11 Facts about Nurse Sharks, AKA the Housecats of the Sea

2. Nurse sharks have between 58-76 small, serrated teeth to crush their prey’s hard shells

Nurse sharks need to break up the hard shells of lobsters, conches, and crabs. They have between 30-42 along their upper jaws, and 28-34 along their lower jaws, so 58-76 teeth total. They are very small, but serrated, and very sharp. Sometimes, a nurse shark will thrash its head while chomping on a shell to break it down quicker. Unlike other shark teeth, nurse shark teeth are lined single file in their mouths, so they fall out easily as newer teeth push forward.

3. Nurse sharks have very small eyes, so they use their two sensory barbels to sniff out prey along the ocean floor

Nurse sharks have very small eyes. They can only see about 10m (30ft) in front of them. But since their prey is usually hidden under sand or gravel, they don’t need powerful eyesight. Instead, nurse sharks rely on two small barbels around its mouth to detect prey. The small barbells are chemoreceptors. They allow the shark to sniff out prey.

4. Nurse sharks only bite humans if they are provoked, but they are safe to swim near

Nurse sharks are not aggressive. But many divers act uncautious around them. These divers try to pet, grab, or play with them. If a nurse shark feels provoked, it will bite a person in self defense. It’s teeth are too small to make a deep bite, but they are sharp enough that it will be very painful. But divers who admire these sharks without bothering them have nothing to fear.

5. Unlike other sharks, the nurse shark does not have to keep swimming to breathe because it pumps water over its gills

Many shark species need to constantly swim in order to breathe. But nurse sharks breathe via the buccal pumping method, which allows them to stay still. For the buccal pumping method, nurse sharks use muscles in their mouths to suck in water and push it over their gills. The water supplies the gills with oxygen. Other sharks need to swim non-stop to keep passing water over their gills.

6. Nurse sharks are very sedentary. They find favorite spots in coral reefs to sleep and lay down

Nurse sharks are known as lazy sharks. Like house cats, they find their favorite spots in their habitat and spend their days lounging around. Other sharks need to keep moving to pass oxygen in water over their gills. If these sharks stop swimming, they will drown. But nurse sharks can be sluggish. They spend all day laying around, and hunt at night. At dawn, they return to their favorite spots to rest until the next hunt.

Nurse sharks sometimes sleep in groups of 40

It seems like nurse sharks don’t like sleeping alone. During the day, they sleep in large piles with several other sharks. Sometimes, 40 sharks will sleep together in a single pile. One theory suggests that this is for protection. Tiger sharks and lemon sharks sometimes eat nurse sharks. Since nurse sharks are much slower, they may pile together for safety. It’s like fish that form schools to avoid predators.

7. A nurse shark’s caudal fin accounts for ¼ of its total length

Nurse sharks usually grow between 9-10ft (2.7-3.0m). The largest individuals recorded were between 13-15ft (4-4.5m), but these accounts have not been confirmed. Nurse sharks have very large caudal fins. A nurse shark’s caudal fin accounts for ¼ of its total length. So a 10ft nurse shark has a 2.5ft caudal fin. Nurse sharks slam their fins into the ground while sucking up prey. The rising sand either hides them, or forces up prey.

8.  Nurse sharks can “walk” along the ocean floor using their large pectoral fins

Nurse sharks have two muscular pectoral fins. Typically, pectoral fins are like arms that fish use to swim. But nurse sharks use theirs to swim or “walk” along the ocean floor. Since these sharks are bottom feeders, using their pectoral fins like legs is an advantage when hunting. They can slowly move across the sand with their barbels low enough to detect prey.

9. Female nurse sharks give birth to 21-29 pups at a time, but their mating cycle takes two years

Female nurse sharks give birth to 21-29 pups in a single litter. Once the mom’s eggs are fertilized, it only takes 4-6 months for the embryos to fully develop. Once the 4-6 months have passed, the female gives birth. Each pup is about 30cm (11in) long. But it takes another 18 months for the mother to develop another batch of eggs. So 18 months for eggs to develop plus 6 months of pregnancy means the mating cycle takes up to 24 months, exactly two years!

Nurse sharks return to the same breeding grounds every time they mate

As reef sharks, nurse sharks usually return to the same breeding grounds each time they mate.  This could be due to their sedentary lifestyle.  They spend their whole lives in shallow, coastal waters because swimming is not necessary to their survival.  Instead, they stick to a familiar spot for mating.  Since these sharks sleep in large piles, they can likely count on plenty of potential mates to be at the breeding grounds.

10. More than one male nurse shark can fertilize a female’s eggs, but females can prevent certain males from doing so

During mating, multiple males can fertilize one female’s eggs. So the 20-30 pups in a single litter will have different fathers. To initiate mating, a male will bite down on a female’s pectoral fins. He does this to hold her in place while he releases his sperm. However, females can be selective. To avoid a male, the female will bury her pectoral fins in the sand. This prevents a male from grabbing onto her.

11. Nurse sharks only grow 4-6in each year, so it takes over a decade for them to reach sexual maturity

Nurse sharks are slow moving and slow growing. Baby nurse sharks are born 30cm (11in) long. They double in size the first year of their lives. After that first year, the average nurse shark will only grow 10-15cm (4-6in) every year. Males do not mate until they are 210-220cm (6.9-7.2ft) long. Females do not mate until they are 220-230cm (7.5ft) long. So it can take up to fifteen years for nurse sharks to reach sexual maturity.

Nurse Sharks: The Ocean’s favorite housecat

In many ways, a nurse shark is basically a coral reef’s housecat. One has barbels; the other has whiskers. Both are skilled hunters, but prefer to spend their days laying around, basking in the sun. They are most active at night, and have their favorite spots for naps. Humans adore them, but it's usually a pretty one sided relationship. When a human tries to unleash its affection on one, he gets bitten. Besides, have you ever seen a housecat and a nurse shark in a room together? Think about it.

• IUCN List of Threatened Species Status: Vulnerable
• In the 20th century, commercial fisheries targeted nurse sharks for their skin, to use in leather.
  Today, commercial fisheries do not target nurse sharks because they no longer have market value

• Nurse sharks are still often caught as “bycatches,” and killed because they are considered pests.
• These sharks are popular in ecotourism, and constant human disturbances in their habitats can be harmful.
• Climate change contributes to habitat degradation, and the decline in populations of their prey.
• Certain Brazilian populations are considered functionally extinct.