Animal Spotlight: Nurse Shark

Taxonomy:

animals-nurseshark-slide1-web
Credit: George Grall, National Aquarium
  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Chondrichthyes
  • Order: Orectolobiformes
  • Family: Ginglymostomatidae
  • Genus: Ginglymostoma
  • Species: cirratum
  • Average length: 12 ft (3.7 m)
  • Average Weight: 200-330 lbs (90-150 kg)
  • Average Lifespan: 25-35 years

Nurse sharks are slow-moving bottom-dwellers with small mouths. For the most part, they are harmless to humans. Because of their small mouths, they are unable to consume large fish. Instead, they will catch their prey on the seafloor, mostly by sucking their prey into their mouths. While doing this, the shark will make an unmistakable “slurping” or sucking sound, which is completely unique to this species.

These sharks can be found in the warm, shallow waters of the western Atlantic, from Rhode Island in the United States to southern Brazil, eastern Atlantic, from the Cape Verde islands to Gabon, and eastern Pacific oceans, from the Gulf of California to Peru. They are abundant throughout their range and have no special conservation status. But, the closeness of their habit to human activities is putting pressure on the species.

Physical Description:

A nurse shark’s long, flexible body is yellowish-brown to grey-brown, with two spineless, rounded dorsal fins, and a long tail fin that can be over a quarter of the whole body length. The first dorsal fin is much larger than second. The large, rounded pectoral fins are flexible and muscular, and can be used as limbs to clamber along the sea bottom.

Its head is broad and flat, with small but strong jaws housing thousands of tiny, serrated teeth. The eyes are placed behind the sub-terminal mouth. Other facial characteristics include minute spiracles and moderately long barbels that reach the mouth. Nasoral grooves are present, but there is no perinasal groove. It will use its strong jaws to crush and eat its prey.

animals-nurseshark-slide2-web
Credit: George Grall, National Aquarium

Its unique suction feeding method is because of its small mouth and large pharynx. By cupping its mouth over a hole or crevice and expanding its throat, it creates a vacuum that sucks prey out of their hiding. This suction-feeding is also useful for extracting snails from their shells. It will also dig in the sand to root out prey sensed by its fleshy

Unlike most other sharks, nurse sharks are smooth to the touch.

Juveniles up to 23 in (60 cm) have small black spots, with an area of lighter pigmentation surrounding each spot, covering the entire body. These are bands of lighter and darker pigmentation along the dorsal surface. Juveniles from 28 to 48 in (70 to 120 cm) are capable of limited color changes. In a tank experiment small nurse sharks, covered for just a few minutes became considerably lighter than individuals exposed to full sunlight. Unusually pigmented individuals (e.g. brilliant yellow or milky white) have been reported several times.

Diet:

Nurse sharks’ preferred prey includes sea snails, crustaceans, mollusks, and other small fish. They will feed on bottom invertebrates such as spiny lobsters, shrimps, crabs, sea urchins, squids, octopuses, snails, and bivalves. For fish, nurse sharks feed on catfishes, mullets, puffers, and stingrays. In addition, they have been seen grazing for algae and ground corals.

Behavior:

These sharks are nocturnal creatures, actively roaming the sea bottom and reef for prey. Its nocturnal nature enables it to prey on resting fish that would be too active during the day to capture. During the daylight hours, nurse sharks are sluggish, spending most of their time resting on sandy bottoms or in crevices in rocks and coral reefs. They are often seen congregating when resting, sometimes even piling on to of one another, in groups as large as 40 individuals. While in these groups, they mostly remain hidden under submerged ledges and around reefs. Scientists believe they rest together for protection purposes only, with very little dominance or hierarchy.

The nurse shark is one of the few sharks in which courtship behavior is relatively well known by scientists. The male swims alongside the female, grabs one of the pectoral fins in his mouth, rolls her over, and they mate. Often times, a large number of males will attempt to mate with a single female. As a result, the females will bear numerous bite-scars and bruises received during mating attempts. Therefore, it is not surprising that females frequently try to avoid males by swimming in very shallow water, where they can bury their pectoral fins in the sand.

nurse-shark-pup
Newborn nurse shark pup. Credit: Wes Pratt

This shark species is ovoviparous, meaning that the eggs develop within the uterus of a female until birth. The eggs are retained within the body of the female in a brood chamber where the embryo develops, receiving nourishment from a yolk sac. This is the method of reproduction for the “live-bearing” fishes where pups hatch from egg capsules inside the mother’s uterus and are born soon afterward. The pregnancy period usually lasts about six months with births occurring generally in the months of June and July. For the next 18 months after birth, an adult female is unable to produce eggs.

On average, 20 to 30 pups are born from one pregnancy. Inside the womb, cannibalism between pups does occur, with the larger and stronger pups eating the underdeveloped ones. When born, the pups are only about 12 inches (30 cm) long and have a spotted colored skins. Similarly with tiger shark pups, they will lose the spotted patterns with age.

Like many sharks the nurse shark is slow growing, with males not reaching maturity until 10 to 15 years of age, and females 15 to 20 years.

Conservation and Threats:

In general, nurse sharks are not considered threatened and are safe from the extinction that many other species experience.

This species is not typically hunted by commercial fishers or for sport fishing. However, some small operations in some parts of its range capture this fish for its skin, which is used in high quality leathers. Their liver is also harvested for certain types of oils. Due to their relatively slow speeds, they are very easy to catch. They are also incidentally captured in many coastal fisheries, for the aquarium trade, and is occasionally the target of spear fishermen.

The main threat is overexploitation for the reasons above. Because the nurse shark grows slowly and matures late, this can cause populations to decline rapidly and recover slowly.

nurse-shark-habitat-map-1
Nurse sharks’ range

This threat is compounded by the impact of  humans on the coastal and reef habitats of the nurse shark. Coral reefs are a particularly vulnerable habitat, being impacted by pollution, sedimentation, global climate change, and disturbance from tourism. Extreme population reductions have already been recorded in the southern Western Atlantic. It is possible that the nurse shark is declining, unnoticed, in other areas where there is a lack of data, subsequently making it difficult to assess the status of populations and implement appropriate conservation measures.

IUCN lists the nurse shark as “data deficient” because of the lack of studies and research. More specific research has been done on the Western Atlantic subpopulation and IUCN lists this specific subpopulation as “near threatened”. Countries like the United States consider the nurse shark as “least concern” for extinction. But, the nurse shark is included as a vulnerable species in the Official List of Endangered Animals in Brazil, fisheries are managed within United States’ waters, and the Colombian government is considering a ban on the nurse shark fishery along with an extensive habitat protection campaign.

Interesting Facts:

  • While most fish, including sharks, must keep moving in order to breath, nurse sharks can remain motionless while resting on the seafloor. They do this by facing against the flow of water and pumping the water through their mouths and gills.
  • Its name is a mix of Greek and Latin meaning “curled, hinged mouth” to describe this shark’s somewhat puckered appearance.
  • The term “nurse” might also be derived from the archaic word “nusse” meaning cat shark. However, the most likely theory is that the name comes from the Old English word for sea-floor shark, “hurse”.
  • Nurse sharks are nocturnal and will often rest on the seafloor during the day in groups of up to 40 sharks, sometimes piled on top of each other.
  • The nurse shark can use its large front fins to “walk” along the ocean floor.
  • Nurse sharks are caught and killed by fishermen in some regions because they are considered a nuisance animal that takes bait intended for other species In the Lesser Antilles, where it often raids fish traps, the shark is considered a pest. Commercial fishers in the United States routinely release nurse sharks alive.

References + For More Reading:

The Nurse Shark

NatGeo: Nurse Shark

Arkive: Nurse Shark

National Aquarium: Nurse Shark

Animal Corner: Nurse Shark

IUCN: Ginglymostoma cirratum

Ginglymostoma cirratum

National Park Service: Nurse Shark

15 Fascinating Facts About Nurse Sharks

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