In an earlier column we looked at Golf Ball Sponges. These regular little animals are odd ones out among sponges where many species cannot be easily identified by colour or shape.

Many sponges tend to grow to suit their habitat. Where seas are rough they may take on encrusting forms, and in calmer water they are erect. Within the one species, colours can vary. There are around a thousand species of sponge in southern Australian waters, so how do the experts tell which is which?

Researchers have to resort to the microscope to classify sponges according to the shape of the spicules and what these are made of. Spicules are inorganic crystals embedded in the tissue of the sponge. They come in a wide variety of shapes from needles, to barbed arrows, to spiky balls. Most of our common sponges have spicules made of silica.

As well as helping giving the sponge shape, the spicules are a great deterrent to predators. Most sponges also produce toxic chemicals, some of which are being studied for their potential as drugs or use in fighting fungi and bacteria. Only a few predators, some nudibranchs among them, will eat nasty-tasting splinter-filled sponges.

Sponges are usually hermaphrodites producing both eggs, which are temporarily retained in the cells, and sperm, which are released. Sperm from a neighbouring sponge attach to the outer cells or are drawn into the animal and attach to the lining cells.

The fertilized egg is released and develops into a planktonic larva that swims with the help of flagellae (whip-like projections) on most of its surface cells. It eventually settles, attaches itself firmly, loses the outer flagellae and grows into a mature sponge.