Thursday, August 4, 2011

Life of Grey Nurse Shark

Life of Sea | Grey Nurse Shark (Carcharias taurus) | Grey nurse shark is also know as Sand tiger shark and Spotted ragged-tooth shark. Grey Nurse Sharks prefer to live in shallow inshore waters. They like to stay in habitats that have sandy-bottomed gutters or rocky caves and are close to inshore rocky reefs or islands. In Australia, grey nurse sharks have been regularly reported from Mooloolaba in southern Queensland, around most of the southern half of the continent (excluding the Great Australian Bight), and northward to Shark Bay in Western Australia. Grey nurse sharks have been recorded as far north as Cairns in the east, the North West Shelf in the west and also in the Arafura Sea. In Australia, they are included as an endangered marine species.

Grey nurse sharks have two large dorsal fins which are of similar size. The tail is distinctive, with the top lobe being larger than the bottom. Their upper body is bronze-coloured, while the underside is paler. They are grey to grey-brown on top, paler underneath and sometimes have reddish or brownish spots on their backs. Juveniles tend to have darker spots on the lower half of their body which fade as they get older. Despite their appearance, Grey nurse sharks are not a threat to divers or swimmers and actually have a very placid nature.The species has a conical snout, long awl-like teeth in both jaws (with single lateral cusplets), similarly sized first and second dorsal fin and an asymmetrical caudal fin. Grey nurse sharks grow to at least 360 cm total length.

They have large, sharp teeth, but they are not very strong and break easily. Grey nurse sharks is a slow but strong swimmer and is generally more active at night. Grey nurse sharks are more active at night, when they feed upon fish, smaller sharks, rays, squid and crustaceans. Male sharks reach sexual maturity at 4 - 6 years of age, and females at 6 - 8 years. Both males and females mature at about 2.2m and reach a total length of about 3.6m. Pups measure an average of 1m in length at birth.

The breeding of Grey nurse sharks is quite unusual. Mating occurs mainly in autumn and is followed by a 9 - 12 month gestation period, and the young are born in winter. Towards the end of the gestation period, the more fully developed embryos eat the less developed embryos and unfertilized eggs within the female shark’s uterus. As a result, only two pups are produced per litter – one in each uterus. Grey nurse sharks tend to breed only once every two years. This is the lowest reproductive rate of any shark and makes it more susceptible to external pressures that increase mortality.    

Many sharks lead extremely active lives, so they need an efficient supply  of oxygen to their muscles and organs. Sharks breathe out  oxygen from the water by taking it in through their mouth, allowing it to flow over their gills and then expelling it through their slits. Sluggish, bottom-living species such as Catsharks and Wobbegongs use special muscles to pump water over their gills even while at rest. But fast, highly active species such as Mackerel sharks use their forward motion to force water through their gills; a  process known as ram-jet ventilation. These sharks must keep swimming in order to breathe. Between these extremes are species like the Grey nurse sharks which pump water over its gills at rest, but switches to ram-jet ventilation when swimming to save energy.  

Grey nurse sharks have suffered a decline over recent years which has led to them being listed as “Critically Endangered” under the Commonwealth Government’s Environment Protection and Biodiversity Conservation Act (1999). Based on a New South Wales Fisheries survey in 2000, it is thought that the number of Grey Nurse Sharks in New South Wales could be as low as 292.

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