Featured Elasmobranch – Nurse Shark

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Featured Elasmobranch – Nurse Shark




Identification: Characteristic of sharks within this family, the nurse shark has nasal barbells that extend to the mouth, nasoral grooves, a small mouth positioned in front of the eyes, and the 4th and 5th gill-slits very close together. The nurse shark has rounded pectoral and dorsal fins, and the second dorsal fin origin occurs in front of the anal fin origin. It has a long asymmetrical tail and is tan to dark brown in color. The teeth bear three or more cusps, with the central point longest, and are used in two to three series simultaneously.

Size: Maximum reported length is 4.3 m, with a weight of 150 kg, but most adults are less than 3 m total length.

Distribution: A wide ranging species nurse sharks are distributed across the tropical and subtropical waters of the Pacific and Atlantic Oceans. The eastern Pacific Ocean distribution ranges from Baja California to Peru. In the Atlantic Ocean it ranges from Cameroon to Gabon in the eastern Atlantic and from Rhode Island to Brazil in the western Atlantic, including the islands of the Caribbean Sea.

Habitat: An inshore, bottom-dwelling shark that inhabits the continental and insular shelves. It is frequently observed in waters from <1 to 12 m depth, though it has been observed to 130 m. Nurse sharks prefers patch reefs, reef faces, mangrove swamps, and sandflats.

Biology: Reproductive mode is aplacental viviparity with litters of between 21-28 pups. Fully developed, 30 cm-long, neonates are born following a six month gestation period. The juveniles have black-spotted coloration patterns that fade with age. Mating is biennial, as it takes18 months for female ovaries to produce another batch of eggs. Females mature at about 2.3-2.4 m total length at an age of 15 to 20 years, and males at about 2.1 m and an age of 10 to 15 years. The growth rate is about 10 cm and 2.7 kg per year.

Mating season occurs from late June to late July during daylight hours. These sharks form breeding assemblages in shallow waters, allowing for thorough documentation of the mating process. Courtship and copulation begins with females and males swimming along side one another. As the male lags behind, he bites the posterior edge of the female’s pectoral fin. Consequently the female pivots perpendicular to the male and rolls on her back. The male then lifts and pushes the female with his rostrum so the two bodies are parallel. The male moves on top of the female, inserts his claspers, rolls onto his back, remains in the position for 2 minutes, after which they both swim away. Sometimes a third individual is involved in the initial courting. Mating occurs either at the water surface, resulting in caudal fin slapping of the surface, or on the seafloor.

Nurse sharks feed on a wide range of fish and invertebrates, including crustaceans, mollusks, tunicates, bivalves, sea urchins, teleosts, stingrays, octopus, squid, and cuttlefish. Their small mouth somewhat limits the prey size that can be swallowed whole, but a large throat (= buccal) cavity allows these sharks to suck in prey items larger than their jaw openings (which is the origin of their name). The nasal barbels located near the front of the mouth are covered with taste receptors to help them locate food; additionally the powerful jaws aid in crushing shells of crustaceans and mollusks. Nurse sharks hunt nocturnally and prey predominately upon dormant fish that cannot swim away. These sharks have reportedly been observed grazing on algae and coral.

General interest: A nocturnal species, these sharks often spend the day in large inactive groups of up to 40 individuals resting at the seafloor. Juveniles are often observed resting on their pectoral fins, rolled underneath their bodies, to mimic a home for shelter-seeking fish and shrimp. The juveniles then consume the attracted prey items. These sharks have fixed home ranges; tagged individuals have been recaptured in the same area after 4 years at liberty. Nurse sharks breathe by actively pumping water over their gills by opening and closing their mouth. This enables them to breathe while resting on the seafloor.

Although sluggish during the day, this is a well-adapted shark. The dilation of pupils takes 24-30 seconds (when light intensity is low), and constriction only takes 5-13 seconds (when light intensity is high), allowing them to be keen predators at night. Two individuals kept at the Naval Ocean Center, San Diego CA, learned to work rings over their heads and swim to a trainer to receive food, similar to dolphins or sea lions. Due to the large abundance and easy ability of nurse sharks to adjust well to captivity, this species is frequently displayed in public aquariums. The longest living specimen was in captivity for 25 years. Although not fished in targeted fisheries its tough skin is prized for leather.

The nurse shark is considered a docile species, although their have been a small number of unprovoked, non-fatal, attacks on divers and swimmers. Most provoked incidents with this shark occur when people attempt to ride, spear, grab, or otherwise harass these sharks.
By Kristin Hunter-Thomson
Moss Landing Marine Laboratories
8272 Moss Landing Road, Moss Landing, CA 95039


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