The deepwater stingray or giant stingaree ( Plesiobatis daviesi) is a species of stingray and the sole member of the family Plesiobatidae. It is widely distributed in the Indo-Pacific, typically over fine on the upper continental slope at depths of . This species reaches in length and 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 .
Preying on , , and , the deepwater stingray may hunt both on the sea floor and well above it in open water. It is likely aplacental viviparous, with the mother supplying her gestating young with histotroph ("uterine milk"). Captured rays merit caution due to their long, stingers. This species is taken by deepwater commercial fisheries, but in numbers too small to significantly threaten its population. Therefore, the International Union for Conservation of Nature has assessed it as Least Concern.
Taxonomy and phylogeny
The first scientific description of the deepwater stingray was authored by John H. Wallace, as part of a 1967 Investigational Report from the Oceanographic Research Institute,
Durban. He named the new species
daviesi in honor of David H. Davies, the late director of the ORI, and placed it in the genus
Urotrygon based on its long, low caudal fin and lack of a
dorsal fin. The
were collected during September 1996 near the
Limpopo River mouth in
Mozambique: the
holotype is a mature male across, and the
paratype is an immature male across.
Other
for this species include Davies' stingray and giant stingray.
In a 1990 morphological phylogenetic study, Kiyonori Nishida concluded that the deepwater stingray and the sixgill stingray ( Hexatrygon bickelli) were the most basal clade (suborder Myliobatoidei). Therefore, he moved this species to its own genus, Plesiobatis, and family, Plesiobatidae; the name is derived from the Ancient Greek plesio ("primitive") and batis ("ray").
Distribution and habitat
Records of the deepwater stingray come from a number of locations scattered widely in the
Indo-Pacific:
KwaZulu-Natal in South Africa and Mozambique, the Gulf of Mannar, the northern
Andaman Islands, the South China Sea, the
Ryukyu Islands and the Kyushu–Palau Ridge, along the southern coastline of Australia, northwestern Australia from the
Rowley Shoals to
Shark Bay, northeastern Australia from Townsville to Wooli,
New Caledonia, and
Hawaii.
This
benthic fish species generally inhabits the upper continental slope at depths of , over muddy or silty substrates.
An anomalous record from only deep off Mozambique was made. It seems to be locally common in tropical Australian waters, but may be rarer elsewhere.
Description
The deepwater stingray has a flabby body,
with enlarged
forming a disc usually longer than it is wide. The leading margins of the disc converge at a broad angle. The snout is thin and measures over six times as long as the diameter of the orbit; the snout tip protrudes slightly from the disc. The small eyes are positioned just ahead of the spiracles, which have angular posterior rims. The large, circular nostrils are placed close to the mouth, to which they are connected by a pair of broad grooves. Between the nostrils is a broad curtain of skin with a strongly fringed posterior margin. The wide, straight mouth contains 32–60 tooth rows in either jaw, increasing in number with age. Each tooth is small with a low, blunt cusp; in adult males the teeth at the center are sharp and backward-pointing. The five pairs of
are small and placed beneath the disc.
The are small and have blunt outer corners. The moderately thick tail measures 93–102% as long as the disc and lacks lateral skin folds and . One or two serrated stinging spines are present atop the tail, just ahead of the halfway point. The slender caudal fin originates a short distance behind the sting; it is symmetrical above and below, and terminates in a rounded leaf-like shape. The skin is densely covered by fine , that become sparse to absent on the pelvic fins, towards the ventral disc margin, and around the mouth. The deepwater stingray is purplish brown to blackish above; some rays also have irregular darker blotches and spots. The underside is white, with a narrow dark border along the lateral disc margins. The tail is entirely dark, and the caudal fin is black. This large species grows up to long, across, and in weight off southern Africa, though it is not known to exceed long off Australia.
Biology and ecology
The diet of the deepwater stingray consists of
,
(including
Penaeidae,
, and
), and
(including
).
Its long, flexible snout is well-suited for rooting through
sediment, while the presence of
mesopelagic species in its diet suggest that it may also hunt well above the
sea floor.
One recorded individual was found severely gouged by
(
Dalatias licha), which are capable of excising plugs of flesh,
cookiecutter-like, from larger animals.
The deepwater stingray is presumed to be similar to other stingrays in being aplacental viviparous, and having the developing
nourished by maternally produced
histotroph ("uterine milk"). Given its large size and deepwater habits, it is probably not highly prolific, with a small litter size and a long
gestation period. The young are apparently born at close to long, as evidenced by the capture of a free-living specimen of that size that still bore a
yolk sac scar. Males and females
sexual maturity at and long, respectively. The maximum size, and likely also the maturation size, varies between geographic regions.
Human interactions
When captured, the deepwater stingray flails its powerful tail violently, and its long,
sting can inflict a serious injury to a
fishery worker. It is
bycatch by deepwater
and
longline fishing; the meat may be sold, but is poorly regarded.
None of the deepwater commercial fisheries operating within its range (including those off South Africa,
Taiwan,
Indonesia and Australia) are extensive, and thus only small numbers of deepwater stingrays are landed. As a result, the International Union for Conservation of Nature has determined this species to be minimally threatened by human activity, and listed it under
least concern. However, should deepwater fisheries expand in the future, it may be susceptible to depletion due to its probable rarity and low reproductive rate.
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