Actinopterygii: Life History and Ecology

While everyone knows that fish live in water and breathe through gills, the simile "like a fish out of water" does not always apply to ray-finned fish. A few, like the walking catfish and the mudskipper, are able to crawl about on land, to find food or new habitats. Some others, like the Siamese fighting fish, are capable of breathing air in addition to extracting oxygen from the water with their gills. These fish are not close relatives of the sarcopterygian lineage that led to air-breathing land vertebrates, but have independently evolved limited abilities to move on land and/or breathe air.

Many fish lay eggs, but some -- including common aquarium fish such as guppies, swordtails, and mollies -- retain the eggs inside their bodies until hatching. In a few, such as the seahorse, the male receives the eggs and keeps them inside a special pouch in his body. Such fish are said to be ovoviviparous; unlike mammals, these fish do not nourish the embryos during development. Many fish do not provide any care for their offspring, but a number do protect them in various ways. In some African cichlids, for instance, the female keeps her eggs in her mouth -- and after they hatch, the young are kept in her mouth as well.