Nurse sharks are a bottom dwelling species of shark that humans often don’t give a second thought. The nurse shark won’t hesitate to give a defensive bite though and at as long as 14 feet they can leave a significant wound behind. In this article we will cover everything you ever wanted to know about the nurse shark including: how the nurse shark got its name, where it prefers to live, what it feeds on and the normal behavior of these bottom dwellers.
How Did the Nurse Shark Get its Name?
Nurses are a part of a helping profession that we rely upon for our own health and safety, and yet when this term is mentioned in reference to a shark this is definitely not the image that is conjured up. So why is the nurse shark called a nurse shark? There are multiple theories that abound in reference to how this shark species got its name, the most favored of which is that it is derived from the old English word “hurse” which translates to “sea-floor shark.” Others believe that the name derives from the Archaic word “nusse” which translates to mean “cat shark.” The third theory states that the nurse shark got its name from the sucking sound that it makes when hunting prey, a sound that is likened to that of a nursing baby.
The Basic Taxonomy of the Nurse Shark
The scientific name for the nurse shark is Ginglymostoma cirratum, placing it within the Ginglymostomatidae family of sharks. The Ginglymostomatidae family of sharks are characterized for their habit of living and feeding on the bottom of the ocean floor, hence the term “carpet sharks.” Sharks within this family are recognized not only for their bottom dwelling habits but also for their sluggish and docile nature. Ginglymostomatidae sharks are found most commonly in tropical and subtropical waters due to their preference for warmer waters, and more often than not they are found in shallower areas of the ocean. The family name Ginglymostomatidae is of Greek origin and also characterizes these sharks as having a hinge-like mouth with “gynglimos” meaning hinge and “stoma” meaning mouth.
There are three genera of sharks to be found within the Ginglymostomatidae family, the Ginglymostoma, Nebrius and Psudoginglymostoma. The nurse shark is a member of the genus Ginglymostoma.
A Physical Description of the Nurse Shark
The Teeth of the Nurse Shark
The nurse shark is something of a large shark and while they more commonly grow to be anywhere from 7 1/2 feet to 9 3/4 feet long, some individuals in the species have been seen as large as 14 feet long. Despite their size most nurse sharks are recognized as being harmless to humans but with such large tooth filled jaws these sharks can cause a significant amount of damage if they do bite a human. Nurse sharks have particularly strong hinged jaws which contain thousands of small serrated teeth and they will not hesitate to bite humans in an attempt to defend themselves when threatened. Most commonly the nurse shark, due to its nature as a bottom dweller, is pushed to defending itself when it is stepped upon in shallow waters. The jaws are particularly powerful and in an undisturbed habitat they are utilized to crush shellfish and coral and shred squid, shrimp and fish. The nurse shark can weigh in at anywhere from 200 to 330 pounds which puts an awful lot of power behind those serrated jaws if they should lock on in a defensive bite.
The individual tooth structure of the nurse shark is particularly interesting but also particularly damaging. Each tooth is built-in a fan-shaped structure with a number of serrated ridges. In general there are thousands of these individual teeth within the shark’s mouth and these teeth are replaceable. The teeth are arranged in rows and as one tooth breaks or is lost during a struggle, a fight or during feeding, another of the teeth rotates in to take its place. This system of replaceable teeth ensures that the nurse shark does not starve due to tooth loss which is a common occurrence even for a shark that prefers to feed on smaller fish and crustaceans.
The Color and Texture of the Nurse Shark
Nurse sharks are a grayish brown color and their skin texture is somewhat unique for a shark. While most sharks have a rough texture similar to sandpaper, the skin of the nurse shark is much more like that of the dolphin and has a smooth texture to it. Younger nurse sharks often can be recognized from small spots on their skin. The body is rather stout and the head is particularly wide with barbells that can be seen well. The nurse shark utilizes these “barbells” for both sensing touch and taste as they hunt along the ocean floor. One of the more distinctive features of the nurse shark is the tail fin, an elongated and pointed fin that can grow to be as much as 1/4 the total body length of the shark. As mentioned before the body length of these sharks can range from 7.5 to as long as 14 feet long making those long tail fins anywhere from 1.9 feet to 3.5 feet long! The dorsal fins of the nurse shark are nowhere near as large as the tail fins but the two dorsal fins that can be seen on the back of the shark are approximately the same size. These dorsal fins are much more rounded than they are in other species of shark and this fact along with the length of the tail fin can help significantly in shark species identification.
Behavior of the Nurse Shark
While the nurse shark is classified as being a particularly docile species of shark they are also active nocturnal hunters that tend to take advantage of dormant fish who are not prepared for attack during the night. During the day, while most fish are active shoals of nurse sharks can be found hidden under ledges or in ocean crevices where they can rest peacefully during daylight hours. As many as 40 nurse sharks have been noted in particular resting locations at any one time, piled on top of each other. Commonly the nurse shark returns to the same location to rest during the day. Despite the fact that many times the nurse shark can be found resting in larger groups, during the day these are solitary hunters that sweep the ocean floors for unsuspecting prey.
When it comes to feeding, the nurse shark is limited by the size of its mouth and as such it cannot take in large prey items due to the risk of them becoming lodged in the mouth. As the nurse shark hunts along the bottom of the ocean it sucks in prey as it moves, occasionally stopping to feed on coral and algae. The barbell structures on the face of the nurse shark are what help it to detect prey items as it moves along the ocean floor. These fleshy organs that look like whiskers to many, are located on the lower jaw and are used both to taste and as fingers to feel out prey. Since most hunting is done at night, the nurse shark feels along the bottom of the ocean floor for prey items and attacks them with its thousands of serrated teeth when an item is located. Unlike some other oceanic creatures, the nurse shark is able to remain stationary and continue to breathe underwater by sucking water in through the mouth and releasing it out of their gills.
The Natural Habitat of the Nurse Shark
As mentioned before the nurse shark prefers to swim along the bottom of tropical and subtropical waters. Often these sharks prefer warmer shallower waters and can often be found thriving in mudflats, sand bars, on reefs, channels between mangrove islands and on sandy beaches. The natural territory of these sharks is the Western Atlantic Ocean running from Rhode Island down to South Brazil and the Eastern Atlantic from Cameron to Gabon. In addition, the nurse shark can be found in the Eastern Pacific from Southern Baja California to Peru and in the islands of the Caribbean.
Reproduction in the Nurse Shark
Nurse sharks are officially capable of reproduction at some point between the ages of 15 to 20 years old and much like with humans, this age varies from individual to individual. Reproduction in the nurse shark is a particularly interesting process and is referred to by biologists as aplacental viviparity. Aplacental viviparity refers to a process where eggs develop inside the female of the species after fertilization. As the eggs develop within the female nurse shark they eventually hatch and the mother gives birth to small live nurse sharks.
The nurse shark pups resemble the fully grown nurse sharks with the exception of, of course, their size. A newborn nurse shark measures in at approximately 12 inches long. The average nurse shark mother gives birth to around 20 to 30 pups in a single birthing session. The usual mating season for the nurse shark is from the end of June to the end of July and the gestation process takes approximately six months in order for the nurse shark pups to develop fully. It is not uncommon to see cannibalistic behavior in the nurse shark even in the womb as the stronger of the pups vie to compete for food against other siblings. Female nurse sharks are capable of producing eggs every 18 months but this does not guarantee that they will mate every 18 months like clockwork.
Migrating Behavior in the Nurse Shark
Some species of shark are known for migrating behavior that takes place as ocean currents shift and the waters the sharks inhabit become cooler. Unlike migrating shark species however, the nurse shark copes with dips in water temperature in a different way. As the waters become colder, the nurse shark simply becomes less active requiring less energy to function. This docile behavior is made possible through the ability of the nurse shark to breathe through its mouth and out of its gills.
The Nurse Shark and Humans
Unlike other species like the infamous great white shark, the nurse shark is not as widely known by the general population. Those who live in areas where the nurse shark inhabits the waters are generally familiar and on the lookout for the species but for those unfamiliar with the shark accidental interactions are more likely. As mentioned previously attacks upon humans by nurse sharks are particularly unlikely; although in the past a handful of unprovoked attacks have been noted. In many cases these unprovoked attacks have elements that could explain the attack as being self-defense since often they take place at diving sites where sharks may see foreign invaders as a threat.
Nurse Sharks and Aquariums
Nurse sharks are one species of shark that tend to be able to thrive in a commercial aquarium setting where many other shark species have been noted for their inability to thrive in captivity. The ability of this shark species to endure captivity has led to the sale of juvenile nurse sharks in the saltwater aquarium trade. There are a number of problems that occur when this species is kept in a personal saltwater aquarium however, the largest of which being the size of the shark once it has matured. In some cases the nurse shark can grow to an amazing 14 feet in length and it is not common for a saltwater aquarium keeper to have a tank that is capable of holding a shark of this length. What happens in most cases is that a saltwater aquarium keeper will purchase a juvenile nurse shark and when it becomes too large for its tank will attempt to sell, donate or release the shark in the wild. All of these solutions pose significant problems for the shark however.
Selling a nurse shark poses difficulty in that there are few saltwater aquarium owners who have the resources to take on such a large shark. Not only does the fully grown shark require an extremely large tank but it also requires a significant amount of food to remain healthy.
Donating a fully grown nurse shark to a local aquarium is also not a particularly feasible solution as most aquariums already have an established shark population and are unable to take on new individuals with their limited resources and financial support.
Releasing a captive bred nurse shark in to the wild is another solution that many aquarium hobbyists turn to but it is one that generally results in the death of the shark in question. Like any other animal that is raised in captivity, nurse sharks are unable to adapt to life in the wild because of a lack of the skills demanded to survive. Captive bred nurse sharks are not given the opportunity to learn how to hunt for themselves, nor are they able to distinguish natural predators or other naturally occurring threats to their existence in the ocean.
The long and short of it is that unless you have a commercial sized aquarium and a budget large enough to keep a nurse shark fed healthily through its potentially 25 year long captive lifespan, this is not the “pet” for you.
Nurse Sharks and the Fishing Industry
Where many species of shark are fished intentionally, there is not a particularly wide trade for commercially fishing the nurse shark. With that said however, the nurse shark is easily fished due to its slow movement and often becomes the target of local fishermen. There are a number of uses for the nurse shark when fished locally but one of the most common is the unique skin of this shark. Where many other sharks possess a sandpaper like texture to their skin, the smooth texture of the nurse shark gives it a leather like quality that is utilized in the textiles industry. The skin of the nurse shark is not the only part that is utilized commercially; the flesh and liver are also used. When salted the flesh of the nurse shark is consumed and the liver of the shark is used for the oil that it produces.
The Conservation Status of the Nurse Shark
Currently the conservation status of the nurse shark is unknown because there is an insufficiency of data available to determine whether these sharks are at risk for extinction. It is doubtful that at the moment this particular shark species is threatened with extinction; however, if they continue to be fished there is a possibility that one day they may face endangerment.
Video: Shark Academy
Watch this minute and a half video to see the nurse sharks in action.
Why is the Nurse Shark Important?
There are many people who understand the importance of any animal species to the ecosystem in which it lives, but for those who have a fear of sharks the question of the importance of these creatures is often raised. The nurse shark is one of the biggest predators that our oceans will ever see and this is crucial to the health of the ocean marine life. While there are many species of sharks that thrive in our oceans, each have different hunting and feeding habits that keep other marine life populations in check. Were the nurse shark species to become extinct the smaller marine life that these sharks feed upon would soon begin to grow out of control and put other species in danger as they succumbed to large populations of these creatures. One perfect example of this is the reefs that many smaller marine organisms feed on, if these smaller organisms are left unchecked by nurse shark populations, they can quickly decimate reefs of the ocean – something that will affect ocean life as we know it. So while the nurse shark may not yet be an endangered species, it is our responsibility to ensure that overfishing and fear of these creatures does not lead to its eradication.
Have you ever seen or swam with a nurse shark?Tagged With: Sharks