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Molluscs Including Nudibranchs And Giant Clams

NudibranchWith over 100 000 known species in the world, molluscs are one of the largest divisions in the animal kingdom. They take many forms, but all share certain characteristics. They have soft bodies with a muscular foot and mantle - a flap of tissue covering the body. In a space between the mantle and the body, the mantle cavity, are gills, used for breathing. Most molluscs have a shell, but there are a few which have internalised it or do without one altogether. There are four main groups, gastropods, bivalves, cephalopods, and chitons.

Gastropods

Whelk This is by far the largest mollusc group with 80 000 different species. The name means "stomach-foot" and most species conform to the basic pattern of one shell with a large foot. To feed, the Gastropod uses a flexible ribbon-like tongue, covered with rows of tiny teeth. Like a file, this "radula" is used to scrape up food such as algae. Many Gastropods also sport a horny or shell-like stopper called an operculum, which can be pulled into the opening of the shell when the animal is retreating from danger or desiccation. "Eyes" are usually tiny black dots on the head, which are a little more than light sensitive spots.

Some of the more well know Gastropods are the Cowrie, in which there is over 200 different species, the Cone Shell which are loaded with venom capable of killing a human. Strombs which include the heavily spined Spider or Scorpion shells. Limpets, which are commonly found, stuck tight to rocks, as well as Top Shells and Turban Shells, and the Drupella, which may be small but are numerous and have caused extensive damage to reefs in Western Australia.

Nudibranch The most unexpected Gastropods are the ones with no shell at all. They belong to a branch of the Gastropod class, the sea slugs. Included in this group are bubble shells, which may have external, internal or minimal shells, as well as various sea slugs and sea butterflies, which swim with flexible wings and feed on plankton. The most favourite amongst divers in this group is the nudibranch, meaning naked gill. They are marvellously bright and have colourful patterns, which enables them to camouflage with the reef. They are small but simply beautiful and the varieties are endless.

Bivalves

This is the second largest group of molluscs with 15,000 species. It's name, meaning "two shells", accurately describes the basic difference from the Gastropods. Bivalves are usually sedentary, either attaching themselves to something or burrowing into it. They lack the radular teeth of other molluscs and are mainly filter feeders, sieving plankton from water which is passed through their greatly enlarged gills. They have also lost their heads so their eyes or more accurately, light sensitive spots are arranged on the mantle, at the edge of the shell, alongside tentacles which are sensitive to touch and smell.

The best known bivalve would have to be the humble oyster. They cluster together in intertidal zones and glue themselves to anything from rocks and coral reefs, to cement pipes and mangrove roots. Oysters were very heavily harvested from the reef in the late nineteenth century. These oysters were the Pearl Shells. Both the shell itself "Mother of Pearl", was used for a variety of things and a bonus was an actual pearl. Pearls are now cultured on farms. The muscle inside the shell is a delicacy worldwide.

Another delicacy is the Scallop. The scallop is a mover and if disturbed can simply shoot out a jet of water, which causes it to leap away.

Pipis are a common site on the waters edge. Uncovered briefly by a wave, they quickly disappear drawn back into the sand by their muscular feet. Pipis are also eaten by humans but are more commonly used by anglers as bait. Another Bivalve is the shipworm, which is not, really worms at all; they like to bury themselves into dead wood, like wharfs and of course wooden ships.

Giant Clam Probably the most popular on the Great Barrier Reef, and belonging to the bivalve family, would have to be the giant clam. There are 8 species of giant clams in the world, and 6 of them occur on the Great Barrier Reef. Not all are huge. The adult shells of the smallest are only 12-15cm long, while the largest can produce a shell over a 1 meter in length. Weighing up to half a tonne, it is the largest bivalve that has ever existed. This enormous size is due to its special relationship with certain plants.

Young clams grow slowly at first, the larger species growing fastest. Size is not an indication of great age, most adults living about 10-30 years. The oldest reliably-dated shell (calculated by the number of growth bands) indicated an age of about 50 years.

In recent years clam farms have been set up in Australian and several other Pacific countries, one of the aims being to restock depleted reefs in areas of the Pacific where clams are an important food resource.

Many areas of the Great Barrier Reef off North Queensland have excellent species, which can be viewed easily. It is a great thrill for a diver to swim over a clam. The light sensitive spots sense that there is danger near and the giant muscle will close up.

"It is not unusual for a diver to be caught in the jaws of one of these giant clams, which clamp shut with the suddeness and strength of a bear trap." Astonishingly, this nonsense was written not as fiction, but in a 1939 natural history magazine! Still today many cartoons portray divers being caught in clams jaws, and maybe this fostered illusion of danger may save the lives of many giant clams, as they really do not like being touched.

Even a soft prod damages the tissues of our mantles, and jewellery and sunscreen even worse. Divers have even been observed dropping things inside our shells. Once in, an object stays, the clam then develops an infection and not too much later there is an empty shell on the reef. - Just remember, don't touch just observe then both of you will remain intact!

Cephalopods

The "head-foot" molluscs are the brainiest of all invertebrates and among the most surprising of molluscs. Most have no shells and extremely well-developed eyes. Using their siphons to force out water they can propel themselves rapidly (and usually backwards) through the water, positioning the siphon to control direction.

The nautilus belongs to an ancient family, which evolved about 550 million years ago. For active predators a heavy shell, although providing protection, restricted movement. The nautilus evolved a way of overcoming this, by floating. The animal occupies only the part of the shell closest to the opening. The rest of the shell is a series of empty chambers, which the animal fills, with gaps of gas, or floods, depending on how buoyant it wants to be. It's molluscan foot has evolved into 60-90 arms with which it can grasp it's food, crustaceans and carrion. Nautiluses inhabit deep water. If it is nautilus you especially want to see, only one company, Undersea Explorer, specialise in these unique creatures, diving and capturing, studying and releasing them.

Cuttlefish have retained an internal shell. Like the nautilus it can also fill gaps with gas and use its shell as a flotation device. Squids and Cuttlefish have 10 arms including two large ones which actually reach out and pull prey such as prawns into it's parrot like beak. They are very mobile and their eyes are thought to better than a humans. Some species grow to enormous sizes.

Octopus Octopuses have dispensed with shells altogether, except for the related female Argonaut, which produces a nautilus-like shell as an egg capsule. Although the octopus cannot jet around quickly if necessary they prefer to walk on their eight arms and lurk inconspicuously in rock and coral crevices. Like squids they are able to squirt black ink into the water creating a visual, and possibly scent, screen while they make a swift escape. Both animals can also change colour, to camouflage themselves or to communicate.

Chintons

These "Coat Of Mail" molluscs are a very ancient group, found in all seas from tropical to polar and from intertidal zones to deep depths. They have eight articulated shell plates surrounded by a leathery girdle. If dislodged from its home, a Chinton can curl it's shell around to protect it's vulnerable underside. The most common is found stuck fast to rocks, however others can be found on seagrasses, sand, mangroves roots or even boring onto coral.

There are over 4000 shell-producing molluscs on the Great Barrier Reef, so it is not surprising that shell-collecting is popular with tourists in search of a souvenir of their visit. In most cases these casual collectors pick up a few dead, empty shells on the beach but after many years of cumulative small-scale collecting some popular beaches are becoming depleted of shells - leaving hermit crabs with a housing shortage crisis.

Shell Collecting

Shell collecting in the Marine Park like other activities, is managed through zoning, which allows for limited collection in the General Use Zone. The Giant Clams, Large Helmut Shell and Triton, or Trumpet Shell are protected and must not be collected.

Visitors are encouraged to enjoy viewing live shells in their natural environment, without collection. For those who wish to collect, take dead shells wherever possible and observe the "Shell Club" collecting code

  • Do not break coral in search for shells
  • Return all overturned rocks carefully to their original position
  • Do not remove juveniles or eggs
  • Collect only sufficient for your own needs

A great saying amongst the operators who use the Marine Park is "Take nothing but photographs and leave nothing but footprints".

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Diving Cairns - The ultimate guide to a diving holiday on North Queensland's Great Barrier Reef