(full classification at end)
Nurse sharks have more in common with catfish than what's traditionally associated with the word shark. The nurse shark's name is likely derived from Nusse, the common name given to "catsharks" of the Scyliorhinidae family, of which the nurse shark was originally thought to belong. The earliest description of a nurse shark dates to 1788, when it was classified as Squalus cirratus. The nurse shark's current classification of Ginglymostoma cirratum was assigned in 1841. In the Caribbean, the nurse shark is still referred to by some as tiburon gato, which is Spanish for "cat shark."
The relatively small mouth of the nurse shark is located on the ventral side of its head, farther forward than its eyes on the dorsal side. The shark has barbels extending outward from between its nostrils (similar to catfish) which are long enough to reach the mouth. Inside the creature's mouth are teeth arranged in what's called independent dentition. Independent dentition is the simplest type of dental arrangement found in sharks: Nurse shark teeth don't overlap each other at all and the forward movements of the teeth used to shed them doesn't depend on other teeth in the mouth. Tooth replacement occurs more quickly in the summer and more often for juvenile nurse sharks.
The nurse shark makes up for its small mouth with a powerful pharnyx which allows the creature to suck in its food at high speeds. Nurse sharks are largely nocturnal animals and this quick and powerful sucking mechanism likely aids them greatly in preying upon resting fish and heavily-shelled conches. Nurse sharks feed on bivalves, catfish, crabs, mullets (not the hair style), octopi, puffers, sea urchins, shrimps, snails, spiny lobsters, squids, sting rays, and, occasionally, algae and coral. Younger nurse sharks have been observed at rest with their snouts pointed towards the surface and their bodies supported by their pectoral fins on the sea floor. Some researchers have suggested the goal of resting in this posture is to lure crabs and small fish beneath the nurse sharks in search of shelter, only to end up being eaten by the sharks.
The dorsal fins of the nurse shark are spineless and the first is much larger than the second. The first dorsal fin begins at roughly the same point on the shark's body as the pelvic fins. The caudal (tail) fin of the nurse shark accounts for over 25% of its total length. Nurse sharks also have small anal fins and huge, muscular pectoral fins (which are larger than even the first dorsal fin). The average adult female nurse shark ranges in size from 220 to 270 centimetres (7.5 to 9.0 feet) in length and 75 to 105 kilograms (167 to 233 pounds) in weight. The average adult male nurse shark is slightly smaller, ranging from 210 to 260cm in length and 90 to 120 kilograms in weight. The females reach maturity at about 225cm, while the males do so at about 210cm.
The mating habits of nurse sharks have been observed extensively in waters off the Florida Keys and Dry Tortugas archipelagos. Nurse shark mating generally occurs between the end of June and July. Nurse sharks begin their mating habits with a male approaching a resting female on the sea floor. The male bites and holds one of the female's pectoral fins, while simultaneously pushing her onto one side. Once in this position, the male can easily insert his clasper (an extension of a pelvic fin which aids in sperm transmission) into the female's cloaca. Some females have been observed with severe scarring and bruising on their fins due to large numbers of males attempting to mate with them. Females can avoid mating with the males by swimming into very shallow waters and burying their pectoral fins in the sand.
The gestation period for developing nurse sharks is six months (and it will be another eighteen months for the mother before the her ovaries produce mature eggs again). Mature female nurse sharks give birth to a brood of between 30 and 40 pups once every two years. Inside their mother, the pups experience aplacental viviparity (the young hatch from their eggs while still inside the mother, feeding on the yolk of the eggs rather than a placenta, and are eventually born live). The size of the pups at birth is usually between 28 and 30cm (11 and 12 inches). The sharks will grow at a rate of about 13cm (5 inches) and 2.3kg (5 pounds) per year until they reach maturity, after which time their growth rates decrease. Juvenile nurse sharks up to 60cm (23 inches) have small black spots encircled by lighter pigmentation covering their bodies. Larger nurse sharks range in colour from a light yellowish tan to dark brown, though brilliant yellow and milky white specimens have been reported several times.
Nurse sharks are commonly found in the Atlantic and eastern Pacific Oceans, particularly in shallow tropical and subtropical waters between 44°N and 35°S latitude. Despite a preference for warmer waters, nurse sharks have been spotted as far north as Rhode Island and as far south as southern Brazil. Nurse sharks have also been spotted in France's Gulf of Gascogne, though it's thought they were introduced to this habitat by humans. G. cirratum hasn't been found in Indo-Pacific waters, though related species have successfully evolved in that area.
Nurse sharks are nocturnal and usually rest on the sandy sea floor or in caves and crevices during the day. Nurse sharks have occasionally been observed resting in groups as large as 40 sharks, all lying close together (sometimes atop one another). When active, the sharks swim along on or very close to the sea floor, using their pectoral fins as limbs to sort-of crawl along. Nurse sharks up to 170cm (6 feet) are usually found in shallow coral reefs, grass flats, mangrove islands, limestone solution holes, and under rock ledges in 1 to 4m (3 to 13 foot) deep water. Larger juvenile and adult nurse sharks are typically found in deeper reefs and rocky regions at depths between 3 and 75m (10 and 250 feet) during the day but move to waters less than 20m deep during the night. Nurse sharks don't migrate and have shown a strong preference for certain resting sites, repeatedly returning to the same areas during the day.
Nurse sharks have no regular predators, though a nurse shark can be an occasional meal for a larger species of shark. There isn't a very significant nurse shark market. Nurse sharks are edible but typically only sold as bait for crabs (and even this isn't common). In the past, nurse shark liver oil was used as fuel and to calm the water's surface for sponge fishing, while nurse shark hide was used to make high-quality leather. Some fishermen are known to kill any nurse sharks they catch because the sharks are considered a nuisance, sometimes eating the bait intended for other fish. Commercial fisheries in the United States routinely release them alive though if they catch any. Nurse sharks are non-aggressive fish, usually swimming away if approached by a human, though if provoked (either deliberately or by accidentally being stepped on) will likely bite. Nurse sharks have a powerful bite. There are some cases of nurse shark bites reported where surgical instruments were required to break the creatures hold on their victims.