Nurse sharks
Ginglymostomatidae
The nurse shark (Ginglymostoma cirratum) is a species of elasmobranch fish belonging to the family Ginglymostomatidae. According to the IUCN List of Threatened Species, the nurse shark is globally categorized as Vulnerable. While it is considered to be a species of least concern in the 🇺🇸 United States and The 🇧🇸 Bahamas, it is classified as near threatened in the western Atlantic Ocean due to its vulnerable status in South America and reported threats in Central America and the Caribbean. The nurse shark is directly targeted in some fisheries and is considered by-catch in others.
Nurse sharks play a significant role in shark research due to their robust nature and ability to tolerate capture, handling, and tagging. Despite their seemingly harmless demeanor, they rank fourth in documented shark bite incidents involving humans. This is likely due to the incautious behavior of divers, who may underestimate the nurse shark's calm and sedentary nature.
Characterized by two rounded dorsal fins, rounded pectoral fins, an elongated caudal fin, and a broad head, adult nurse sharks have a maximum documented length of 3.08 m (10 ft 1+1⁄2 in). Past reports of larger sizes, up to 4.5 m (15 ft) in length and weighing up to 330 kg (730 lb), are likely exaggerations. Adult nurse sharks have a brownish coloration, while newborns display a spotted pattern that fades with age. Newborns are approximately 30 cm in length.
The nurse shark has a patchy geographic distribution along tropical and subtropical coastal waters of the Eastern Atlantic, Western Atlantic, and Eastern Pacific. In the Eastern Atlantic, it ranges from Cape Verde to Gabon, with some accidental sightings as far north as France. In the Western Atlantic, including the Caribbean, its range extends from Rhode Island to southern 🇧🇷 Brazil. In the East Pacific, it can be found from Baja California to 🇵🇪 Peru. Nurse sharks primarily inhabit inshore bottom areas, with juveniles typically found in shallow coral reefs, seagrass flats, and around mangrove islands, while older individuals reside in and around deeper reefs and rocky areas. They seek shelter in crevices and under ledges during the day and venture out at night to feed on the seabed in shallower areas. Nurse sharks can display piebaldism, a genetic condition resulting in partial pigmentation loss and a speckled appearance.
As opportunistic predators, nurse sharks primarily feed on small fish (such as stingrays) and some invertebrates (including crustaceans, molluscs, and tunicates). They are solitary nocturnal animals, searching for food by sifting through bottom sediments at night. However, during the day, they can form large sedentary groups. Nurse sharks are characterized by their ability to generate high suction forces, allowing them to be obligate suction feeders. While their small mouths limit the size of prey, they can employ suck-and-spit behavior or shake their heads vigorously to reduce the size of food items. Unlike most shark species, nurse sharks are exceptionally sedentary. They exhibit strong site fidelity, returning to the same breeding grounds and showing mating site fidelity.
In certain coastal habitats, nurse sharks may occasionally fall prey to American alligators (Alligator mississippiensis) and American crocodiles. Photographic evidence and historical accounts suggest that encounters between these species are common in their shared habitats.
Nurse sharks are ovoviviparous, with fertilized eggs developing 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 occurs from late June to the end of July, and the gestation period lasts for six months. A typical litter consists of 21-29 fully developed pups measuring approximately 30 cm in length.
During the mating season, nurse sharks engage in multiple paternity, as observed in a ten-year study. The study found that broods of nurse sharks had more genotypes when compared to broods with only one father. Analysis of a examined brood revealed 14 separate genotypes, implying that multiple fathers fertilized the mother's eggs. This behavior promotes genetic variation within the population.
