Sharks have strong digestive juices and a specialized type of intestine called a spiral valve that increases the secretory and absorptive surfaces considerably. An epithelial fold spirals the length of the small intestine. Sharks exhibit all of the different kinds of reproduction. In all sharks fertilization of the eggs occurs internally. During copulation only one clasper has been observed to be inserted into the female.
Gestation varies from a few months to more than two years.
The longest known gestation period of any vertebrate, 20-24 months, is that of the spiny dogfish. Unlike bony fish, which usually lay enormous numbers of tiny eggs that develop into minute young, sharks produce small numbers of large young. The largest litter contains less than 100 young, while the usual litter is far smaller. Some species such as the sand tiger produce only two young. At birth all sharks are completely developed and ready for the predacious life of a carnivore.