Nurse shark (Ginglymostoma cirratum)
|Size||Length: up to 3 m (2)|
Classified as Data Deficient (DD) by the IUCN Red List. The Western Atlantic subpopulation is classified as Near Threatened (NT) (1).
The strange looking nurse shark is not the heavy, fearsome fish we normally expect a shark to look like. Its 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 (2). The large, rounded pectoral fins are flexible and muscular, and can be used as limbs to clamber along the sea bottom (2). The head is broad and flat, with small jaws housing small teeth (3) and fleshy, sensory projections (barbels) hang down by its mouth (2). It is not entirely clear where the nurse shark got its strange name from, perhaps from the ancient English name for a dogfish, ‘huss’ (4). More suitable is its scientific name which translates in Latin as ‘the shark with the flexible curly mouth’ (4).
Occurs in the Western Atlantic, from Rhode Island in the United States to southern Brazil; the Eastern Atlantic, from the Cape Verde islands to Gabon; and the Eastern Pacific, from the Gulf of California to Peru (2).
The nurse shark inhabits inshore tropical and subtropical waters, where it is found at depths of less than one meter down to 130 metres (2). It is frequently found on rocky and coral reefs, and in channels between mangroves keys and sand flats (2).
During the day the nurse shark is a sluggish animal, spending most of its time resting on sandy bottoms, or in crevices in rocks and coral reefs. Often several nurse sharks may congregate when resting, sometimes even piling on top of one another (2). At night however, the nurse shark actively roams the sea bottom and reef for prey. It feeds on bottom invertebrates such as spiny lobsters, shrimps, crabs, sea urchins, squid and octopi (2). It is also feeds on fish, its nocturnal nature enabling it to prey on resting fish that would be too active during the day to capture (2). The nurse shark’s small mouth and large pharynx enable it to feed by a unique suction method, effectively making the nurse shark the ‘hoover’ of the ocean floor. By cupping its mouth over a hole or crevice and expanding its throat, it creates a vacuum that sucks prey out of their hiding (4). This suction-feeding is also useful for extracting snails from their shells (2), and it will dig in sand to root out prey sensed by its fleshy barbels (2).
The nurse shark is one of few sharks in which courtship behaviour is relatively well known (4). The male swims alongside the female, grabs one of the pectoral fins in his mouth, rolls her over and they mate (4). A large number of males will often try to mate with a single female, and females often bear numerous bite-scars and bruises received during mating attempts. It is therefore 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 (5).
Nurse sharks are ovoviviparous (2), a method of reproduction whereby the young develop inside a weakly-formed egg shell within the mother, receiving nourishment from their yolk sac, for five to six months (2). The females move to shallow seagrass beds and coral reefs to give birth to 20 to 30 pups in late spring and summer (2). 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 (2).
Nurse sharks are commonly caught in small-scale local fisheries in some parts of its range, and are incidentally captured in many coastal fisheries (1) (2). Its tough, thick hide makes good leather, the flesh is consumed by humans and used for fishmeal, and oil is extracted from the liver (2). The nurse shark is also captured for the aquarium trade, and is occasionally the target of spear fishermen (1). As the nurse shark grows slowly and matures late, this exploitation can cause populations to decline rapidly and recover slowly (6). The threat of overexploitation is compounded by the impact of humans on the coastal and reef habitats of the nurse shark (1) (6). Coral reefs are a particularly vulnerable habitat, being impacted by pollution, sedimentation, global climate change and disturbance from tourism (1). Extreme population reductions have already been recorded in the southern Western Atlantic, and it is possible that the nurse shark is declining, unnoticed, in other areas where there is a lack of data (1). For this reason, the World Conservation Union (IUCN) has classified the nurse shark as Data Deficient (1).
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 (1). However, in regions outside of the Western Atlantic, the lack of data makes it difficult to assess the status of populations, and subsequently to implement appropriate conservation measures. Measures recommended include the regulation of spear-fishing and the marine ornamental fish trade, compulsory release of incidentally caught sharks, and the establishment of no-fishing areas encompassing mating and breeding grounds (1). Countries should be motivated to take action to prevent over-fishing of the nurse shark as it is likely to be far more valuable alive for dive-tourism than as fisheries products (6).
For further information on sharks and their conservation see:
- Save Our Seas Foundation:
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- Dorsal fins: the unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Nocturnal: active at night.
- Ovoviviparous: method of reproduction whereby the egg shell is weakly formed and young hatch inside the female; they are nourished by their yolk sac and then ‘born’ live.
- Pectoral fins: in fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
- Pharynx: the part of the throat that begins from behind the nose to the beginning of the oesophagus and trachea, or windpipe.
IUCN Red List (December, 2009)