Baby Seal

There is also environmental pressure for a pulmonary surfactant system capable of controlled surfactant secretion even under long periods of hydrostatic pressure. Alveolar epithelial type II cells, which secrete surfactant when stretched, appear to have decreased sensitivity to pressure in California sea lions compared to terrestrial mammals so that constant levels of surfactant are maintained under long periods of pressure. Furthermore, the gene for leptin, a hormone that has a role in surfactant phospholipid synthesis, was found to have nonsynonymous substitutions that are under positive selection in phocids within otherwise highly conserved regions in mammals. The sites of substitution are predicted to be on the exposed sides of leptin where the changes likely alter leptin function. The modified leptin can then affect surfactant production in phocids.
Other adaptations
Pinniped eyes are well adapted for seeing both above and below the surface. The animal has a clear membrane that covers and protects its eyes underwater. In addition, its nostrils close and blood circulation stops to most of its organs when diving. Testicles and mammary glands are located in slits under the skin to maintain the pinniped’s streamlined shape. They also have whiskers to help navigate, and sensors in their skull to absorb sounds underwater and transmit them to the cochlea.