| Sixgill stingray | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Order: | Myliobatiformes |
| Family: | Hexatrygonidae Heemstra & M. M. Smith, 1980 |
| Genus: | Hexatrygon Heemstra & M. M. Smith, 1980 |
| Species: | H. bickelli |
| Binomial name | |
| Hexatrygon bickelli Heemstra & M. M. Smith, 1980 |
|
| Synonyms | |
|
Hexatrematobatis longirostris Chu & Meng, 1981 |
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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 slits (all other rays have five). It is the sole member of the genus Hexatrygon and the family Hexatrygonidae.[1] Although several species of Hexatrygon have been described historically, it is now believed that they may represent variations in a single, widespread species.
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The holotype 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.[2] 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.[3]
The sixgill stingray is widely distributed in the Indian and Pacific Oceans, 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 photographed feeding in shallow water off Japan. It is found over soft bottom substrates.[4]
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, heart-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 fins, 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.[3][4]
The ampullae of Lorenzini are well-developed, arranged in longitudinal rows underneath the snout. There are six pairs of small gill slits 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.[3][4]
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 mineralization of their skeleton likely precludes them from taking hard-shelled organisms.[5] 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 fetuses. The size at birth is 48 cm; males are estimated to mature at 110 cm long.[4]