Intertidal Zone: Life Between the Tides
Let’s just come right out and say it: it’s amazing what males will endure for an opportunity to mate, and male Nurse Sharks (Ginglymostoma cirratum) are no exception.
Just the Facts:
Habitat: Intertidal, Estuaries, Sandy Plains, Rocky Reefs, Coral Reefs
Depth: Intertidal to 165 ft (50 m)
Distribution: Tropical Eastern Pacific, Chilean, Western North Atlantic, Caribbean, Amazonian, Argentinean, Eastern North Atlantic/Mediterranean?, West African, Southern African?
Nurse Sharks are among the most commonly encountered of sharks. During daylight hours in Florida and the Caribbean, divers and snorkellers frequently discover one or more Nurse Sharks lying quietly on the bottom, often with their heads tucked under a coral ledge. Highly social, Nurse Sharks often rest together during the day, packed close together like a cluster of pinkish-brown, fat-bellied pollywogs.
Due to their apparently docile nature, Nurse Sharks have gained a reputation for being ‘harmless’. They are not. If touched, grabbed, poked, or otherwise molested, Nurse Sharks often turn on their human tormentors with astonishing speed. Clamping on with their short, powerful jaws and multi-cusped teeth, Nurse Sharks typically roll over and over in the water — like a crocodile in a ‘death roll’ — causing nasty bruises and sometimes massive tissue damage. Rescuers often must carry the ‘victim’ out of the water with the shark still attached, the animal stubbornly refusing to let go until it suffocates or is killed. It is therefore always best to let a ‘sleeping’ Nurse Shark lie.
At night, Nurse Sharks become quite active, demonstrating themselves to be very capable and formidable predators. These sharks are powerful suction feeders, using their small mouths and thick throat and gill muscles to create a partial pharyngeal vacuum. Nurse Sharks employ several remarkable behaviors to secure a meal. Juveniles of this species often lie propped up on their pectoral fins, which are curled under the chest to create a false cave that crabs seem to find almost irresistible. A duped crab barely has time to nestle into its new ‘cave’ before the duplicitous Nurse Shark performs a comical little ‘push up’, dipping its head and hoovering up the hapless crab by pharyngeal suction. Adults of this species typically employ more brutal feeding methods. Mature Nurse Sharks have been observed using bottom topography to break apart the articulated external skeleton of spiny lobsters and flipping over conchs (a type of large marine snail), extracting the soft mollusc from its heavy shell through a combination of biting and suction. Nurse Sharks have even been observed using their heads to dig under coral rubble in search of concealed prey.
Tagging studies in the Florida Keys have indicated that Nurse Sharks have very fixed home ranges, with some individuals known to have remained in the same area for at least four years. This species is most commonly encountered in waters less than 20 feet (6 metres) deep, but rarely enters intertidal areas except to breed.
Only very recently have Nurse Shark courtship and mating been studied in the wild. In the Florida Keys from mid-June to early July, large groups of Nurse Sharks enter extremely shallow intertidal areas. The water here is so shallow, the sharks’ backs are often out of the water and there is barely room to swim. However, the milling Nurse Sharks are so completely single-minded about their procreative purpose, they seem oblivious to virtually everything, allowing scientists and film-makers to enter the water to study and document Nurse Shark nuptials in intimate detail.
Four distinct stages of Nurse Shark mating behavior have been identified. During the first stage, one or more male Nurse Sharks approach a female resting on the bottom or — if she is actively swimming — follow close behind the female at about the level of her pectoral fins. Coupling begins with a male Nurse Shark grasping one of the female’s pectoral fins between his teeth, sometimes taking the entire fin into his mouth. On several occasions, up to four males have been observed attempting to secure a grasp on a single female and sometimes a ‘popular’ female has a male attached to each of her pectoral fins. The next stage begins with the male Nurse Shark forcefully rolling the female onto her back, the grasped pectoral fin twisting in his mouth so that the male remains upright. At this stage of courtship, the female Nurse Shark often actively resists being rolled. To overcome her recalcitrance, a courting male Nurse Shark may require the assistance of up to three other males. Positioning continues with the male Nurse Shark rolling to bring his belly alongside that of the female so that his claspers can be aligned with her vent. The final stage, begins with the male Nurse Shark flexing his far-side clasper across the mid-line of his body and inserting it into the vent of the now co-operative and quiescent female; during copulation, the male thrusts vigorously while wrapping his tail around the female’s body. Nurse Shark copulation concludes with both sharks co-operating to slowly withdraw the clasper, the male releases the female’s pectoral fin, and the sharks part company.
After copulation, the male Nurse Shark often lies on the bottom actively pumping water over his gills. He seems ‘breathless’, apparently exhausted by the exertions of mating. This is almost certainly due to the fact that throughout most of the courtship and mating — a period of 15 minutes or more — the male’s mouth is effectively plugged by the pectoral fin of his mate, preventing oxygen-bearing water from flushing over his gills. Combined with the female’s initial resistance to mating and the typically low dissolved oxygen content of warm intertidal waters, this suggests a possible evolutionary explanation for why Nurse Shark mating takes place here.
Mating is tough on male Nurse Sharks. Fewer than 10% of all male Nurse Sharks’ mating attempts result in successful copulation, and the shallower the water the poorer a male’s chances of copulatory success. Males must therefore bodily drag a female into deeper water for successful copulation to occur. To make matters even more difficult, mature female Nurse Sharks are typically 12 to 15% longer and up to 60% heavier than males. A female Nurse Shark can thus test a male’s stamina to assess the ‘quality’ of prospective mates: those vigorous males which can manage to drag her to deeper water while enduring prolonged oxygen starvation may have the ‘best’ genes to pass on to her offspring.
In November or early December, Nurse Shark pups are born in the same shallow waters where mating took place less than half a year before. Each newborn pup is a perfect miniature of its mother, but is colored soft pinkish-orange and peppered with small brownish spots. After giving birth, a mother Nurse Shark will not mate again for about 18 months, re-building her energy stores to tempt and test the next crop of lusty males.