Pelagic stingray

Appearance

The pelagic stingray has a very thick, distinctively wedge-shaped pectoral fin disc one-third wider than long, with broadly curved leading margins, rather angular outer corners, and nearly straight trailing margins. The snout is short with a rounded tip. The eyes are minute and, unlike in other stingrays, do not protrude above the body; the spiracles follow immediate behind. There is a short but broad curtain of skin between the nostrils, with a weakly fringed rear margin. The mouth is small and gently arched, with deep furrows at the corners and a tiny projection at the center of the upper jaw that fits into an indentation on the lower jaw. There are anywhere from 0 to 15 forked papillae in a row across the floor of the mouth. There are 25–34 upper tooth rows and 25–31 lower tooth rows; the teeth of both sexes have single, pointed cusps, but those of adult males are longer and sharper than those of adult females. The margins of the pelvic fins are nearly straight in front and become rounded at the tips and in back.

The whip-like tail measures up to twice as long as the disc; it is thick at the base and tapers significantly to the extremely long, serrated spine placed approximately one-third to halfway along its length. Two spines may be present if a replacement grows in before the original drops off. Posterior to the spine origin, there is a low ventral fin fold that does not reach to the spine tip. Young rays are completely smooth-skinned; with age small prickles appear over the center of the back, as well as a row of small thorns along the midline from between the eyes to the origin of the spine. This species is a plain dark purple to blue-green above, extending onto the tail fold, and a slightly lighter shade below. When captured and handled, it exudes a thick black mucus that covers its body. The pelagic stingray typically grows to 1.3 m long and 59 cm across. The largest individuals on record are from a captive rearing experiment conducted from 1995 to 2000, in which a male reached 68 cm across and 12 kg, and a female 94 cm across and 49 kg.

Distribution

The distribution of the pelagic stingray extends nearly worldwide in tropical to warm-temperate pelagic waters, between the latitudes of 52°N and 50°S. In the western Atlantic, it has been reported from the Grand Banks of Newfoundland to North Carolina, the northern Gulf of Mexico, and the Lesser Antilles, Brazil and Uruguay. In the eastern Atlantic, this species has been recorded from the North Sea to Madeira, including the Mediterranean, as well as around Cape Verde, in the Gulf of Guinea and off South Africa. In the Pacific, it is known from Japan to Australia and New Zealand in the west, British Columbia to Chile in the east, and around many oceanic islands including Hawaii, the Galápagos, and Easter Island. There have been no reports of this species from much of the Indian Ocean, but it is known to be common in the southwestern portion and around Indonesia.

Unique amongst stingrays in inhabiting the open ocean rather than the sea floor, the pelagic stingray is generally found from the surface to a depth of 100 m over deep water. It has also been caught at a depth of 330–381 m over the Kyushu–Palau Ridge, indicating that it at least occasionally approaches the bottom. This species prefers water temperatures above 19 °C, and will die if the temperature drops to 15 °C.

The pelagic stingray performs seasonal migrations following warm water masses. In the northwestern Atlantic, it is found in or near the Gulf Stream from December to April, and moves north of the Stream to gather near the continental shelf from July to September. A similar migration seems to occur in the Mediterranean, though the specifics are unknown. In the Pacific, this species apparently spends the winter in oceanic waters near the equator and move into higher latitudes and towards the coast in spring. Two Pacific populations are known: one migrates from near Central America to California, and the other from the central Pacific to as far as Japan and British Columbia. Off southeastern Brazil, pelagic stingrays are displaced towards the coast by upwellings of cold water in late spring and summer; in some years they may even be pushed into inshore waters less than 45 m deep.

Behavior

In adopting a midwater lifestyle, the pelagic stingray exhibits several characteristics different from those of its bottom-dwelling relatives. While most stingrays propel themselves by undulating their disc margins, this species swims by oscillating its pectoral fins in a manner approaching the "underwater flying" employed by eagle rays. Oscillatory fin motions generate lift, thus improving cruising efficiency in open water at a cost to maneuverability. The pelagic stingray is adept at swimming backwards, which may compensate for the lower fine control offered by its swimming mode.

Vision seems to be more important to the pelagic ray in finding food than in other stingrays. Compared to other members of its family, this species has less than one-third the density of electroreceptive ampullae of Lorenzini on its underside. The ampullae also cover a smaller area, though not as small as in eagle rays, and are more evenly distributed between the ventral and dorsal surfaces. This ray can detect an electric field of well under 1 nV/cm at a distance of up to 30 cm, and may be able to pick up the minute electric fields produced by moving sea water. The lateral line of the pelagic stingray, a complex system of mechanoreceptors that detect nearby movement and ocean currents, remains similar to other stingrays in covering a greater area on the ventral than the dorsal surface. However, this species is less responsive to mechanical than to visual stimuli.

Pelagic stingrays may segregate by sex, vertically in that males are found in deeper water than females, and perhaps horizontally as well. Captive individuals often act highly aggressively towards ocean sunfish, biting and harassing them, particularly if they are hungry. This species is preyed upon by oceanic whitetip sharks, great white sharks, toothed whales, and other large, ocean-going carnivores. Its all-around dark coloration likely serves to camouflage it against its featureless habitat. The venom on its tail spine is also quite potent, causing it to be avoided by other fishes. Known parasites of this species include the tapeworms "Acanthobothrium benedeni", "A. crassicolle", and "A. filicolle", "Rhinebothrium baeri" and "R. palombii", and "Tetragonocephalum uarnak", and the monogenean "Entobdella diadema".

Food

The pelagic stingray is an active predator that captures prey by wrapping its pectoral fins around it, before manipulating it to the mouth. It is the only stingray in which both sexes have pointed teeth, for grasping and cutting into slippery prey. A wide variety of organisms are represented in its diet: crustaceans including amphipods, krill, and larval crabs, molluscs including squid, octopus, and pteropods, bony fishes including herring, mackerel, sea horses and, comb jellies and medusae, and polychaete worms. Off California, pelagic stingrays hunt large mating aggregations of squid that form from November to April. Off Brazil, this species follows groups of Atlantic cutlassfish towards the coast in January and February, with both predators seeking small schooling fishes. Juvenile rays consume 6–7% of their body weight in food per day, which declines to just above 1% in adults.