The term Nurse Shark is probably derived from “Nusse”, an old common name for cat sharks belonging to the family Scyliorhinidae. Today, the Nurse Shark is considered a part of the family Ginglymostomatidae. This family is probably rather closely related to the whale shark family Rhincodontidae, since they have similar reproductive cycles. The taxonomic history of the Nurse Shark has taken some turns before the scientific community agreed on calling the Nurse Shark Ginglymostoma cirratum and placing it in the family Ginglymostomatidae. Bonnaterre was the first one to describe the Nurse Shark, and he named it Squalus cirratus in 1788. Lapepéde changed the name to Squalus punctulatus in 1800, and just one year later the Nurse Shark was called Squalus punctatus by Bloch and Schneider. Bancroft called the same animal Squalus argus in 1832, Poey coined Ginglymostoma fulvum in 1861, and in 1867 Capello suggested Ginglymostoma caboverdianus. The current name, Ginglymostoma cirratum, was introduced in 1841 by Muller and Henle. The term Ginglymostoma is a combination of two Greek words – gynglimos and stoma. Gynglimos means hinge and stoma mouth. The species name, carratum, is not a Greek word but the Latin word for curl.
The average size varies between male and female Nurse Sharks. The males are averagely 7-8.5 feet (210-260 centimetres) long and typically weigh around 200-267 lbs (90-120 kg). The female Nurse Sharks grow longer than the males, and average at 7.5-9 feet (220-270 centimetres). Females typically weigh between 167 and 233 lbs (75 and 105 kilograms). The size at maturity is around 7.5 feet (225 centimetres) for female Nurse Sharks and 7 feet (210 centimetres) for males. Both male and female Nurse Sharks are recognized by their two spineless, rounded dorsal fins. The second dorsal fin is much smaller than the first. The caudal fin makes up more than 25 percent of the total length of the Nurse Shark. The spiracles are very small on a Nurse Shark. Nurse Sharks are without any perinasal grooves, but do have nasoral grooves.
An adult Nurse Shark typically display light yellow to dark brownish colours, but there are also many reports about brilliant yellow and milky white Nurse Sharks. The young Nurse Sharks, those who are smaller than 23 inches (60 centimetres), have small dark spots on the entire body. Each of these spots is encircled by an area of lighter pigmentation. Young Nurse Sharks are capable of performing colour changes in order to adapt them selves to the surrounding environment. Being exposed to full sunlight tends to make them darker than when they are covered from the light.
When a Nurse Shark looses or hurt a tooth, or when the tooth is worn down from use, the tooth will be replaced. The replacing process is faster during the summer, when temperatures are higher, and juveniles replace teeth faster than adult Nurse Sharks do. The Nurse Shark is equipped with teeth that do not overlap each other. This is called independent dentition, and is the simplest variant of tooth arrangements found in sharks. If a shark has overlapping dentition, a tooth can not be replaced before outer blocking teeth are lost. In the Nurse Shark, the forward movements of teeth that lead to shedding are never depending on other teeth.
The Nurse Shark feeds primarily on fish, such as stingrays. They also eat octopus, squids, clams and other molluscs. Crustaceans are also popular and corals and algae are sometimes found in the stomach of Nurse Sharks. The Nurse Shark is a nocturnal hunter and often feed on small fish that rests during the night. It has a relatively small mouth, but is also equipped with a large pharynx that makes it possible for the Nurse Shark to suck in food items at a very high speed. The Nurse Shark has also been seen eating heavily shelled conches by flipping them over and extracting the snails by sucking and biting. Young Nurse Sharks are sometimes seen staying very still in the water, supporting them selves on their pectoral fins. They point their snouts upwards, and we still do not know for sure why the do this. One theory is that the young Nurse Sharks use their bodies to provide a shelter for small fishes and crabs, animals that can then be easily surprised and eaten by the sharks.
The Nurse Sharks mate during the summer, usually in June or July. The male Nurse Shark moves near the female when she is resting on the sea floor, or when she is swimming slightly above it. The male Nurse Shark will bite one of her pectoral fins, and try to turn her over by pushing her. When she is pushed over onto one side, it is easier for the male Nurse Shark to penetrate her with his clasper. He will still need to vigilantly bend the lower part of his body towards her cloaca for penetration to be possible. A female Nurse Shark is often courted by a lot of male Nurse Sharks that try to mate with her, and you can therefore often see a large number or scars and injuries on her body. The females are often reluctant, and will try to avoid mating by swimming in very shallow waters where they can hide their pectoral fins by burying them in the sand. After mating, the embryos will develop in an egg case inside the shark's ovary. The Nurse Shark is an ovoviviparous shark, which means that the embryo will not receive any nutrition from a placenta or similar. Instead, each Nurse Shark embryo is equipped with its own yolk sac. One brood usually consists of 30-40 embryos, and birth occurs in November or December the year after mating. The reproductive cycle of the Nurse Shark is biennial, and the gestation period lasts around six months after mating. After that, it will take an additional 18 months before the ovaries are ready to produce mature eggs for a new breeding. When the young Nurse Sharks are born, they measure 11-12 inches (28-30 centimetres). They grow around 5 inches (13 centimetres) a year and gain roughly 5 lbs (2.3 kg) during one year. Once they have reached maturity, the growth rate will slow down considerably.
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