MadSci Network: Zoology |
Okay! Sharks and rays are one of my favourite marine animals. Here's a bit about them. I'll deal with the question about the dwarf shark in a sec (1) How are skates, rays and sharks alike and different? Sharks, rays and chimaeras (ghost sharks) are grouped together because they're "cartillagenous" - they don't have bones like other fish, but a skeleton of cartillage. They're a very old group - dating back to more than 400 million years ago. No mean feat, when you think about crocodiles being pretty ancient (they're only 220 million years old). Sharks and rays are two separate groups, with "skates" being a family (or sub-grouping) of rays (Family Rajidae). Quickly, the three groups have these features: - SHARKS Fusiform shape (ie. streamlined, fishy shape). Have an obvious and well developed caudal fin to swim. Over 370 species worldwide. - RAYS Probably evolved from sharks. "Dorsoventrally flattened", which means they're a very flat animal, rather than the classic fishy shape you see with a typical shark. They also have enlarged pectoral fins (bigger than a true shark), and eyes on the top of the head and a mouth and gill slits on the underside. -CHIMAERAS An ancient shark group. They're different from true sharks because: (a) They have the top jaw completely fused to the skull (sharks have a moveable jaw - watch JAWS for more idea on how a modern shark can roll out it's jaws to grab food) (b) One external gill opening (modern sharks and rays have 5-7 gill slits) (c) Smooth, denticle (shark scales) free skin. They generally eat other animals that live on the sea floor. Well, I hope this explains the nature of the shark family. They're a very diverse and interesting group. Now I've been unable to find anything on dwarf sharks. Do you mean the little cookiecutter sharks, or dwarf catsharks? Let me know a species name and I can find out more for you. cheers, Alastair
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