Replacement teeth are larger that tooth being replaced. The rate of tooth replacement varies with species, may be from eight days to a full year, with the teeth replaced singly or complete row at once.
Sharks lack the air bladder of bony fish to maintain buoyancy.
Some sharks such as the sand tiger are gulpers and surface periodically to swallow air to assist in buoyancy. Adds buoyancy because oils of liver have lower specific gravity than surrounding water. In fact, production of liver oils is controlled by the shark to help compensate for buoyancy changes. Still, sharks are slightly negative and will sink if they stop swimming. The longest gestation period for any animal is that of the spiny dogfish, Squalus acanthias, at 22-24 months.