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What are sharks?
The fishes is the most species rich group among the
vertebrates, with more than 25000 species described. One can divide
the jawed fishes in to two different groups, the bony fishes and the
cartilaginous fishes (we leave out the few fishes that lack jaws,
for the sake of simplicity).
(© J. Lokrantz/Azoteimages)
The bony fishes, or Osteichthyes, are fishes with a skeleton
made out of bone. They are the largest group amongst the fishes and
outnumber the cartilaginous fishes, or Chondrichthyes, which
only constitutes of about 850 species. Of these 850 species less than
400 are sharks, which means that only 1-2 % of all fishes are sharks.
That does not sound like a lot, but for being such a small part of
the fishes, they are quiet fascinating, don't you think? Beside the
differences in the skeleton there are many other features that make
sharks special. Here are some of them:
- Asymmetric tail
- Placoid scales
- Mouth underneath the head
- Spiral shaped intestines
- 5-7 pare of gill slits
- Large liver rich in oil
- Renewably teeth throughout their life
- Receptor cells sensitive to electric fields
- Internal fertilization (some exceptions)
- High concentration of urea in the blood
(© J. Lokrantz/Azoteimages)
Ref: Hickman CP, Roberts LS and Larson
A (2000), Animal Diversity, McGraw-Hill Higher Education, USA -- Springer
VG and Gold JP (1989), Sharks in quetion: the Smithsonian answear
book, Smithsonian Institution Press, Washington, D.C.
London
 Further reading
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