MadSci Network: Zoology
Query:

Re: How are skates, rays and sharks alike and different?

Date: Tue May 8 19:26:58 2001
Posted By: Alastair Lyon, Science Information Officer
Area of science: Zoology
ID: 988139697.Zo
Message:

	  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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