Nurse shark
Ginglymostoma cirratum
FAMILY
Ginglymostomatidae
TAXONOMY
Ginglymostoma cirratum Bonaterre, 1788, type locality not specified.
OTHER COMMON NAMES
None known.
PHYSICAL CHARACTERISTICS
Recognized by its conspicuous, long nasal barbels on the anterior margins of the nostrils and first dorsal fin originating over or posterior to the pelvic fin origin. Very wide head that gives it a tadpole appearance from above. Mouth is full of minute teeth, which are similar in both jaws. Teeth have one large cusp flanked on each side by two or three cusplets. Coloration ranges from rich yellowish to grayish-brown, with most specimens being reddish-brown. Yellow and even white specimens have been reported. Newborn nurse sharks have small black spots over the entire body, with an area of lighter pigmentation surrounding each spot and with bands of lighter and darker pigmentation alternating along the dorsal surfaces. The spots disappear by the time the specimens reach 20 in (50 cm) in length. Capable of limited color changes according to light intensity. Although the size and weight attained by the nurse shark often have been exaggerated in both the scientific and popular literatures, no specimen measured by researchers has exceeded 110 in (280 cm) in length. The largest specimen measured and weighed by Castro in Florida was 106 in (268 cm) long and weighed 243 lb (110 kg). The sizes of 132–144 in (335–365 cm) found in the literature must be considered exaggerations caused by Fowler's inaccurate estimates.
DISTRIBUTION
Distributed widely in littoral waters on both sides of the tropical and subtropical Atlantic. Ranges from tropical West Africa to the Cape Verde islands in the east and from southern Brazil to North Carolina and Rhode Island in the west. Also found on the western coast of America from the Gulf of California to Panama and Ecuador. Abundant in the shallow waters of tropical Florida and the Caribbean.
HABITAT
Small juveniles are found in shallow coral reefs and grass flats. Juveniles ranging in size from 47 to 67 in (1,200–1,700 mm) are found around shallow reefs and mangrove islands. Larger juveniles and adults are found near reefs and rocky areas at depths of 66–75 ft (20–75 m) during the daytime and in much shallower areas at night.
BEHAVIOR
Can be found resting on the bottom in small groups during the daytime, concealed under ledges or among boulders and rocks. These sharks often are in very close proximity to and
sometimes almost on top of one another. At dusk they disperse to search for food.
FEEDING ECOLOGY AND DIET
Said to feed chiefly on invertebrates (squids, shrimps, crabs, spiny lobsters, and sea urchins) and small fishes. A recent study of Florida nurse sharks showed that it is an opportunistic predator that consumes a wide range of species of small bony fishes, 3–9 in (8–23 cm) in length, primarily grunts. Prey typically is captured through a powerful sucking action. This suction accounts for the coral debris and solitary corals occasionally encountered in the stomach contents. Castro's study did not support the common belief that the nurse shark preys heavily on spiny lobsters and other crustaceans.
REPRODUCTIVE BIOLOGY
The nurse shark is a viviparous species without a placenta. In Florida and the Bahamas the mating period apparently extends from the last week in May to July, but most observations of mating have been made in mid-June. The embryos are enclosed in horny egg capsules for the first weeks of gestation. Embryos hatch out of the egg cases when they reach 8.9–9.2 in (21.8–23.3 cm). The embryos are lecithotrophic, that is they are fed from yolk stored in a yolk sac, and there is no evidence of any other mode of embryonic nourishment.
The embryos are in different developmental stages through the first four months of gestation. In females examined in October, some embryos measuring 8.5–9.2 in (21.5–23.3 cm) were still inside the egg cases, while others measuring 10.6–10.9 in (27–27.8 cm) had fully absorbed yolk sacs, open yolk sac scars and appeared ready for birth. During the last month of gestation all the embryos were in the same stage of development, that is, they all had absorbed the yolk sac and apparently were ready for birth. These embryos at different stages may have been a result of ovulation of the very large oocytes (2.3–2.4 in, or 5.9–6 cm, in diameter) and encapsulation of the eggs, lasting for two or three weeks, and of the very rapid development of the embryo thereafter. Females expel the empty egg cases after the embryos have hatched out of them.
The embryos measure 11.4–12 in (29–30.5 cm) at birth, after a gestation period estimated at about five to six months. Brood sizes are large, ranging from 21 to 50 young, with a median of 32. Aquarium observations on parturition suggest that the young are released over a period of a few days, usually at night. The reproductive cycle of the nurse shark encompasses a five- to six-month gestation period and a biennial reproductive cycle. After the gestation period of five to six months and birth in late November or early December, a female does not mate again until 18 months later, in June; thus reproduction is biennial.
CONSERVATION STATUS
Not listed by the IUCN.
SIGNIFICANCE TO HUMANS
The greatest value of the nurse shark probably lies in ecotourism. The nurse shark is the species of shark most often seen by recreational divers in Florida and the Caribbean. Because it is a large but harmless species, the nurse shark thrills divers that see one unexpectedly or at close range. In the last decade, numerous shark-watching operations have emerged in Florida and the Bahamas. In some locations nurse sharks have become habituated to being fed by divers. Although the long-term consequences and risks of these operations are still unclear, one can hope that public awareness and concern may result in some form of protection for these interesting animals. This species is one of the most common sharks in aquaria, because of its hardiness and its ability to survive in confinement for many years. According to Clark, one specimen survived 25 years at the Shedd Aquarium in Chicago, and another lived for 24 years at the Government Aquarium in Bermuda. The nurse shark is one of the most important species in shark research.
