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
Stingrays are a group of sea rays, which are cartilaginous fish related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes and consist of eight families: Hexatrygonidae (sixgill stingray), Plesiobatidae ( ...
and the only extant member of the
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
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
gelatin
Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
ous substance. It is brownish above and white below, and lacks
dermal denticle
A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...
s.
Benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
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
seamount
A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abru ...
s at depths of . It has been recorded from scattered locations in the
Indo-Pacific
The Indo-Pacific is a vast biogeographic region of Earth.
In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
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 stat ...
. 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, sand an ...
. 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
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...
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
Gqeberha (), formerly Port Elizabeth and colloquially often referred to as P.E., is a major seaport and the most populous city in the Eastern Cape province of South Africa. It is the seat of the Nelson Mandela Bay Metropolitan Municipality, Sou ...
, 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
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. 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
In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon) and these groups are given ...
therefore concluded tentatively that there is only a single species of sixgill stingray,
though
genetic analysis
Genetic analysis is the overall process of studying and researching in fields of science that involve genetics and molecular biology. There are a number of applications that are developed from this research, and these are also considered parts of ...
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'', "dawn ...
(49–37 million years ago).
Description
The sixgill stingray has a bulky, flabby body with 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 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
gelatin
Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
ous 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
spiracle Spiracle or spiraculum may refer to:
* Spiracle (arthropods), opening in the exoskeletons of some arthropods
* Spiracle (vertebrates), openings on the surface of some vertebrates
* Spiraculum, a genus of land snails in family Cyclophoridae
Cycl ...
s. 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 (dep ...
pattern; the teeth are more numerous in adults. Six pairs of small
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 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 Selachimo ...
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 Hexanchidae. These sharks are characterized by a broad, pointed head, six pairs of gill slits, comb-like, yellow lower teeth, and a long tail. The largest species ca ...
'').
One recorded specimen had six gill slits on the left side and seven on the right.
Their
pelvic fin
Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods.
Structure and function Structure
In actinopterygians, the pelvic fin consists of two en ...
s 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 se ...
that is nearly symmetrical above and below. The skin is delicate and entirely lacking
dermal denticle
A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as we ...
s. 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
The Indo-Pacific is a vast biogeographic region of Earth.
In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
. In the Indian Ocean, it has been reported from South Africa off Port Elizabeth and
Port Alfred
A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ha ...
, 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
Shark Bay (Malgana: ''Gathaagudu'', "two waters") is a World Heritage Site in the Gascoyne region of Western Australia. The http://www.environment.gov.au/heritage/places/world/shark-bay area is located approximately north of Perth, on the ...
. 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
Flinders Reef is a small isolated reef near Moreton Island, north-east of Cape Moreton in South East Queensland, Australia. It has the highest number of coral species of any subtropical reef system along Australia's east coast and is the neares ...
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
, established_ ...
,
New Caledonia
)
, anthem = ""
, image_map = New Caledonia on the globe (small islands magnified) (Polynesia centered).svg
, map_alt = Location of New Caledonia
, map_caption = Location of New Caledonia
, mapsize = 290px
, subdivision_type = Sovereign st ...
, and
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 stat ...
.
This
bottom-dwelling species typically inhabits 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
seamount
A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abru ...
s 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
![Isistius brasiliensis](https://upload.wikimedia.org/wikipedia/commons/7/7d/Isistius_brasiliensis.jpg)
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
Ampullae of Lorenzini (singular ''Ampulla'') are electroreceptors, sense organs able to detect electric fields. They form a network of mucus-filled pores in the skin of cartilaginous fish (sharks, rays, and chimaeras) and of basal actinopterygia ...
arranged in longitudinal rows, which are capable of detecting the minute
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
s 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 reco ...
(''Isistius brasiliensis'').
Reproduction in the sixgill stingray is
viviparous
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...
, 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
Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juve ...
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 towing ...
s. The catch rate seems to have decreased in recent years, leading to concerns that it may be locally
overfished
Overfishing is the removal of a species of fish (i.e. fishing) from a body of water at a rate greater than that the species can replenish its population naturally (i.e. the overexploitation of the fishery's existing fish stock), resulting in the ...
, 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