How Nurse Sharks Work

Any shark looks huge when you're not expecting to meet one, even the more modestly sized nurse shark. While some claim to have seen nurse sharks up to 14 feet (4.3 meters) long, marine biologists who have actually put a measuring tape against the species cite more conservative lengths [source: Castro]. What scientists do agree on is that the female is always slightly larger than the male of the species. A more realistic estimate puts female nurse sharks anywhere from just over 7 feet to just under 9 feet (2.2 to 2.7 meters) [sources: Guarracino, National Aquarium]. The males are generally about one-third of a foot shorter [source: Guarracino].

Males, however, weigh a bit more than female nurse sharks, with males tipping the scales at 200 to 267 pounds (90 to 120 kilograms) and females weighing in anywhere from 167 to 233 pounds (75 to 105 kilograms) [source: Guarracino].

The nurse shark can be many different colors, from yellowish tan to dark brown, although most fall closer to the dark brown side of the spectrum. Juvenile nurse sharks sometimes have additional pigmentation in the form of small black spots and bands.

Let's take a look at the nurse shark's fins. Instead of the pointy dorsal fins that we might see in movies when sharks swim near their prey, nurse sharks have rounded dorsal fins on their back, the first one bigger than the second. Nurse sharks have extremely long caudal fins, or tails; the caudal fin makes up more than 25 percent of the shark's entire length [source: Guarracino]. The bottom fin, the pectoral fin, helps the nurse shark to hover, and sometimes even to "walk" along the ocean floor. This hovering motion sometimes proves useful to nurse sharks looking for a bite to eat; we'll discuss nurse shark prey in a later section.

The nurse shark has a few other anatomical features that help with eating, including a pharynx, which is a muscular cavity that can suck in food like a vacuum. You might also notice barbels, or whiskers, on the nurse shark's snout; these help the nurse shark sense nearby food.

Because they rely on sucking in food, nurse sharks have a small mouth and fairly simple teeth. The small mouth limits the size of the prey that's available to the nurse shark, and the teeth are barely used in feeding because of the shark's ability to draw food in with the pharynx. The teeth are small and non-serrated, which means they're not ideal for cutting or tearing into their prey's flesh [source: Matott, Motta, Hueter]. That's not to say that the nurse shark's bite isn't strong -- surgical instruments are sometimes required to extricate a human who is in the grasp of a nurse shark's bite [source: Guarracino].

As we mentioned on the last page, nurse sharks are slow and sluggish, and they don't put a lot of effort into moving around. While some sharks must move continually to breathe (a process known as ram-jet ventilation), nurse sharks have respiratory systems that can pump water over their gill slits. This allows them to breathe without moving.

Maybe the nurse shark never moves because it just loves where it is. Find out where the nurse shark calls home on the next page.

The nurse shark lives in warm waters in the Atlantic and eastern Pacific oceans [sources: Guarracino, National Geographic]. It's often seen in Florida, especially around the Keys. They've been spotted as far north as Rhode Island (though this is uncommon) and all the way down to Brazil. On the west coast of North and South America, they are found from Mexico to Peru. They're also abundant in the Caribbean and the Gulf of Mexico, and along the eastern Atlantic coast of Africa [source: Busch Gardens].

But you may not need to go far to spot a nurse shark; they're commonly found in aquariums because they're very easy to care for. A director at a marine laboratory commented on their hardiness by saying that you could almost roll them up in wet newspaper, and they'd be okay [source: Stevens].

If you were to see them in their natural habitat, there'd be very little going on. Nurse sharks are nocturnal hunters and rest during the day. They usually dwell at the foot of the ocean floor, hovering above the sandy bottom. Groups of nurse sharks are also known to seek out reefs or caves, and these groups have been observed to be almost lying on top of each other [source: Castro]. Though they go out at night to hunt, nurse sharks are creatures of habit. They return to certain resting sites, and they rarely migrate.

But staying in one place can have repercussions when it comes to mating, and some scientists have wondered how the nurse shark has avoided the dangers of inbreeding. It turns out that one way nurse sharks encourage genetic diversity is by having more than one male fertilize a litter. In 2002, a study showed that the DNA of 32 pups born to one mother yielded at least four possible fathers [source: Saville et al.].

When male nurse sharks want to mate, they bite the female on her pectoral fin to hold her into position. Scientists have observed female sharks avoiding males by swimming to shallow water and burying their pectoral fin in the sand [source: Castro].

The nurse shark is ovoviviparous, which means that the female shark carries the fertilized eggs in egg cases within her ovaries. The embryos receive nourishment from the yolk in the egg cases, and once that's finished, the shark pups hatch from the egg case and are soon born. The gestation period for nurse sharks is about six months [source: Castro]. Nurse shark pups start life at about a foot long [source: Guarracino]. Once a female has produced a litter, it takes her 18 months to produce more eggs [source: Guarracino].

Nurse shark pups are nourished by a yolk sac, but what do adult nurse sharks eat? Find out on the next page.

Because the nurse shark doesn't do a whole lot of moving around, it might be hard to believe that they can catch any prey. And based on studies of nurse shark stomachs, they may not have to do much eating to maintain their low energy level. Frequently, when scientists examine the stomach contents of nurse sharks, they don't find anything; in three such experiments since the 1960s, food was found in less than half of the nurse sharks used in the test sample [source: Castro].

But when nurse sharks do eat, it appears they enjoy fish, mollusks such as octopus and squid, and crustaceans including lobsters and shrimp. Sometimes algae and corals have been found in nurse shark stomachs as well. This may have to do with their method of eating. With their muscular pharynx, the nurse shark sucks in food, sometimes hard enough to dismember it [source: Motta, Wilga]. Algae and coral might sneak in when the nurse shark sucks in its prey.

Putting up with algae and coral in their diet is another mark of how little effort the nurse shark puts into eating. Their hunting methods also seem to be ways to get food without expending much energy. For example, nurse sharks are mostly nocturnal creatures, which allows them to eat fish that are resting and would otherwise be too quick to catch. They also trap their own food. As we mentioned, the nurse shark can hover off the ocean floor with the support of its pectoral fin. Some nurse sharks have been observed hovering with their snouts pointed up, which they may do so that little fish and crabs swim under the nurse shark, seeking shelter. The nurse shark then descends and slurps up his dinner.

Although the nurse shark doesn't have any predators that routinely make a meal out of them, they have occasionally been found in the stomachs of lemon sharks and tiger sharks. Although they were once hunted for their liver oil, flesh and skin, they are rarely caught today, except by fishermen who see them as pests for taking the bait meant for other fish. While data are limited on the number of nurse sharks, they are not considered to be endangered or threatened.

To learn more about nurse sharks and other creatures of the sea, swim over to the links on the next page.