Talk about the laziest species of sharks; Nurse sharks (Ginglymostoma cirratum) are the first that comes to mind. This shark prefers to spend most of its day time cruising on the seafloor and then emerge at night to hunt. Hence, earning it the name “Sea couch potato” from people.
Apart from its inactivity nature, there are other exciting features about this shark that we would like to share with you in this article. Let’s dive right in!
Scientific Classification
Kingdom – Animalia
Phylum – Chordata
Class – Chondrichthyes
Order – Orectolobiformes
Family – Ginglymostomatidae
Genus – Ginglymostoma
Species – G. cirratum
Taxonomy
Bonnaterre first described nurse sharks in 1788 as Squalus cirratus. The new scientific name, which is Ginglymostoma cirratum, was described by Muller and Henle, in 1841.
Scientists derived Gynglymostoma from the Greek words Gynglimos, meaning “Hinge” and Stoma, meaning “Mouth.” The species name cirratum is from Latin meaning “Curled Ringlets” in Latin. Similarities in the molecular phylogenetic and the reproductive cycle data indicate a close relationship between the nurse shark (Ginglymostoma cirratum) and the whale shark (Rhincodon typus).
Common Names
Where the name Nurse sharks originated from is still not clear. However, some people think it might be from the sloppy sound nurse sharks make that sounds like that of a nursing child.
Other common names include carpet shark, catshark.
How to Identify Nurse Sharks
These sharks are one of the most observed sharks in both the Pacific Ocean and the Atlantic ocean. You can identify nurse sharks by their broad head, rounded pectoral fins, and an elongated caudal fin. These amazing sharks do not have the typical shark greyish coloration, instead, they are yellowish-brown.
Nurse sharks have two rounded, spineless dorsal fins. The first dorsal fin is more massive than the second one. They also have barbels between their nostrils, which they use when searching for their prey at night.
Their eyes are unusually small, and they reach a relatively large size of about 10 ft. Behind each of their eyes is a spiracle, which is an organ that enables them to take in water used for breathing when they rest at the sea bottom.
Coloration
Adult nurse sharks are known to exhibit a little difference in coloration when compared to their juveniles. Their color usually ranges from light tan to dark brown. Nurse sharks juveniles that are up to (22 In) have black spots covering their body with an area of lighter pigmentation around each black spot. These black spots tend to fade as they age.
When small nurse sharks were used in a tank experiment and covered for a few minutes, they showed considerable lighter coloration when compared to individuals exposed to sunlight. This is likely due to the tanning effect. Even more, there have been reports of unusually pigmented nurse sharks.
Dentition
Nurse sharks have the simplest type of tooth arrangement found in sharks. They have an independent dentition, i.e., there is no overlap between their teeth. This kind of teeth arrangement allows forward movement of replacement teeth that is independent of adjacent teeth in the jaw.
This goes with the fact that sharks with overlapping dentitions or imbricate dentitions can only replace their teeth when they lose the adjacent “blocking” teeth. Teeth replacement happens faster for juvenile nurse sharks than adults. Also, tooth replacement occurs much more for these sharks, in summer when the temperature is higher.
Nurse Sharks Size, Age and Growth
Females nurse sharks reach maturity at about 7.5 to 7.9 feet (2.3 to 2.4 m) and their males, at 6.8 to 7 feet (2.10 to 2.14 m). The size of nurse sharks at birth is usually between 10.6 to 11.8 inches (27 to 30 cm). The largest recorded size of the nurse shark is 10.1 feet (3.08 m).
The Age of female nurse sharks at maturity is between 15 to 20 years. Meanwhile, that of males is between 10 to 15 years.
Typical Behavior
During the daytime, nurse sharks swim in groups as large as 40 individuals. While in groups, they mostly remain hidden around reefs and submerged ledges. Scientist believes these sharks stay in groups for protection. At night, nurse sharks prefer to be solitary, often found wandering the waters alone.
Hunting Behaviour
Nurse sharks have small mouths; therefore, they are not able to consume large fish. They go for small prey like stingrays, crabs, and squids. These sharks like to hunt alone and catch their prey on the seafloor, most times by sucking them into their mouth. While sucking, nurse sharks unconsciously make a slurping or sucking sound, which many believes sounds like that of a nursing baby.
These sharks are nocturnal predators that hide under submerged crevices within the reef. They tend to rest during the day after spending most of the night hunting. These sharks are docile animals known to take advantage of dormant small fish that are sleeping at night but too active for them to catch during the day.
Food/Feeding Habits
Nurse sharks feed mainly on fish, mollusks (octopi, clams, squids), stingrays, and crustaceans. Algae and Corals are also sometimes found in their stomach content. They spend most of their time foraging through the bottom sediments for food.
Although nurse sharks have small mouths, their large pharynx allows them to suck in food items efficiently.
Small nurse sharks who are not big enough to hunt for long tend to sleep with their snouts pointing upward, and their bodies supported off the bottom of their pectoral fins. This posture provides false shelter for small fishes and crabs that they can ambush and eat.
Courting/Mating
These sharks have one of the most complex rituals when it comes to courting and mating. It starts with the male shark biting the female on its pectoral fins to hold her in a mating position, but the female shark avoids the male nurse shark by swimming to shallow waters and burying its pectoral fins in the sand.
However, a persistent male will continue to chase the female and even form a co-operative group with other males to block off her route until it can get the female nurse shark into a mating position. With this position, it is easier for the male to insert his clasper, bending the lower portion of his body towards the female’s cloaca. Also, many males can fertilize litter.
Reproduction
Nurse sharks are ovoviviparous, i.e., the eggs develop and hatch within the body of the female shark where the hatchlings grow further until live birth happens. Their mating cycle is biennial. It takes 18 months for female nurse sharks to produce another batch of eggs.
These sharks mating cycle runs from late June to the end of July, and the gestation period often lasts for about six months. Nurse sharks produce about 21 to 29 pups at once. However, from birth, there is competition for survival, with larger pups consuming the smaller ones from the beginning. Therefore, only a small number of them reach maturity eventually.
Baby nurse sharks are born fully developed at about 30cm in length. The young ones possess a spotted coloration that fades as they age. Nurse sharks reach about 35 years in the wild and 25 years or more in captivity.
Distribution Range
Nurse sharks are generally bottom dwellers, typically found in tropical to warm temperate latitudes in Eastern Pacific and the western Atlantic Ocean. In the east of Atlantic Ocean, they range from Cape Verde to Gabon, and in the west of Atlantic Ocean, their range moves from Rhode Island to Southern Brazil.
Also, in the Eastern Pacific Ocean, these sharks range from Baja California to Peru. You can find their juveniles on the bottom of Seagrass flats, mangrove islands, and coral reefs at a depth of 3 to 75 meters and their adults, around the deeper reef and rocky places.
Nurse sharks seek shelter in crevices and under ledges during the day and leave there to go hunting at night. They display a strong preference for specific resting sites, frequently returning to the same crevices and caves after nocturnal activities.
Natural Habitat
Nurse sharks are nocturnal animals that prefer to rest on sandy bottoms during the day. They sometimes occur in groups of up to 40 individuals, where you can see them cruising on the seafloor, sometimes stacked upon each other.
You can find these sharks swimming near the bottom or clambering across the seabed, with their muscular pectoral fins as limbs. Most of their large juveniles and adults swim at a depth of 3 to 75 meters during the daytime and 20 meters at night.
Do Nurse Sharks Migrate?
Nurse sharks are not migratory species. They only reduce their activity level when water becomes colder.
Biology and Ecology
These lazy sharks are opportunistic predators that majorly feed on prey like stingrays, crustaceans, and crabs. Nurse sharks prefer to be solitary at night, rifling through bottom sediments in search of food. During the daytime, you can find them in large sedentary groups, resting together.
They are suction feeders, capable of generating suction forces that are among the highest recorded for any aquatic vertebra to date. Unlike most sharks, nurse sharks are exceptionally sedentary.
They exhibit strong site fidelity, thought to be typical of Reef sharks (one of the few shark species known to display mating site fidelity). Nurse sharks often return to the same breeding grounds time and time again.
Adaptations
Nurse sharks have slender, fleshy, and whisker-like organs on their lower jaw, below the nostrils that can sense, taste, and touch. It is an adaptation for detecting potential prey on the seafloor.
Social Groups
Nurse sharks gather in schools. They are sluggish species found piled up on top of each other on the sea bottom.
Predators of Nurse Sharks
There are no specific species that hunt these sharks. However, they are occasionally preyed upon by larger fish such as Lemon sharks, Bull sharks, and Tiger sharks. There have also been cases of American alligators preying on nurse sharks where they share the same habitat. Recent research based on historical accounts and photographs indicates these encounters may be more rampant than scientists initially thought.
Parasites
Aquarium keepers found several Nematodes on the gills of nurse sharks in captivity, at the New York Aquarium. These are the popular parasites of nurse sharks.
Nurse Sharks and Humans
In history, Humans targeted nurse sharks for their liver oil and their skin for leather. Presently, there is no commercial fishery for these species. Regionally, they are popular because of their meat, fins, and skin, mostly in areas like Brazil, Panama, Venezuela, and the Caribbean.
In other places, Fishermen catch and kill nurse sharks because they consider them as pests that take the bait meant for other species. In the lesser Antilles, where these sharks often raid fish traps, they are also considered as pests.
Nurse sharks adapt well in captivity and are an essential species for research. These sharks are responsible for the fourth highest shark bites on humans.
Dangers to Humans
Nurse sharks are not aggressive. They usually swim away when they see humans approaching. However, there have been several cases of attacks on divers and swimmers. But, pieces of evidence show that these are often provoked attacks.
When frightened, nurse sharks bite with a sturdy vice-like grip, capable of inflicting fatal injury. In some cases where the bites are deep, the jaw locks on the victim’s body, only surgical instruments separated it. The rate of these sharks bites increased in recent years due to ecotourism feeding operations according to ISAF 2018.
Can Nurse Sharks Serve as Pets?
No, These sharks cannot be kept as pets because most Home aquariums are too small to support them. Moreover, it is costly to provide nurse sharks with proper care. They are mostly better off in zoos and public aquariums.
Domestication
Humans have not been able to domesticate these sharks in any way.
Nurse Sharks care
Nurse sharks do well in public aquarium and zoos. These sharks are quite sedentary, so aquarium keepers do not have to worry about them bumping into aquarium walls. However, it is critical to provide a lot of hiding places for them in the tank to sleep. As surprising as this may seem, nurse sharks are social fish when housed with several other sharks. They readily eat squid, fish, and crustaceans.
Conservation
Nurse sharks are not widely commercially fished. However, their sluggish behavior is often an easy target for local fisheries. Their skin is hard and prized for leather. Humans get oils from their liver and also consume nurse sharks’ flesh both fresh and salted.
Although there have been many attacks on humans by these sharks, they do not still pose a major threat. Nurse sharks are famous as bait and sport fish with crab fishers and anglers. These sharks are also popular species for research and display in public aquaria due to their status as a shark, docile behavior, and ease of care.
Due to anthropogenic activities and fishers taking nurse sharks as pests in some areas, there are now practical measures in place for conserving them. In the united states, Whenever you catch a nurse shark by accident, you must keep it alive and release it back into the Ocean.
The Population of Nurse sharks
Nurse sharks are naturally rare species. There is no much information about its population size.
IUCN Assessment
On the IUCN Red List of Threatened Species, these sharks appear as “Data Deficient (DD)” species. The nurse shark may not yet be an endangered species. However, the abundance in the littoral waters of Florida has decreased because of anthropogenic activities. The Assessment comes from the fact that not much information is available on the population, migratory behavior, and connectivity of this species.
In the eastern Pacific Ocean, there is also a noticeable decrease in the population of these sharks. And, in the southern ranges of brazil, nurse sharks have become extinct. The effect of human interactions with these sharks in coastal waters is still not known.
Interesting Facts About Nurse Sharks in a Glance
1. Possession of Barbells
One unique thing about these sharks is their barbells. Barbells are fleshy appendages that hang below the nostrils of nurse sharks. It provides a sense of touch that assists in the locating of prey in the sea bottom.
2. Ranked Fourth on Human Attackers List
Although these sharks are not aggressive and are naturally docile, they ranked fourth on the list of most attacks on humans. Now, this may get you wondering how?
Humans provoke these sharks until attacked. The ability of nurse sharks to lay motionless in the ocean bottom has led divers/swimmers into false security, leading them to poke, prod nurse sharks occasionally until these sharks feel threatened.
3. Buccal Pumping Method of Expelling Water
Unlike most sharks that require continuous movement, to move water over their gills, otherwise, they die. Nurse sharks instead make use of a method referred to as “Buccal pumping” to pump water over their gills without moving. This is possible through the constant opening and closing of its mouth.
4. They do Well in Captivity
Nurse sharks thrive well in captivity; there are records of them living up to 25 years or more in captivity. As a result, they are among the few shark species to live long as captives.
5. Nurse Sharks has the Capacity to Learn
In a learning experiment, these sharks displayed the ability to learn to swim to get food by a handler.
6. These sharks can Walk Across the Ocean Floor.
Nurse sharks can walk across the ocean floor. They are nocturnal predators, found in shallow waters. They tend to rest during the day and hunt at night. Instead of swimming to look for their prey, they use their pectoral fins to walk across the sea bottom.
7. Nurse Sharks Suck Up Their Food.
There is a cavity within their throat that generates a powerful suction that vacuums their prey into their mouth. Then, the tiny rows of teeth crush up the prey.
8. They have Varying Colors.
Nurse sharks have different colors; adult nurse sharks are usually brown, grey, or yellowish. In 1992, a milk-white nurse shark with brown splotches was captured and photographed near key largo in Florida.
Another adult nurse shark that has this same coloration was caught in 2014. Their adults do not usually have dark spots, but their juveniles do, and it fades as they Age.
9. Nurse Sharks Like to Unwind in Groups
You will always find these sharks in a group of about 40 at the bottom of the sea. Although they don’t like hunting in groups, they love to rest in groups, piled up on top of each other.
10. They have Certain Behaviors in Common with Whale Sharks.
The whale shark on consideration is the most prominent fish presently, and like the nurse sharks, it eats via suction. However, that is not the only thing nurse sharks have in common with whale sharks. Thus, they are both members of the order Orectolobiformes. And, this is a group that consists of 39 other shark species, mainly in temperate and tropical oceans, popularly known as Carpet sharks.
You can identify Carpet sharks by their small mouths and the two dorsal fins on their back. Also, their five sets of gills slits. Species in this order seem to have striking patterns on their skins, although adult nurse sharks are an exception.
11. Reproduction is Biennial
Their females do not give birth every year, it takes up to about 18 months for them to reproduce, and six months for gestation.
12. The Origin of their Name is a Linguistic Mystery
How these sharks got the name “nurse sharks” is still a linguistic mystery because we all know they are not capable of caring for patients in the hospital. However, historians seem to have their theories. Some suggest it was from the sound they make when they suck in their prey that reminds sailors of nursing infants.
But, others think the “Nurse” in Nurse sharks could be from an old name “huss” given to an unrelated bottom dwellers. Over time “huss” involved to “Nuss,” a word that came to mean Large Fish. So maybe the name came from either of the two.
13. Scientists Just Recently Named a New Species of Nurse Sharks
In 2012, a recent study found that the population of nurse sharks living in the tropical eastern pacific was anatomically and genetically different from Atlantic nurse sharks to be a specie on its own. Therefore, scientists named it Pacific nurse shark or Ginglymostoma unami.
Further Reading
- “Discover Fishes: Ginglymostoma cirratum, Nurse Shark”, Florida Museum.
- “Animals: Nurse Shark”, SeaWorld Parks and Entertainment.
- “Nurse shark”, Wikipedia (Acessed March 28, 2020).
- “Nurse Shark”, Animals Network.
- “13 Facts About Nurse Sharks”, Mental Floss.
- “The Nurse Shark”, Shark Sider.