the caudal fin is long and leaf-shaped, nearly symmetrical above and below the tail. The snout is highly distinctive; it is soft and filled with jelly, and is greatly elongated and broadly angular, measuring over six times the inter-orbit distance in length. The snout is much longer in adults than in young, and can shrink significantly when exposed to air or preservatives, likely contributing to taxonomic confusion.
The ampullae of Lorenzini
Ampullae of Lorenzini
The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also been...
are well-developed, arranged in longitudinal rows underneath the snout. There are six pairs of small gill slit
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, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s beneath the disc. The mouth is straight and moderately broad, containing small, bluntly rounded teeth numbering 44-102 rows in either jaw. The anterior nasal flaps are fused into a very broad, very short nasal curtain that reaches to just forward of the mouth. The coloration in life is purplish brown above and white below with dark disc and pelvic fin margins. The snout is lighter and translucent on the underside, and the tail is dark. The maximum known length is a 168 cm female.
Biology and ecology
Little is known of the life history of the sixgill stingray. They are capable of protruding their mouths downward more than 100% of their head length, suggesting that they feed on benthic prey. The poor mineralizationMineralization
Mineralization may refer to:* Mineralization , the process through which an organic substance becomes impregnated by inorganic substances...
of their skeleton likely precludes them from taking hard-shelled organisms. The long snout is prehensile and may be used to poke around the bottom searching for food. A specimen has been found with a wound from a cookie-cutter shark. Reproduction is ovoviviparous; one female examined contained three term fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
es. The size at birth is 48 cm; males are estimated to mature at 110 cm long.