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World wide, there are at least 15 000 species of sponges, with probably over 5000 in Australian Waters and around 1500-2000 in Queensland waters alone. Apart from those species used in the bath, until recently sponges were considered of little economic importance and were largely neglected by researchers. However, their popularity is on the rise.

A sponge has no organs, mouth, digestive tract, tissues, muscles, blood or nervous system. It is loosely organised collection of different cells with different functions which, if broken up by being put through a sieve, are unable to reorganise themselves into the sponge form again.

This form is basically a network of tubes through which water is filtered for food and oxygen. In one-day approximately 5000 litres of water passes through the body of a sponge the size of a tennis ball. However, food is not processed in common body cavity, as in most animals, but is caught eaten, digested and excreted within certain cells. Yet despite functioning in this way, a sponge is an individual with continuous "skin" surrounding it, not a colony.

The familiar bath sponge is actually a supporting skeleton of fibrous material known as spongin, from which the living cells have been removed. Most sponges would not be so comfortable to touch, however. Their skeletons include numerous needles (spicules) made from glass like silica or limestone. These are produced by the sponge's cells and sometimes arranged in interlocking patterns. How the individual, and largely independent cells of such loosely organised animal manage to cooperate to produce sometimes intricate lattices of spicules is one of nature's wonders.

Sponges vary greatly in their forms. Some are thin encrusting mats while huge barrel sponges grow over a metre high, resembling upturned cement mixers. Particularly common are sponges, which grow as bunches of tubes or chimneys while those, which obtain symbiotic algae often, adopt flattened and vase like light capturing shapes.

The shape or colour of a sponge is not a good clue for identification since these can vary considerably, within the same species. Shape may change according to local conditions, such as water currents, while colours are thought to be influenced by the amount of light to which sponges are exposed - and perhaps related to the presence or absence of micro-organisms on the surface. The most accurate method of identification involves studying the spicules since their size and type and the way in which they are arranged varies according to species.

Invertebrates such as sponges, ascidians and bryozoans that cannot generally (as adults) move, are potentially vulnerable. Without teeth, claws, feet, or shells with which to fight, flee or hide they would appear to be at the mercy of predators, parasites, bad tenants and competitive neighbours. However they are unarmed - their speciality is chemical weaponry. Functioning virtually as chemicals factories, they produce a huge variety of compounds, many of which may meet human needs, particularly in the medical field.

Sponges living in crowded tropical communities, such as the Great Barrier Reef produce many more chemicals than those in more isolated, colder regions. Presumably the diversity of enemies and competitors leads to greater variety of defences and offences. The Great Barrier Reef, therefore, may well be the source of medical cures for the future.

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