Species distribution can be understood through many factors except variation of sea levels.
 Some vertebrates such as seabirds, seals and sea turtles return to the land to breed but fish, cetaceans and sea snakes have a completely aquatic lifestyle and many invertebrate phyla are entirely marine
 There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas or particularly as a mediterranean sea) or wholly (as inland seas) enclosed by land

