The nurse shark (Ginglymostoma cirratum) is common in the warm, shallow waters of the Atlantic and Eastern Pacific Oceans. This slow-moving, bottom-dwelling species is often found resting on the seafloor or in coral crevices during the day. Widely recognized for its docile nature, the nurse shark’s reproductive method is a point of public curiosity. The question of whether this familiar shark lays eggs or gives live birth touches on the diverse reproductive biology found across all shark species.
How Sharks Reproduce
Sharks, skates, and rays belong to the class Chondrichthyes, and they exhibit internal fertilization, unlike most bony fish. The method of embryonic development and birth varies significantly among the different groups. Scientists categorize these reproductive strategies into three main types based on how the young are nourished and delivered.
The first method is oviparity, where the female deposits fertilized eggs encased in a tough, leathery shell known as a “mermaid’s purse.” These egg cases are typically secured to the seabed or seaweed, and the embryo develops externally, solely nourished by a yolk sac, as seen in species like the horn shark.
Conversely, viviparity involves true live birth, where the young develop inside the mother with a placental connection, similar to mammals. This advanced method provides continuous nourishment and oxygen, resulting in fully developed, independent pups at birth, a strategy used by hammerheads and bull sharks.
The third category is ovoviviparity, which represents a middle ground between the other two. The female produces eggs, but they are retained internally within the oviducts, where they hatch. The developing pups are sustained entirely by their yolk sac, without a direct placental link to the mother. Once the pups are fully developed, the mother gives birth to live, free-swimming young.
Nurse Shark Reproduction Explained
Nurse sharks belong to the ovoviviparous group, meaning they give birth to live young after an internal gestation period. The fertilized eggs develop within a thin egg case inside the mother’s oviduct, but they are never laid externally. The embryo’s initial nourishment comes from a large yolk sac, which is absorbed throughout the development process.
The gestation period for nurse sharks lasts approximately five to six months. Females follow a biennial reproductive cycle, breeding only once every two years. A single litter can be quite large, with females giving birth to anywhere from 20 to 40 pups, though some studies report up to 50.
Survival of the Pups
Newborn nurse shark pups are independent and are born at a length of about 27 to 30 centimeters. They are fully formed miniature versions of the adults and must immediately fend for themselves without parental care. The coloration of juveniles is distinct from the adults, as they display a pattern of small black spots that are lost as the shark matures.
These small pups seek out protected nursery habitats to increase their chances of survival. They are frequently found in very shallow waters, such as around mangrove islands, seagrass flats, and coral reefs, where they find cover from larger predators. Their initial diet consists of small invertebrates and bony fish found on the ocean floor, which they vacuum up using their strong suction feeding mechanism. The rapid growth rate during their first year highlights their immediate need for a robust food supply in these sheltered environments.