Fish Facts » Nurse Shark
Nurse sharks have two spineless, rounded dorsal fins with the first dorsal fin much larger than second, and one anal fin. The origin of the first dorsal fin is about over the origin of the pelvic fin. The caudal fin is more than ? of the total animal length. The sub-terminal mouth is placed well in front of the eyes, the spiracles are minute, and moderately long barbels reach the mouth. Nasoral grooves are present, but there is no perinasal groove. Adult nurse sharks generally range from light yellowish tan to dark brown in color. Juveniles up to 60 cm (23 in) have small black spots, with an area of lighter pigmentation surrounding each spot, covering the entire body. These are bands of lighter and darker pigmentation along the dorsal surface. Juveniles (70-120 cm / 28-48 in) are capable of limited color changes. In a tank experiment small nurse sharks, covered for just a few minutes became considerably lighter than individuals exposed to full sunlight. Unusually pigmented individuals (e.g. brilliant yellow or milky white) have been reported several times.
The nurse shark is a nocturnal animal that rests on sandy bottoms or in caves or crevices in rock in shallow waters during the day. They occasionally occur in groups of up to 40 individual, as they lie very close together sometimes even piling upon one another. Nurse sharks are very active during the night. In addition to swimming near the bottom or well off it, the nurse shark can clamber on the sea floor, using its flexible, muscular pectoral fins as limbs. Large juveniles and adults are usually found around deeper reefs and rocky areas at depths of 3-75 meters (10-250 ft) during the daytime and migrate into shallower waters of less than 20 meters (70 ft) deep after darkness. Juveniles up to 170 cm (6 ft) are generally found around shallow coral reefs, grass flats or mangrove islands in 1-4 meters (3-13 ft) of water. They often lie in groups within limestone solution holes or under rock ledges. Nurse sharks show a strong preference for certain resting sites, and repeatedly return to the same caves and crevices after a nocturnal activity.
A nocturnal predator, the nurse shark feeds mainly on fish especially stingrays, molluscs (octopi, squids and clams) and crustaceans. Algae and corals are occasionally founded in the stomachs as well. The nurse shark has small mouth, but its large, bellows-like pharynx allows it to suck in food items at high speed. This system probably allows the species to prey on small fish that are resting at night, species that are too active for the sluggish nurse shark to catch during the day. Heavy-shelled conches are flipped over, and the snail extracted by use of suction and teeth. Young nurse sharks have been observed resting with their snouts pointed upwards and their bodies supported off the bottom on their pectoral fins. Some suggest this posture may possibly provide a false shelter for crabs and small fishes that the shark ambushes and eats.
At present there is not a fishery for this species. The fins are not marketed and the meat, although edible, is not often retained for human consumption. However it is sometimes sold as crab bait. Nevertheless, nurse shark are caught and killed by fishermen in some regions because they are considered a nuisance animal that takes bait intended for other species. In the Lesser Antilles, where it often raids fish traps, it is considered a pest. Commercial fishers in the United States routinely release nurse sharks alive.
Averaging 220-270 cm (7.5-9 ft) in total length and weighing 75-105 kg (167-233 lbs), adult females reach a larger size than adult males (210-260 cm / 7-8.5 ft ; 90-120 kg / 200-267 lbs). Size at maturity is also larger for females, about 225 cm (7.5 ft) in females vs 210 cm (7 ft) for males. Size at birth is in the 28-30 cm (11-12 in) range, with growth rates for juveniles of about 13 cm (5 in) and 2.3 kg (5 lbs) per year. Once maturity is reached, growth rates are usually much lower.
Common in the Atlantic and in the eastern Pacific, in coastal tropical and sub-tropical waters. Reported from Senegal to Gabon, Rhode Island to Southern Brazil, and Mexico to Peru. Also, some individuals have been reported in the Gulf of Gascogne in southwest France. This species is locally very common in shallow waters throughout the West Indies, south Florida and the Florida Keys. Apart of the eastern Pacific, the nurse shark is absent from the Indo-Pacific area, where other related groups have successfully evolved.
Florida Museum of Natural History
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