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Relatives of sharks: skates and rays


 
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Dr Vic Peddemors, from the NSW Department of Primary Industries, looks at the other species of cartilaginous fish (think shark), especially skates and rays and chimaeras (also known as ratfish).     
Chimaeras usually inhabit the deeper parts of oceans. Bizarre looking, they are well represented in fossil records. They also have only one gill slit (sharks mostly have at least five) and have external pigmentation areas which glow in the dark, luring prey. Living at those dark depths means that little is known about them but it is known that there are ten species in Australian waters. I
n fact Australia has a very diverse collection of cartilaginous fish, with over 300 identified species of sharks, skates, rays and chimaeras. They are generally small but manta rays, for instance, have wing spans of four or five metres.
Rays are a large group of fish. They have a long whip-like tail without fins, usually with a barb at the end. Devil rays and manta rays do not have these barbs whereas all other stingrays do.
Skates, on the other hand, have a thicker, fleshy tail with fins. Stingarees are skates. Skates and rays are bottom dwellers, living on the sea bed.
Most skates and rays lay eggs. Skates and rays have relatively small mouths with crushing teeth for devouring prey like molluscs. The teeth also have pointy, grasping attachments which are used to grab invertebrates such as worms and prawns.
Sharks, skates and rays do not compete for food, each species being adapted to eat different prey. Many skates have thorns on the tops of their bodies, which helps to stop them from being predated. They are also well camouflaged and will bury themselves in the sea bed when they are not feeding.
The barb on a ray is a formidable defence tool. There is a thin fleshy covering over the pointy end and this also encases a poison sac in many species. The barb itself is made of many barblets which point backwards. When the barb is used it usually snaps off as the barblets hold the barb in place. The barbs are so efficient some ancient cultures used them as arrowheads.
Trawl fishing is hazardous for skates and rays. They appear to be surviving well off the NSW coast but there have been no observers on those fleets for some time, although observations are planned. Shovel-nosed rays are popular in fish shops and used for fish and chips, with some 120 tons caught each year.
However, at this stage, the species itself seems to be coping.
That is a ray of hope in itself.

Text: V.B.

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