SAND TIGER SHARKS:
Characteristics

• Cartilage skeleton
• 5-7 gill slits
• Short ribs possible, never a full rib cage - Fusiform in body shape generally, some species dorso-ventrally flattened
• 1-2 dorsal fins
• 1 pair pectoral, 1 pair pelvic fins
• Protruding snout with nostrils
• Cresent-shaped mouth
• Spiracle opening in posterior to eyes (evolutionary remnant of bony fish gill slit, not present in some species) - Dermal denticles in place of scales like bony fish. - Multiple rows of teeth, up to eight functional rows. The structure/shape of teeth varies with species and their diets. 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.
• Large liver, can occupy as much as 90% of the body cavity or be 25% of the body weight in some species. 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.

Most sharks give live birth (young born from oviducts of female instead of an egg that has been laid) after a long gestation period. The longest gestation period for any animal is that of the spiny dogfish, Squalus acanthias, at 22-24 months. It is believed that the male grasps the females’ fins with his mouth and uses his two claspers to mate with her. This violent meeting leaves deep cuts and gashes behind the females head. These injuries usually heal within a week however. After mating, shark reproduction occurs in one of three ways:

1. Ovoviviparity-most common. Internal egg laying with gestation and live birth. Sand tigers have a unique variation of ovoviviparity called intrauterine cannibalism. The first young in each of a sand tigers’ two oviducts to reach 60mm tears open its egg and swims to the uterus where it feeds on its siblings, embryos, and the other unfertilized eggs for the rest of gestation. Usually two 3 foot long live pups are born from a mother that is only about 9 feet long. Great whites are also believed to exhibit intrauterine cannibalism.
2. Viviparity-The yolk in the yolk sac is first consumed by embryo, then uterine milk produced by female is absorbed by an embryo through the means of a placenta that is formed from the yolk sac to wall of uterus. Sharks are born live and ready to fend for themselves outside the female. Viviparous species include the hammerhead, bull shark, blue shark and lemon sharks.
3. Oviparity-An egg case with yolk sac inside for nourishment is laid by the female on the sea floor or on rocks. "Mermaids purse" is a common name for swell shark and cat shark egg cases. the Horn sharks’ egg cases are screw shaped and are often found wedged tightly into rocks. Oviparous sharks include the Swell shark and Horn shark.