13 Facts About Nurse Sharks
Known as the “couch potato of the shark world,” the nurse shark (Ginglymostoma cirratum) leads a sedentary life. By day, it rests, and by night, it creeps over the sandy floors and coral reefs of its shallow-water habitat, slurping up little animals along the way.
But though it's not a fast or aggressive fish, you should give it plenty of space: People who act carelessly around nurse sharks risk serious injuries. Here are 13 things that every ocean-lover ought to know about the nurse shark.
1. IT USES A METHOD CALLED BUCCAL PUMPING TO BREATHE.
For certain sharks, lying on the ocean floor is an impossibility. Species like the great white and the whale shark breathe by swimming nonstop; as they travel around, water is constantly flowing into their open mouths and across their gills, supplying oxygen en route. If the fish stop moving for too long, that flow ceases and they die. But other species are perfectly capable of breathing while sitting still—including the nurse shark. By using oral muscles to actively suck water into the mouth—what's called buccal pumping—it can supply oxygen to the gills without needing to swim anywhere.
2. THEY CAN "WALK" ACROSS THE OCEAN FLOOR.
Wild nurse sharks are usually found in shallow, coastal waters. The fish are nocturnal predators who tend to hunt within 65 feet of the ocean’s surface (although adults sometimes rest in deeper waters during the daylight hours). They spend their lives around coral reefs and coastal shelves, and most of their hunting takes place right on the ocean floor, where these slow-moving carnivores look for prey in or near the sand. Instead of swimming, they sometimes use their pectoral fins to “walk” across the bottom.
3. THE TWO LITTLE KNOBS ON THEIR FACES ARE CALLED “BARBELS.”
4. THEY SUCK UP THEIR FOOD.
Nurse sharks eat a variety of sea life, from conchs, squid, and sea urchins to bony fish. A cavity within the throat generates a powerful suction which vacuums hapless animals up into the nurse shark’s mouth, where rows of tiny, backward-curving teeth crush up the food.
The mouth works like a dental conveyor belt; new rows of teeth pop up towards the back and gradually push older ones forward until they fall out. How long an individual row lasts depends on the season. During the winter, a nurse shark will acquire a fresh row of teeth every 50 to 70 days. But in the summer, tooth row replacement occurs every 10 to 20 days.
5. THE SPECIES COMES IN A FEW DIFFERENT COLORS.
Full-grown nurse sharks are usually brown, but they can also be grey or yellowish. In 1992, a “milk white” individual with brown splotches was caught and photographed near Key Largo, Florida. The fish might have been piebald, which is a genetic condition that’s similar to albinism. Piebald animals have a combination of normally colored skin and patches of pigment-deficient white skin. Another mature nurse shark who fit this general description was filmed in 2014. Adult specimens don't normally have spots, but as juveniles, the fish are covered in little black dots that fade as they age.
6. IT LIKES TO UNWIND IN BIG GROUPS.
Shark snuggle parties are a thing. By day, the nocturnal nurse shark becomes inactive; for hours on end, it just lies around and pumps water over its gills. Crevices, ledges, and piles of boulders are popular downtime locations for this species. Although the sharks don't socialize on hunting trips, they often recline en masse. Nurse sharks are known to rest communally, with groups of two to 40 individuals piling up on top of each other.
7. ADULTS CAN BE OVER 10 FEET LONG.
The maximum reliably-measured length for this species is 10.1 feet. As far as weight goes, the heaviest adult ever reported to the International Game and Fish Association was a 263.8-pounder caught by two fishermen (a father and his 15-year-old son) in 2007. Day-old pups are 7.8 to 12 inches long—and a batch of premature nurse sharks who were measured by scientists after being born near-term weighed between 4.2 and 5.3 ounces apiece. Big things can start out small.
8. NOBODY KNOWS WHERE THE NAME “NURSE SHARK” CAME FROM.
It’s definitely not qualified to care for hospital patients, so why did people start calling this barbel-faced sea critter the “nurse” shark? That’s a linguistic mystery, but historians have their theories. Maybe the suction-based feeding methods reminded sailors of nursing infants. Alternatively, the nurse in nurse shark could be descended from “huss,” an archaic name given to an unrelated family of bottom-dwellers. (We now call them “catsharks.”) Over time, huss evolved into nuss, a word that came to mean “shark” or “large fish.” So perhaps the nurse shark moniker is based on a corruption of nuss.
9. THE WHALE SHARK IS A RELATIVE.
Approaching 40 feet in total length and weighing several tons, adult whale sharks are the biggest fish presently alive. Like the nurse shark, this species eats via suction, and that’s not where the resemblance stops. Whale and nurse sharks are both members of the order Orectolobiformes, a group of 39 shark species largely in temperate and tropical oceans. Also known as “carpet sharks,” they're characterized by having small mouths that—when viewed in profile—do not extend behind the eyes. All of these fish have two dorsal fins on their backs and five sets of gill slits. Species within this order tend to have striking patterns on their skins, with grown-up nurse sharks being an obvious exception. Barbels are another common feature.
The strangest member of Orectolobiformes might be the shaggy wobbegong sharks, who lie still on tropical sea beds and use brilliant camouflage to ambush unsuspecting fish from below.
10. NURSE SHARKS ARE PARTIALLY MIGRATORY.
Plenty of well-known sharks embark on huge migrations; hundreds of whale sharks from across the Atlantic visit Mexico’s Yucatan Peninsula every summer and Pacific great whites go on winter pilgrimages to a mysterious, mid-ocean site dubbed the “White Shark Café.” Nurse sharks are less prone to wanderlust; many remain in the same general area all year round.
But some of their peers might feel the travel bug from time to time. In January 2018, Environmental Biology of Fishes published a 23-year nurse shark tracking study. The scientists behind it looked at a wild population which uses the Dry Tortugas (part of the Florida Keys) as a mating ground. Altogether, they captured and recaptured 76 adult nurse sharks. Tagging revealed that some of these fish clung to the Dry Tortugas and neighboring islands throughout the year. However, others were venturing as far north as the Tampa Bay area in between mating seasons, making the shark “partially migratory.” That means some individuals within this species migrate, but others don't.
11. FEMALES DON’T GIVE BIRTH EVERY YEAR.
The nurse shark mating season lasts from May to July, during which females will mate with multiple males. Sometimes two, three, or more males will attempt to mate with the same female simultaneously, resulting in violent shoving matches.
Nurse sharks have a five- or six-month gestation period and give birth to litters of 20 to 40 live young. A single batch of newborn pups may include the offspring of up to six different fathers. After she’s given birth, a mother nurse shark won’t mate again for another 18 months.
12. BE WARNED: IT CAN DOLE OUT PAINFUL BITES!
Underestimate this animal at your own risk. Because nurse sharks are sluggish by nature, commonly kept in aquariums, and don't possess large teeth, a lot of people who swim or dive in their natural habitat assume that the fish aren’t dangerous. But these predators can crush clams between their teeth and generate enough suction to rip a full-grown conch right out of its shell—so you don’t want one latching onto your arm.
But that’s just what happened to a swimmer in Boca Raton, Florida, in 2016. The 23-year-old female victim had been out snorkeling with friends when a 2-foot-long nurse shark clamped down on her right arm. (Eyewitnesses reported that another group of beachgoers had been harassing it.) The shark remained there while the snorkeler was driven to a nearby hospital. She survived, but the shark died before the medical team showed up. In another 2018 incident, an Instagram model was bitten while posing in some nurse shark-inhabited shallows.
Nurse shark attacks are uncommon, but they’re certainly not unheard-of—and humans are usually to blame. YouTube is loaded with videos of scuba divers hugging, grabbing, or stroking wild nurse sharks. Docile and shy as nurse sharks are, they may bite when provoked—or if they mistake an arm or finger for food.
“People are playing with fire,” George Burgess, the longtime director of the International Shark Attack File database, told the Palm Beach Post. In an interview with Newsweek, Burgess said that “A nurse shark bite is one of the worst, because their teeth are like cheese graters on each side. When they get onto a human being, it’s like a vacuum cleaner … They leave a concave hole where they’ve turned flesh into hamburger.”
13. A NEW SPECIES OF NURSE SHARK WAS RECENTLY NAMED.
Ginglymostoma cirratum lives in the Caribbean, off the northeastern coast of South America, near Spain, along western Africa, and by the eastern U.S. seaboard. A 2012 study found that a population living in the tropical eastern Pacific was genetically and anatomically different enough from Atlantic nurse sharks to constitute its own species. Named Ginglymostoma unami, or the Pacific nurse shark, it has a couple of noticeable traits that set it apart from G. cirratum. For example, the newly named fish’s second dorsal fin lies closer to the tail [PDF]. The two species may have diverged from one another when tectonic plates collided about 3 million years ago, isolating the ancestral nurse shark populations on either side of the Panamanian land bridge.