Catfish have dorsal fin spines, which help defend against predators, and caudal fins, which help to propel the fish through the water. A swim bladder controls how high or low the fish sits in the water and gills allow the fish to breathe by taking oxygen out of the water. Catfish spend most of their lives at the bottom of freshwater lakes and streams. Which catfish trait may change (over many generations) if the pond habitat in which it lives becomes significantly deeper?