On many of our Nature Coast Marine Group’s rock platform excursions people have been intrigued by the orange blobs about 2-3cm in diameter attached to rocks in shaded crevices near the low tide mark. These are Golf Ball Sponges (Tethya corticata), so called because of their shape. Sponges are animals with a structure and lifestyle very different to most animals we are familiar with.

Sponges have a single outer layer of cells and an inner layer that lines tubes and chambers inside. Small pores in the outer surface of Golf Ball Sponges let in water to cavities below. From there the water travels through inhalant channels to feeding chambers before being expelled through larger exhalent channels.

The lining of the feeding chambers has unusually shaped collar cells that each have a ring of fine projections, called cilia, and a central whip-like flagellum that helps propel the water through the sponge. The cilia trap sub-microscopic food particles from the water. The flagella do such a good job that a sponge can process its own volume in water in ten seconds. Even so sponges grow best in areas where there is a strong current.

Mature sponges are fixed in place, have no nervous system and can’t see or hear what is coming their way. But that does not mean they are defenceless.

The space between the inner and outer layer of cells is filled with organic spongin (found in natural bath sponges) and many have sharp inorganic spicules. In the case of Golf Ball Sponges the spicules are made of silica. Bundles of long straight needle-like spicules radiate from the centre of each ball and near the surface there are huge numbers of tiny spiky spherical ones. Toxic chemicals in the sponge also deter predators.

Golf Ball Sponges can reproduce by budding. They do this by putting out thin strands of tissue, each thicker at the end. The ends then develop into mature balls. One or two larval sponges can eventually develop into the large populations found in some crevices and underwater caves.