| Nurse shark | |
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Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Orectolobiformes |
| Family: | Ginglymostomatidae |
| Genus: | Ginglymostoma |
| Species: | G. cirratum |
| Binomial name | |
| Ginglymostoma cirratum (Bonnaterre, 1788) | |
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| Range in blue | |
Thenurse shark (Ginglymostoma cirratum) is anelasmobranch fish in the familyGinglymostomatidae. Theconservation status of the nurse shark is globally assessed asVulnerable in theIUCN List of Threatened Species.[2] They are considered to be a species of least concern in the United States and inThe Bahamas, but considered to be near threatened in the western Atlantic Ocean because of their vulnerable status in South America and reported threats throughout many areas of Central America and the Caribbean.[2] They are directly targeted in some fisheries and considered by-catch in others.
Nurse sharks are an important species for shark research.[3] They are robust and able to tolerate capture, handling, and tagging extremely well.[4] As inoffensive as nurse sharks may appear, they are ranked fourth in documented shark bites on humans,[5] likely due to incautious behavior by divers on account of the nurse shark's calm, sedentary nature.
The nurse shark genusGinglymostoma is derived fromGreek language meaning hinged mouth, whereas the speciescirratum is derived fromLatin meaning having curled ringlets. Based onmorphological similarities,Ginglymostoma is believed to be the sister genus ofNebrius, with both being placed in aclade that also include speciesPseudoginglymostoma brevicaudatum,Rhincodon typus, andStegostoma fasciatum.[6]
The name "nurse" may have originated from antiquated spelling conventions. TheOxford English Dictionary notes that, inmedieval times, the "n" of the word "an" was frequentlytransferred to a following word that began with a vowel. Huss, husse and hurse were antiquated names fordogfish and other sharks. Nurse survives and so does huss.[7]
The nurse shark has two rounded dorsal fins, rounded pectoral fins, an elongated caudal fin, and a broad head.[8] Maximum adult length is currently documented as 3.08 m (10 ft1+1⁄2 in), whereas past reports of 4.5 m (15 ft) and corresponding weights of up to 330 kg (730 lb) are likely to have been exaggerated.[2] Adult nurse sharks are brownish in color. Newly born nurse sharks have a spotted coloration which fades with age and are about 30 cm in length when nascent.
The nurse shark has a wide but patchy geographical distribution along tropical and subtropical coastal waters of the Eastern Atlantic, Western Atlantic, and Eastern Pacific.[9] In the Eastern Atlantic it ranges fromCape Verde toGabon (accidental north to France).[2] In the Western Atlantic, including the Caribbean, it ranges fromRhode Island to southernBrazil,[10] and in the East Pacific fromBaja California toPeru.[2]
Nurse sharks are a typically inshore bottom-dwelling species. Juveniles are mostly found on the bottom of shallow coral reefs, seagrass flats, and around mangrove islands, whereas older individuals typically reside in and around deeper reefs and rocky areas, where they tend to seek shelter in crevices and under ledges during the day and leave their shelter at night to feed on the seabed in shallower areas.[11]
Nurse sharks are also subject topiebaldism, a genetic condition that results in a partial lack of body pigmentation and a speckled body.[12]
Nurse sharks are opportunistic predators that feed primarily on small fish (e.g.stingrays,teleosts) and some invertebrates (e.g.crustaceans,molluscs,sea urchins,tunicates,anthozoans), as well asalgae.[11][13] They are typically solitarynocturnal animals, rifling through bottom sediments in search of food at night, but are often gregarious during the day forming large sedentary groups. Nurse sharks areobligate suction feeders capable of generatingsuction forces that are among the highest recorded for any aquatic vertebrate to date.[14][15] Although their small mouths may limit the size of prey, they can exhibit a suck-and-spit behavior and/or shake their head violently to reduce the size of food items.[16]
Nurse sharks are exceptionallysedentary unlike most other shark species.[17] Nurse sharks show strongsite fidelity (typical of reef sharks), and it is one of the few shark species known to exhibitmating site fidelity,[18] as they will return to the same breeding grounds time and time again.
American alligators (Alligator mississippiensis) andAmerican crocodiles (Crocodylus acutus) may occasionally prey on nurse sharks in some coastal habitats. Photographic evidence and historical accounts suggest that encounters between species are commonplace in their shared habitats.[19][20]
Nurse sharks areovoviviparous, with fertilized eggs hatching inside the female. The mating cycle of nurse sharks is biennial, with females taking up to 18 months to produce a new batch of eggs. The mating season runs from late June to the end of July, with agestation period of six months and a typical litter of 21–29 pups.[10] The young nurse sharks are born fully developed at about 30 cm long.
Nurse sharks engage in multiple paternity during mating season. A study conducted over a ten-year span found that a brood of nurse sharks had more genotypes than broods with one father. Fourteen separate genotypes were found in the brood examined, which suggests that more than one father fertilized the mother's eggs. Engagement in multiple paternity promotes genetic variation.[21]
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