Bowmouth guitarfish (Rhina ancylostoma)
|Also known as:||mud skate, shark ray, Sharkfin guitarfish|
|Size||Total length: up to 270 cm (2)|
|Weight||up to 135 kg (2)|
Classified as Vulnerable (VU) on the IUCN Red List (1).
Named for its distinctive mouth which undulates like a longbow (3), this deep-bodied guitarfish is unmistakable (2). The species has a broad, rounded snout, a head that is distinctly demarcated from the pectoral fins, and a tail that is much longer than the body (2) (4). The dorsal fins are tall and shark-like, earning the species its alternative common name of sharkfin guitarfish, and heavy ridges of spiky, sharp thorns appear on the bony ridges on the head, used in defensive butting (2) (3) (4). The most distinctive feature of this species is probably its unusual markings and colouration, although these usually become fainter in larger individuals (4). The body is white below and blue-grey above, with white spots on the fins, body and tail, a large blue-edged, black spot above each pectoral fin, and dark bands between the eyes (2) (4). Juveniles are brown with partial ocelli (eye-spots) over the pectoral fins and black bars between the eyes (3). The bowmouth guitarfish uses its heavily ridged teeth in undulating rows to crush crabs and shellfish (2) (4).
The bowmouth guitarfish is widely distributed across the Indo-west Pacific, from South Africa to the Red Sea in the west, to Australia to Japan in the east (3).
A bottom-dwelling species found in shallow coastal areas and inshore coral reefs, at a depth range of 3 to 90 m (2).
Very little is known about the biology of the bowmouth guitarfish (1). This species feeds mainly on crustaceans and mollusks on the sea bed (2), catching them by restraining the prey against the sea bottom using its large head and pectoral fins, and then with a series of short sharp thrusts, moving the prey into its mouth (3).
The bowmouth guitarfish is ovoviviparous, with females typically giving birth to four live young that have hatched within the uterus (2) (5). Like other sharks and rays, this species is thought to have a late onset of sexual maturity and slow reproductive rate (2).
The bowmouth guitarfish is threatened by commercial fisheries throughout its range, being taken both as a target species and as bycatch (1). The pectoral fins are sold for human consumption in Asia, with those of large animals fetching exceptionally high prices (1) (2). Sadly, this high demand and lucrative market creates a powerful incentive to retain bycatch, where it may otherwise have been thrown back into the sea. The species is susceptible to capture by a range of fishing gear types, including trawl nets, gillnets and hooks. Thus, although the exact effects of fishing have not yet been quantified, population numbers are thought to have been locally reduced by fishing throughout its range, and are projected to continue to decline as long as target fisheries remain economical (1). The problem is exacerbated by the species’ slow reproductive rate and population turnover, which makes it extremely vulnerable to over-fishing and slow to recover (2). Habitat destruction is also thought to pose a significant threat throughout much of the species range (1).
Although there are no target fisheries for the bowmouth guitarfish in Australia, the species is known to be caught as bycatch of demersal trawl fisheries. Nevertheless, the introduction of Turtle Exclusion Devices (TEDs) in some Australian trawl fisheries, and the implementation of various shark-finning prohibitions, are assumed to have led to a recent reduction in captures in Australian waters (1).
For further information on the conservation of sharks and rays see:
- ReefQuest Centre for Shark Research
Authenticated (19/06/2006) by R. Aidan Martin, Director of the ReefQuest Centre for Shark Research.
- Bycatch: in the fishing industry, the part of the catch made up of non-target species.
- Demersal: fish that live on or near the ocean bottom. They are often called benthic fish, groundfish, or bottom fish.
- Dorsal fin: the unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Ovovivipary: method of reproduction whereby the egg shell is weakly formed and young hatch inside the female; they are nourished by their yolk sac and then ‘born’ live.
- Pectoral fins: in fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
IUCN Red List (July, 2014)