Sharks & rays
Sharks and rays, or elasmobranchs, form the vast majority of Class Chondrichthyes, the cartilaginous fishes. Their internal skeleton is composed entirely of hardened cartilage. They have 5-7 pairs of external gill openings, and skin with numerous tiny placoid scales, or denticles. The teeth are not fused to the jaws, but are shed throughout life and replaced serially by newly developing rows from back to front, as required.
Sharks differ from rays most obviously through the positioning of the gill slits. In sharks they are located on the sides of the head, while in rays they are on the undersurface. In addition, rays generally have a more flattened appearance, with the greatly enlarged pectoral fins attached to the head in front of the gill openings.
Unlike most bony fishes, fertilisation of the eggs of sharks and rays occurs internally. This is aided by a pair of male ‘claspers’ between the pelvic fins. There is considerable variation in the modes of development and source of nutrition for embryos. In oviparous species, eggs in leathery cases are laid on the sea floor to develop independently of the parent, using the yolk sac for nutrition. In viviparous species, the young are borne fully developed and free-swimming. Among viviparous sharks and rays, nutrition for the embryos is provided within the mother, but may variously come from the yolk sac, uterine secretions, the placenta, or by consuming unfertilised eggs, or other, less developed embryos.
About 500 species of sharks and 600 species of rays are known worldwide. Of these, about 180 sharks and 120 rays are recognised from Australian waters. The vast majority of sharks are harmless to humans. However, in Australia a few species such as the White Shark, Tiger Shark, Bull Shark, and several other whaler sharks have caused fatalities.
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