Friday, 28 October 2011

Shark ray

Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Order: Rajiformes
Family:  Rhinidae
Genus: Rhina
Species: Rhina ancylostoma
Common Names: Bowmouth guitarfish, Mud skate, Shark ray

The shark ray is a species of ray that is closely related to guitarfishes and skates. It is widely distributed over the tropical coastal waters of the Indo-Pacific region in depths of up to 90 meters (300 feet). They are the sole member of the family Rhinidae.

Shark Ray Habitat
Shark rays ray can be found in the Indo-west Pacific, East Africa (Red Sea to South Africa) to Papua New Guinea, north to Japan, across northern Australia from Exmouth Gulf in Western Australia to New South Wales Generally the species prefers sandy or muddy habitats, however it can occasionally be found in rocky habitats, coral reefs and shipwrecks.
They inhabit coastal waters in depths ranging from 3 to 90 m (10–300 feet), typically dwelling on or near the sea floor bottom, although from time to time they may swim well above it.

Shark ray description
Shark rays can reach a maximum reported length of 2.9 m (8.9 feet) long and a maximum reported weight of 135 kg (300 pounds). However sightings of shark rays exceeding the length of 2.4 meters are pretty rare. Their coloration is bluish-gray on the upper side that lightens towards the head and the pectoral fins, and grayish to white on the underside. Usually there are many prominent white spots all over the body as well as a white-edged black marking above each pectoral fin along with two dark transverse bands on top of the head between the eyes. Young shark rays tend to be more vividly colored than adults do, usually having a brownish coloration with a fainter pattern and proportionately smaller black spots.
Their appearance is quite distinctive and they look like a combination of a shark, a ray and a guitarfish.

A female shark ray
Their head is short, wide, and depressed, featuring a broadly rounded snout. Their bodies are wide and thick with shark-resembling dorsal and tail fins.
A thick ridge with massive and sharp thorns runs along the midline of their backs while more such “thorny” ridges can be found in front of the eyes and on the shark ray's "shoulders". These thorns are mainly employed as a means of defense.
Their tail is longer than the body and has a large, crescent-like caudal fin. The nostrils are elongated and oriented in a crosswise manner, with well-developed flaps of skin which separate each opening into inflow and outflow apertures. Their lower jaw has three protruding lobes which fit into three depressions in the upper jaw. The mouth contains 47 upper tooth rows and 50 lower teeth rows with ridged teeth arranged in winding bands. These teeth are primarily used for crushing the shells of their prey.
Shark rays have 5 pairs of gills which are found near the lateral margins of their head.

In the wild their lifespan is unknown however in captivity they have been reported to live to be up to 7 years old. Males are known to reach sexual maturity after reaching a length of 1.5–1.8 m (4.9–5.9 feet).
Shark ray parasites
Shark rays are a common host for many parasites including the following;
  • The tapeworm Tylocephalum carnpanulatum
  • The trematode Melogonimus rhodanometra
  • The monogeneans Branchotenthes robinoverstreeti and Monocotyle ancylostomae
  • The copepod Nesippus vespa
In addition there has been a recorded case during which a shark ray was being cleaned by blue streak cleaner wrasses (Labroides dimidiatus).

Shark ray behavior
They are a very active species (especially during the night) with no territorial behavior ever recorded. According to a few scientists shark rays use their spiked ridges over the eyes, nape, and pectoral fins to “head-butt” potential predators and attackers, however this theory has yet to been proven.

Shark ray diet and predators
They are a carnivorous species mainly consuming bottom-dwelling crustaceans like crabs and shrimps, however on occasions they may also feed on mollusks and bony fishes. They use their smell to locate their prey and after they do, they restrain it with their broad, blunt head easing it afterwards inside their mouth with a series of short but strong thrusts.

Their main predator and enemy is the tiger shark.

Shark ray reproduction
They are aplacental viviparous animals with egg fertilization occurring internally. The fertilized eggs remain inside the female's body with sharks being first nourished by the egg’s yolk (while still inside the egg) and once they hatch by the uterine "milk". This "milk" is secreted from the villi which are appendages in the wall of the uterus. These appendages also help by:
  • Providing oxygen to the embryos
  • Clearing the wastes produced by embryos
Video of a shark ray in the Sydney Aquarium

The litter size is 4, with newborns having an average size of about 45 cm (18 inches), however there is one report (Last and Stevens (2009)) of a pregnant individual bearing 9 mid-term embryos which had a length of 27 to 31 cm (11–12 inches) long. After being born the young shark rays immediately disperse and live by themselves with no parental care at all.

Threats and conservation status
Currently the species is listed by the IUCN as vulnerable while in Australia they are assessed as Near Threatened. For the past decade their overall population is continuously declining. Their main threats are considered to be:
  • Intentional and unintentional fishing
  • Habitat destruction, degradation and pollution (mainly from blast fishing, coral bleaching, and siltation)
Their pectoral fins are highly valued in the market and their meat is also sold (fresh, dried or salted) in Asian countries for human consumption. In Thailand the aforementioned thorns are used for making bracelets.
Unfortunately the species is vulnerable to multiple fishery gear types like
  • Trawl-nets
  • Gillnets
  • Hooks
For this reason regulating bycatchery has proven to be very difficult. In Australia (where they are a frequent bycatch) the installation of TED (turtle excluder devices) has helped with their preservation. It should be noted that although in Australia shark ray fining is illegal, there is an on growing fin black market trade.

Shark rays in captivity
Shark rays are commonly referred to as "the pandas of the aquatic world" due to being rare and facing many different threats. The Newport Aquarium was the first organization to initiate the world's first captive shark ray breeding program for the conservation of the species.

Shark rays interesting facts
Many researchers consider shark rays to be the missing link among sharks and rays. This is because of the ray-like placement of the mouth and gill openings, the disc shape of the front part of the body, the shark-like streamlined appearance the rest of the body has and finally the powerful tail they feature.

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