Sixgill stingray

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Sixgill stingray
Hexatrygon bickelli csiro-nfc.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Superorder: Batoidea
Order: Myliobatiformes
Suborder: Myliobatoidei
Superfamily: Hexatrygonoidea
Family: Hexatrygonidae
Heemstra & M. M. Smith, 1980
Genus: Hexatrygon
Heemstra & M. M. Smith, 1980
Species:
H. bickelli
Binomial name
Hexatrygon bickelli
Hexatrygon bickelli rangemap.png
Range of the sixgill stingray [2]
Synonyms

Hexatrematobatis longirostrisChu & Meng, 1981
Hexatrygon brevirostraShen, 1986
Hexatrygon taiwanensisShen, 1986
Hexatrygon yangiSheng & Liu, 1984

Contents

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.

Taxonomy and phylogeny

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]

Description

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]

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 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 1.7 m (5.6 ft) long. [2] [4] [10]

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 from Exmouth Plateau to Shark Bay. In the Pacific Ocean, it has been found from Japan to Taiwan and the Philippines, as well as off Flinders Reef in Queensland, New Caledonia, and Hawaii. [1] [11] This bottom-dwelling species typically inhabits upper continental slopes and seamounts at depths of 500–1,120 m (1,640–3,670 ft). However, it occasionally ventures into shallower water, with one ray observed feeding at a depth of 30 m (98 ft) off Japan. It can be found over sandy, muddy, or rocky bottom substrates. [1] [10]

Biology and ecology

The cookiecutter shark is known to bite the sixgill stingray. Isistius brasiliensis.jpg
The cookiecutter shark is known to bite the sixgill stingray.

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. [2] 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 organisms. [4] 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. [12] There is a record of a specimen with a wound from a cookiecutter shark (Isistius brasiliensis). [10] Reproduction in the sixgill stingray is viviparous, with documented litter sizes of between two and five pups. [4] Newly born rays measure around 48 cm (19 in) long. Both males and females mature sexually at approximately 1.1 m (3.6 ft) long. [1]

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 trawls. The catch rate seems to have decreased in recent years, leading to concerns that it may be locally overfished, though quantitative data are lacking. [1]

Related Research Articles

<span class="mw-page-title-main">Stingray</span> Suborder of fishes

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, Plesiobatidae, Urolophidae (stingarees), Urotrygonidae, Dasyatidae, Potamotrygonidae, Gymnuridae and Myliobatidae . There are about 220 known stingray species organized into 29 genera.

<span class="mw-page-title-main">Rajiformes</span> Order of fishes in the superorder Batoidea

Rajiformes is one of the four orders in the superorder Batoidea, flattened cartilaginous fishes related to sharks. Rajiforms are distinguished by the presence of greatly enlarged pectoral fins, which reach as far forward as the sides of the head, with a generally flattened body. The undulatory pectoral fin motion diagnostic to this taxon is known as rajiform locomotion. The eyes and spiracles are located on the upper surface of the head and the gill slits are on the underside of the body. Most species give birth to live young, although some lay eggs enclosed in a horny capsule.

<span class="mw-page-title-main">Urolophidae</span> Family of cartilaginous fishes

The Urolophidae are a family of rays in the order Myliobatiformes, commonly known as stingarees or round stingrays. This family formerly included the genera Urobatis and Urotrygon of the Americas, which are presently recognized as forming their own family Urotrygonidae. Stingarees are found in the Indo-Pacific region, with the greatest diversity off Australia. They are sluggish, bottom-dwelling fish that have been recorded from shallow waters close to shore to deep waters over the upper continental slope. Measuring between 15 and 80 cm long, these rays have oval to diamond-shaped pectoral fin discs and relatively short tails that terminate in leaf-shaped caudal fins, and may also have small dorsal fins and lateral skin folds. Most are smooth-skinned, and some have ornate dorsal color patterns.

<span class="mw-page-title-main">Deepwater stingray</span> Species of cartilaginous fish

The deepwater stingray or giant stingaree is a species of stingray and the sole member of the family Plesiobatidae. It is widely distributed in the Indo-Pacific, typically over fine sediments on the upper continental slope at depths of 275–680 m (900–2,230 ft). This species reaches 2.7 m (8.9 ft) in length and 1.5 m (4.9 ft) in width. It has an oval pectoral fin disc with a long, flexible, broad-angled snout. Most of the entire latter half of its tail supports a distinctively long, slender, leaf-shaped caudal fin. Its coloration is dark above and white below, and its skin is almost completely covered by tiny dermal denticles.

<span class="mw-page-title-main">Thorntail stingray</span> Species of cartilaginous fish

The thorntail stingray, black stingray, or longtail stingray is a species of stingray in the family Dasyatidae. It is found off southern Africa, Australia, and New Zealand from the intertidal zone to a depth of 440 m (1,440 ft). This bottom-dweller inhabits soft-bottomed habitats such as lagoons, estuaries, and reefs. Growing to 1.8 m (5.9 ft) across and over 200 kg (440 lb) in weight, the thorntail stingray is among the largest stingrays in the world. Uniformly dark above and light below, it has a diamond-shaped pectoral fin disc and a very long, whip-like tail with a fin fold underneath. The upper surface of the disc and the tail bear numerous stout, sharp thorns.

<span class="mw-page-title-main">Giant freshwater stingray</span> Species of cartilaginous fish

The giant freshwater stingray is a species of stingray in the family Dasyatidae. It is found in large rivers and estuaries in Southeast Asia and Borneo, though historically it may have been more widely distributed in South and Southeast Asia. The largest freshwater fish and the largest stingray in the world, this species grows up to 2.2 m (7.2 ft) across and can reach up to 300 kg (660 lb) in weight. It has a relatively thin, oval pectoral fin disc that is widest anteriorly, and a sharply pointed snout with a protruding tip. Its tail is thin and whip-like, and lacks fin folds. This species is uniformly grayish brown above and white below; the underside of the pectoral and pelvic fins bear distinctive wide, dark bands on their posterior margins.

<i>Rhina ancylostoma</i> Species of cartilaginous fish

Rhina ancylostoma, the bowmouth guitarfish, shark ray or mud skate, is a species of ray and a member of the family Rhinidae. Its evolutionary affinities are not fully resolved, though it may be related to true guitarfishes and skates. This rare species occurs widely in the tropical coastal waters of the western Indo-Pacific, at depths of up to 90 m (300 ft). Highly distinctive in appearance, Rhina ancylostoma has a wide and thick body with a rounded snout and large shark-like dorsal and tail fins. Its mouth forms a W-shaped undulating line, and there are multiple thorny ridges over its head and back. It has a dorsal color pattern of many white spots over a bluish gray to brown background, with a pair of prominent black markings over the pectoral fins. This large species can reach a length of 2.7 m (8.9 ft) and weight of 135 kg (298 lb).

<span class="mw-page-title-main">Thornback guitarfish</span> Species of cartilaginous fish

The thornback guitarfish is a species of ray in the family Platyrhinidae, and the only member of its genus. Despite its name and appearance, it is more closely related to electric rays than to true guitarfishes of the family Rhinobatidae. This species ranges from Tomales Bay to the Gulf of California, generally in inshore waters no deeper than 6 m (20 ft). It can be found on or buried in sand or mud, or in and near kelp beds. Reaching 91 cm (36 in) in length, the thornback guitarfish has a heart-shaped pectoral fin disc and a long, robust tail bearing two posteriorly positioned dorsal fins and a well-developed caudal fin. The most distinctive traits of this plain-colored ray are the three parallel rows of large, hooked thorns that start from the middle of the back and run onto the tail.

<span class="mw-page-title-main">Western shovelnose stingaree</span> Species of cartilaginous fish

The western shovelnose stingaree is a common species of stingray in the family Urolophidae, inhabiting shallow sandy flats and seagrass beds off southwestern Australia from Perth to Gulf St Vincent. Growing to 37 cm (15 in) long, this small ray has a rounded pectoral fin disc and a blunt, broadly triangular snout. Its nostrils have enlarged lobes along the outer rims and a skirt-shaped curtain of skin between them with a strongly fringed posterior margin. Its tail ends in a lance-like caudal fin and lacks dorsal fins and lateral skin folds. This species is colored grayish to brownish above, sometimes with lighter and darker spots, and pale below, sometimes with darker marginal bands and blotches.

<span class="mw-page-title-main">Striped stingaree</span> Species of cartilaginous fish

The striped stingaree is a common but little-known species of stingray in the family Urolophidae, endemic to shallow, inshore waters off southwestern Australia. Reaching 61 cm (24 in) long, this species is characterized by an oval, grayish to brownish disc with darker mask-like markings around the eyes and paired blotches at the center of the disc that are extended posteriorly into horizontal lines. Its nostrils have enlarged lobes on the outer margins and a skirt-shaped curtain of skin with a deeply fringed trailing margin in between. Its tail terminates in a relatively large leaf-shaped caudal fin, and bears a small dorsal fin just before the stinging spine. The rounded, flexible disc of the striped stingaree enable it to maneuver through the rocks, reefs, and seagrass that comprise its favored habitats. The International Union for Conservation of Nature (IUCN) has listed this species under Least Concern; it is seldom caught by fisheries due to its habitat preferences.

<span class="mw-page-title-main">Sandyback stingaree</span> Species of cartilaginous fish

The sandyback stingaree or great stingaree is a little-known species of stingray in the family Urolophidae, endemic to southeastern Australia. It is generally found offshore around the edge of the continental shelf, at a depth of 65–265 m (213–869 ft). A relatively large species reaching 89 cm (35 in) long, the sandyback stingaree has a diamond-shaped pectoral fin disc wider than long, usually with a dorsal pattern of numerous fine lighter marks on a yellowish to brownish background. Its short tail terminates in a deep, leaf-shaped caudal fin, and bears a sizable dorsal fin just in front of the stinging spine.

The circular stingaree is an uncommon, little-known species of stingray in the family Urolophidae. Endemic to the coastal waters of southwestern Australia, it prefers a rocky and/or vegetated habitat. Reaching 60 cm (24 in) in length, this species is characterized by an oval pectoral fin disc bearing a striking dorsal pattern of lighter spots and rings, and a central circle of white-margined black spots, on a bluish gray background. Between its nostrils is a skirt-shaped curtain of skin, with the posterior corners drawn out into lobes. Its tail bears a rather large dorsal fin in front of the stinging spine, and ends in a deep, lance-like caudal fin. Negligibly affected by human activity, the circular stingaree has been listed under Least Concern by the International Union for Conservation of Nature (IUCN).

The patchwork stingaree is a little-known species of stingray in the family Urolophidae, with a disjunct distribution off northwestern and northeastern Australia. It usually inhabits the outer continental shelf, at a depth of 60–320 m (200–1,050 ft). This species has a diamond-shaped pectoral fin disc much wider than long, and a short, flattened tail with a prominent dorsal fin and leaf-like caudal fin. There is a skirt-shaped curtain of skin between its nostrils. Its dorsal color pattern resembles a mosaic of dark brown rings with light-colored centers, separated by fine reticulated lines. The International Union for Conservation of Nature (IUCN) has listed the patchwork stingaree under Least Concern, as it is subject to minimal fishing pressure.

The lobed stingaree is a common species of stingray in the family Urolophidae, endemic to southern Western Australia in shallow, inshore sand and seagrass habitats. This species is plain sandy in color above and has a broad, rounded pectoral fin disc. It is characterized by an enlarged, semicircular skin lobe of unknown function on the inner rim of each nostril. Its tail is slender, with lateral skin folds and a lance-like caudal fin but no dorsal fin. The maximum recorded width is 27 cm (11 in).

The Coral Sea stingaree is a little-known species of stingray in the family Urolophidae, found at a depth of 171–310 m (561–1,017 ft) around the edge of the continental shelf off northern Queensland. Growing to a length of 48 cm (19 in), this species has a diamond-shaped pectoral fin disc with a protruding snout and a skirt-shaped flap of skin between the nostrils. Its tail bears a low dorsal fin before the stinging spine and terminates in a short leaf-shaped caudal fin. Its upper surface is grayish or brownish, sometimes with tiny dark spots. The Coral Sea stingaree may represent two closely similar species, one large and one small. There is very little fishing activity within its range, and thus it has been listed under Least Concern by the International Union for Conservation of Nature (IUCN).

<span class="mw-page-title-main">Finless sleeper ray</span> Species of cartilaginous fish

The finless sleeper ray is a species of electric ray in the family Narkidae, and the sole member of its genus. It is found over the continental shelf of Southeast Asia from the eastern Andaman Sea to Vietnam and Borneo. Typically growing no more than 15 cm (5.9 in) long, it may be the smallest cartilaginous fish. The finless sleeper ray is the only electric ray that lacks dorsal fins. It has an oval pectoral fin disc that varies from longer than wide to wider than long, depending on age, and a short, robust tail that terminates in a short, deep caudal fin. The trailing margins of its pelvic fins are sexually dimorphic, being more concave in males.

<span class="mw-page-title-main">Batoidea</span> Superorder of cartilaginous fishes

Batoidea is a superorder of cartilaginous fishes, commonly known as rays. They and their close relatives, the sharks, comprise the subclass Elasmobranchii. Rays are the largest group of cartilaginous fishes, with well over 600 species in 26 families. Rays are distinguished by their flattened bodies, enlarged pectoral fins that are fused to the head, and gill slits that are placed on their ventral surfaces.

The Hortle's whipray is a little-known species of stingray in the family Dasyatidae, occurring in shallow estuaries and mud flats off southern New Guinea. This species, growing to 71 cm (28 in) across, has a heart-shaped pectoral fin disc with a long, pointed snout and minute eyes. It has a wide dorsal band of dermal denticles extending from in front of the eyes to the tail, as well as scattered sharp denticles on the snout. The underside of the disc is a distinctive bright yellow in color, sometimes with darker markings around the nostrils, mouth, and gill slits. The Hortle's whipray is threatened by extensive seine fisheries and habitat degradation, leading the International Union for Conservation of Nature (IUCN) to assess it as Near Threatened.

<span class="mw-page-title-main">Leopard whipray</span> Species of cartilaginous fish

The leopard whipray is a little-known species of stingray in the family Dasyatidae, found in the Indian and Pacific Oceans from the Andaman Sea to the Coral Triangle. It is found close to shore at depths shallower than 70 m (230 ft), over soft substrates. Attaining a width of 1.8 m (5.9 ft), this species has a diamond-shaped pectoral fin disc with a pointed snout and an extremely long, whip-like tail without fin folds. Adult rays have a leopard-like dorsal pattern of dark brown rings on a yellowish brown background, as well as a row of enlarged, heart-shaped dermal denticles along the midline of the disc. Newborns and small juveniles have large, solid dark spots and few denticles. The leopard whipray is caught by fisheries in many parts of its range, primarily for meat.

<span class="mw-page-title-main">Indonesian angelshark</span> Species of shark

The Indonesian angelshark is a rare species of angelshark, family Squatinidae, known only from a few specimens collected from fish landing sites in southern Indonesia. It is thought to inhabit the deep waters of the continental slope. Reaching at least 1.34 m (4.4 ft) long, this species has a flattened, ray-like shape and a well-developed tail and caudal fin. It is characterized by the absences of fringes on its nasal barbels and thorns down the midline of its back, as well as by its relatively plain grayish-brown dorsal coloration with dark saddles beneath the dorsal fin bases and a black leading margin on the underside of the pectoral fins. The International Union for Conservation of Nature (IUCN) has classified it as Critically Endangered due to significant fishing pressure.

References

  1. 1 2 3 4 5 6 McCormack, C.; Wang, Y.; Ishihara, H.; Fahmi, Manjaji Matsumoto, B.M.; Capuli, E.; Orlov, A. (2015). "Hexatrygon bickelli". IUCN Red List of Threatened Species . 2015: e.T161674A68626659. doi: 10.2305/IUCN.UK.2015-4.RLTS.T161674A68626659.en . Retrieved 12 November 2021.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  2. 1 2 3 4 Last, P.R.; Stevens, J.D. (2009). Sharks and Rays of Australia (second ed.). Harvard University Press. pp. 396–397. ISBN   978-0674034112.
  3. ↑ Heemstra, P.C.; Smith , M.M. (1980). "Hexatrygonidae, a new family of stingrays (Myliobatiformes: Batoidea) from South Africa, with comments on the classification of batoid fishes". Ichthyological Bulletin (J. L. B. Smith Institute of Ichthyology). 43: 1–17. hdl:10962/d1019701.
  4. 1 2 3 4 5 6 Smith, J.L.B.; Smith, M.; Smith, M.M.; Heemstra, P. (2003). Smith's Sea Fishes. Struik. pp. 142–143. ISBN   978-1-86872-890-9.
  5. ↑ Nishida, K. (1990). "Phylogeny of the suborder Myliobatoidei". Memoirs of the Faculty of Fisheries, Hokkaido University. 37: 1–108.
  6. ↑ McEachran, J.D.; Dunn, K.A.; Miyake, T. (1996). "Interrelationships within the batoid fishes (Chondrichthyes: Batoidea)". In Stiassney, M.L.J.; Parenti, L.R.; Johnson, G.D. (eds.). Interrelationships of Fishes . Academic Press. pp.  63–84. ISBN   978-0-12-670951-3.
  7. ↑ Aschliman, N.C.; Claeson, K.M.; McEachran, J.D. (2012). "Phylogeny of Batoidea". In Carrier, J.C.; Musick, J.A.; Heithaus, M.R. (eds.). Biology of Sharks and Their Relatives (second ed.). CRC Press. pp. 57–98. ISBN   978-1439839249.
  8. ↑ Naylor, G.J.; Caira, J.N.; Jensen, K.; Rosana, K.A.; Straube, N.; Lakner, C. (2012). "Elasmobranch phylogeny: A mitochondrial estimate based on 595 species". In Carrier, J.C.; Musick, J.A.; Heithaus, M.R. (eds.). The Biology of Sharks and Their Relatives (second ed.). CRC Press. pp. 31–57. ISBN   978-1-4398-3924-9.
  9. ↑ Adnet, S. (2006). "Two new selachian associations (Elasmobranchii, Neoselachii) from the Middle Eocene of Landes (south-west of France). Implication for the knowledge of deep-water selachian communities". Palaeo Ichthyologica. 10: 5–128.
  10. 1 2 3 4 5 Compagno, L.J.V.; Last, P.R. (1999). "Hexatrygonidae: Sixgill stingray". In Carpenter, K.E.; Niem, V.H. (eds.). FAO Identification Guide For Fishery Purposes: The Living Marine Resources of the Western Central Pacific. Vol. 3. Food and Agricultural Organization of the United Nations. pp. 1477–1478. ISBN   978-92-5-104302-8.
  11. 1 2 Babu, C.; Ramachandran, S.; Varghese, B.C. (2011). "New record of sixgill sting ray Hexatrygon bickelli Heemstra and Smith, 1980 from south-west coast of India". Indian Journal of Fisheries. 58 (2): 137–139.
  12. ↑ Dean, M.N.; Bizzarro, J.J.; Summers, A.P. (2007). "The evolution of cranial design, diet, and feeding mechanisms in batoid fishes". Integrative and Comparative Biology. 47 (1): 70–81. CiteSeerX   10.1.1.596.306 . doi: 10.1093/icb/icm034 . PMID   21672821.
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