Seahorse is the name given to 54 species of marine fishes in the genus Hippocampus. Seahorses need to eat almost constantly, as they don't have a stomach, so food passes through them very quickly. But what do they eat? Seahorses do not have teeth - they suck in their food and swallow it whole. Thus their prey needs to be pretty small. Seahorses feed on plankton, small fish and small crustaceans such as shrimp and copepods.