The nurse shark, Ginglymostoma cirratum, is common in the warm, shallow waters of the tropical Atlantic and Eastern Pacific oceans. While no wild marine animal is truly “friendly,” this species is considered docile, sluggish, and generally non-aggressive toward humans. Understanding their natural behavior and physical characteristics reveals why this bottom-dweller poses minimal risk.
Temperament and Natural Behavior
The nurse shark is often called the “couch potato of the sea” due to its sluggish, sedentary lifestyle during daylight hours. Primarily nocturnal, it spends the day resting motionless on the sandy ocean floor, often tucked into caves or crevices. Unlike many sharks, the nurse shark actively pumps water over its gills, allowing it to remain stationary.
During the day, groups of up to 40 individuals may rest together, sometimes piled atop one another. Their strategy for dealing with disturbances is avoidance, remaining still in hopes of being overlooked. The sharkâs robust body and muscular pectoral fins allow it to “walk” along the bottom, contributing to its slow-moving nature.
At night, the shark becomes an active hunter, slowly patrolling the sea floor for prey. Because they prefer nighttime foraging, most human interactions occur when the shark is lethargic during the day. This behavioral pattern is the primary reason they are perceived as non-threatening.
Anatomy and Feeding Mechanisms
The nurse shark’s physical structure is adapted for bottom-dwelling and suction feeding, not for attacking large, fast-moving prey. Its mouth is small and positioned ventrally, or downward-facing, which helps it forage along the seafloor. Sensory barbels, whisker-like organs, aid in locating prey hidden in the sand or crevices.
The feeding method involves a powerful suction mechanism that rapidly expands the pharynx, creating a strong vacuum to “slurp” prey. This suction force can dismember prey or pull creatures from their hiding spots. Their diet consists mainly of benthic invertebrates, such as lobsters, crabs, mollusks, sea urchins, and small bottom-dwelling fish.
Instead of the large, triangular, cutting teeth of predatory sharks, the nurse shark possesses small, blunt, pavement-like teeth. These teeth are adapted for crushing the hard shells of crustaceans and mollusks. This specialized anatomy means that even if a nurse shark bites a human, it is not designed for a predatory strike.
Documented Human Encounters and Risk
Adverse interactions between nurse sharks and humans are rare, especially unprovoked incidents. The International Shark Attack File records only a handful of non-fatal, unprovoked bites since 1876, with zero confirmed fatalities. This demonstrates the species poses one of the lowest risks among all sharks.
Most documented bites result from clear human provocation. Incidents occur when divers or swimmers physically bother a resting shark, such as by pulling its tail or attempting to grab it. When disturbed, the shark’s defensive response is a strong bite, which can cause serious injury due to its crushing jaw strength.
The risk increases when humans try to feed the sharks or approach them too closely while they are resting. Observers should maintain a respectful distance and avoid attempts to touch or handle the animals to ensure a safe encounter.