Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on
the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the
clade Selachimorpha (or Selachii) and are the sister group to the rays.
There are over 400 species of sharks.
Did you know?
Until the 16th century, sharks were known to mariners as "sea dogs". This is still evidential in
several species termed "dogfish," or the porbeagle.
Based on fossil scales found in Australia and the United States, scientists hypothesize sharks first
appeared in the ocean around
455 million years ago
.
There are oviparous (egg-laying) species, like the zebra shark (Stegostoma
fasciatum) and viviparous (live-bearing) species, like the blue shark
(Prionace glauca).
Oviparous species lay eggs that develop and hatch outside the mother's body with no parental care after the
eggs are laid.
