For many years, the Greynurse Shark was accused of being a 'maneater'. This is far from the truth.
The Greynurse Shark is a distinctive fish which is usually grey-brown on top and a dusky white underneath. Both dorsal fins and the anal fin are of a similar size.
Juveniles have reddish or brownish spots on the posterior (back) half of the body and tail. These spots often fade as the shark ages, but are sometimes still visible on adults.
The Greynurse Shark lives in shallow coastal waters from the surf zone down to 60 m, although it has been recorded from water as deep as 190 m.
During the day, individuals are usually found in the vicinity of dropoffs, caves and ledges.
The Greynurse Shark occurs in tropical and temperate waters in the Atlantic, Indian and western Pacific Oceans, although the species is known by different common names in different regions. It is recorded from all states of Australia except Tasmania.
The map below shows the Australian distribution of the species based on public sightings and specimens in Australian Museums. Click on the map for detailed information.
Feeding and diet
Greynurse Sharks feed on fishes, which are pierced with the sharp teeth.
In some parts of the world, the species is known as the Spotted Ragged-Tooth. The reason for the this name is obvious. The species has fang-like teeth which are visible when the shark's mouth is closed. Greynurse Sharks are not however the 'maneaters' that some people have considered them to be.
The teeth of the Greynurse Shark are constantly being replaced. Older, damaged or blunt teeth on the exterior surfaces of the jaws are replaced by new teeth. In the whaler sharks, family Carcharhinidae, each tooth is replaced every eight to fifteen days.
The genus name Carcharias comes from the Greek carcharo, meaning sharp pointed or jagged, and refers to the impressive dentition.
Other behaviours and adaptations
Many sharks have a tail with a long upper lobe and a shorter lower lobe. This type of tail is described as heterocercal. When such a shark swims, the large upper lobe tends to push the snout of the shark down. This is balanced by the lift produced by the pectoral fins and the ventral surface of the snout.
The tail of the Greynurse Shark is heterocercal and has a characteristic subterminal notch.
Sharks have the same five senses as humans; taste, touch, sight, hearing and smell, but in addition have a sixth sense; electroreception. The underside of the Grey Nurse Shark's snout is dotted with pores. Each of these leads to an organ (ampula of Lorenzini) which can detect electricity.
Sharks can detect very weak electrical currents. This extra sense gives sharks the ability to detect and attack prey at close range without needing to see the prey item. This can be advantageous in murky water or if the shark is a bottom feeder which relies on finding prey buried in the sediment.
Some sharks, such as the Great White, roll their eyes back in the sockets immediately before attacking prey. At this time, the electrosensory ability is most important because the shark cannot rely on sight.
The electroreception capabilities of sharks also gives them the ability to navigate using the Earth's magnetic field. This is possible because an electrical conductor (the shark) moving through a magnetic field (the Earth's) generates an electrical field through the conductor. Sharks can navigate by responding to changes in this electrical field.
Large Oily Liver
Many bony fishes have a structure called a swim bladder which is used to control buoyancy. Sharks do not have swim bladders. Instead, they have different buoyancy regulating mechanisms that involve the liver and fins. The oily liver of the Grey Nurse Shark has been developed to a huge degree and occupies a large proportion of the body cavity. Shark oil is lighter than seawater and this gives the shark buoyancy.
The Greynurse Shark also swallows air at the surface, and holds it in the stomach. This provides buoyancy and enables the shark to hang almost motionless above the bottom.
In the early 1900s, shark liver oil was used to light the street lamps of Sydney.
Greynurse Sharks are countershaded, the dorsal (upper) part is dark, mostly a grey to bronzy colour whereas the ventral (lower) part of the body is pale.
Many fishes that swim in open water are countershaded. This adaptation results in the fish being difficult to see from above because the dark colour of the dorsal surface of the fish blends into the dark colour of the water below. It helps to make the fish also less visible from below because the light colour of the underside of the fish is less noticeable against the light shining from above.
Life history cycle
After fertilization, the developing young are enclosed in egg cases within each uterus of the female. They hatch from the egg cases at about 55 mm in length and then eat not only unfertilised eggs, but also their siblings. After about nine to twelve months two young are born, one from each uterus.
Greynurse Sharks are ovovivaparous. They produce eggs which hatch inside the female and have no placental connection. Other sharks such as the Port Jackson Shark lay eggs (oviparous sharks), or have a true placental attachment and give birth to live young (viviparous sharks), such as the Blue Shark.
The Greynurse Shark is not the only shark in which the young are oophagous (egg eaters). Others include the Shortfin Mako Shark and Porbeagle Shark.
Internal fertilization occurs in all sharks. Sperm transfer occurs through the claspers of the male.
Pogonoski et. al. (2002), list the conservation status of the species as endangered.
Danger to humans
It is generally a slow-moving species that is not considered dangerous to people, although it should never be provoked by divers.
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