The Port Jackson shark, Heterodontus portusjacksoni, is a nocturnal, oviparous type of bullhead shark of the family Heterodontidae, found in the coastal region of southern Australia, including the waters off Port Jackson. It has a large head with prominent forehead ridges and dark brown harness-like markings on a lighter grey-brown body. Port Jackson sharks can grow up to 1.67 metres (5.5 ft) long.[1]
The Port Jackson shark is a migratory species, traveling south in the summer and returning north to breed in the winter months. They feed on hard-shelled mollusks, crustaceans, sea urchins, and fish. Port Jackson Sharks are quite distinctive blunt headed-sharks that lay eggs. Port Jackson Sharks have harness-like markings which cross the eyes, run along the back to the first dorsal fin, then cross the side of the body. This pattern makes it very easy to identify this species.
[edit] Distribution and habitat
The Port Jackson shark is endemic to the waters around Australia—it can be found in southern Australian waters and west of the south central coast of the continent. It is believed to have originated somewhere off the coast of South Africa.[2] On one occasion it has occurred off the coast of New Zealand, however it is usually limited into two groups found from Northeastern Victoria to Western Australia and from Southern Queensland to New South Wales. It usually lives less than 100 metres (328 ft) underwater, but has been known to go as deep as 275 metres (902 ft).[3]
The shark's territory is habitually sited in stone settings on the bottom where it feeds. Though rocky environments are the most common sandy and muddy ones are also possibly, probably near seagrass.[3] Since the shark is nocturnal, during the day it resides in low resistance areas such as caves.[4]
[edit] Appearance
Port Jackson sharks are similar to other organisms in their genus, bearing a broad, flat head, an anal fin, and crests above its eyes. However, the species possesses characteristics that make them easily identifiable—their teeth and the harness-like markings which run for a majority of their body length. This feature runs from their eyes to their first dorsal fin and then across the rest of their body. Both of its dorsal fins are of close to equal size, each with a spine at the foremost edge. This is rumored to be of poisonous nature.[3] Other features that help distinguish them are their small mouths and their nostrils, which are connected to their mouth.[4]
The sharks have gray-brown bodies covered by black banding, which covers a large portion of their sides and back. One of these band winds over the face and progresses even to the shark's eyes. Another, harness shaped band goes around the back, continuing until the pectoral fins and sides. Thin dark stripes are also present on the back of Port Jackson sharks. These progress from the caudal fin to the first dorsal fin.[4]
Two Port Jackson Sharks, demonstrating "harness" feature.
The teeth of the Port Jackson shark are arguably its most distinguishable feature. Unlike other sharks its teeth are different in the front and back. The frontal teeth are small, sharp and pointed, while the latter is flat and blunt. These teeth are helpful for these species towards it diet, which consists of mollusks and similar organisms; they are perfect for crushing. While they are young, the sharks have sharper teeth.[3]
[edit] Respiratory system
The Port Jackson shark has five gills, the first supports only a single row of gill filaments, while the remaining four support double rows of filaments. Each of the second to the fifth gill arches supports a sheet of muscular and connective tissue called a septum. The shark possesses behind each eye an accessory respiratory organ called a spiracle. Along the top and bottom of each gill filament are delicate, closely-packed, transverse flaps of gill tissue known as secondary lamellae. It is these lamellae that are the actual sites of gas exchange. Each lamella is equipped with tiny arteries that carry blood in a direction opposite to that of the water flowing over them. To compensate for the relatively low concentration of dissolved oxygen in seawater, water passes over the secondary lamellae of sharks some 20 times more slowly than air remains in contact with the equivalent gas exchange sites, such as the alveoli of the lungs found in humans. This delay allows sufficient time for dissolved oxygen to diffuse into a shark's blood.
Port Jackson sharks have the ability to eat and breathe at the same time. This ability is unusual for sharks which mostly need to swim with the mouth open to force water over the gills. The Port Jackson shark can pump water into the first enlarged gill slit and out through the other four gill slits. By pumping water across the gills, the shark does not need to move to breathe. It can lie on the bottom for long periods of time.
[edit] Reproduction
Male Port Jackson sharks become sexually mature between ages 8 and 10, and females at 11 through 14. They are oviparous meaning that their eggs, which are laid annually, are produced in a group, rather than in live birth. Their breeding season begins in August, continuing until the middle of November while the female lays egg pairs off and on for every 10 to 14 days. As many as 8 pairs of offspring can be laid during this period. The eggs mature for 10–11 months before hatching at once from the sac. The babies, known as neonates, have an 89% mortality rate of dying before even being born.[4]
[edit] Digestive system
Digestion of food can take a long time in the Port Jackson shark. Food moves from the mouth to the 'J' shaped stomach, where it is stored and initial digestion occurs. Unwanted items may never get any further than the stomach, and are coughed up again. They have the ability to turn their stomachs inside out and spit it out of their mouths in order to get rid of any unwanted contents. One of the biggest differences in digestion in the shark when compared to mammals is the extremely short intestine. This short length is achieved by the spiral valve with multiple turns within a single short section instead of a very long tube-like intestine. The valve provides a very long surface area for the digestion of food, requiring it to pass around inside the apparently short gut until fully digested, when remaining waste products pass by. The most obvious internal organ in sharks is the huge liver, which often fills most of the body cavity. Dietary items include sea urchins, molluscs, crustaceans and fishes. Black sea urchins, Centrostephanus rodgersi are often eaten. Port Jackson Sharks forage for food at night when their prey are most active. They often use caves and rocky outcrops as protection during the day. When most people think of shark teeth, they think of large, sharp teeth like those in the film 'Jaws'. Not all sharks have teeth like these. The teeth of the Port Jackson Shark are very different. They are not serrated, and the front teeth have a very different shape to those found at the back of the jaws, hence the genus name Heterodontus (from the Greek heteros meaning 'different' and dont 'meaning' tooth The anterior teeth are small and pointed whereas the posterior teeth are broad and flat. The teeth function to hold and break, then crush and grind the shells of molluscs and echinoderms. Juvenile Port Jackson Sharks have more pointed teeth and feed on a higher proportion of soft-bodied prey than adults. They can feed by sucking in water and sand from the bottom, blowing the sand out of the gill slits, and retaining the food which is swallowed.
[edit] Relationship with humans
The shark has no major importance to humans, respectively. It is not an endangered species whatsoever and cannot be utilized as a food supply. It is, however, useful when scientists are hoping to study bottom dweller sharks or can be caught as bycatch. It also does not pose any danger to humans. No recorded attacks have occurred, but there is an existing danger from the organism.[4]
[edit] Conservation
Although listed as "Least Concern" on the World Conservation Union (IUCN) Red List, it is known that the shark's embryos experience very high mortality rates (89%). Its status is otherwise, largely unknown. Predators of the species are also unknown. Though Crested Horn Sharks are known to prey upon Jackson shark embryos, the biggest threat is probably from other sharks such as white sharks and the bluntnose sevengill shark.[4]
[edit] Gallery
| | Detail of Port Jackson shark's jaw |
[edit] References
[edit] Bibliography
- Robert Burton (2002). International Wildlife Encyclopedia. Marshall Cavendish.
[edit] External links
