Nurse sharks are opportunistic predators that feed primarily on small fish (e.g. stingrays) and some invertebrates (e.g. crustaceans, molluscs, tunicates). They are typically solitary nocturnal animals, rifling through bottom sediments in search of food at night, but often gregarious during the day forming large sedentary groups. Nurse sharks are obligate suction feeders capable of generating suction forces that are among the highest recorded for any aquatic vertebrate to date. Although their small mouths may limit the size of prey, they can exhibit a suck-and-spit behavior and/or shake their head violently to reduce the size of food items.