Identification: Sponges are very difficult to identify. A single species may vary in size, shape and colour depending on the conditions where it grows. To help identify them, scientists at the Queensland Museum record them at all stages: collection in the field; laboratory preparation; and storage in the Museum’s scientific collection. More than 1500 species have been named so far in Australian waters, and several thousand other species have been discovered that are thought to be new to science. Describing and naming these new species takes a lot longer than their initial discovery, involving a branch of biology known as taxonomy.
Classification: The current classification of the Porifera is based on features of the organic skeleton (collagen fibres and filaments) and the inorganic skeleton (separate and/or fused spicules composed of calcium carbonate or silicon dioxide). There are three distinct classes of living sponges: the Calcarea; Hexactinellida; and the Demospongiae.
Obtaining Food: Sponges in two orders of the Class Demospongiae, rely on symbionts to produce food for them. In the case of these sponges, it is cyanobacteria that produce photosynthetic products that are used as a food source for the sponge. These ‘autotrophic’ sponges live in a symbiotic association with the cyanobacteria and are found in the reef flats and shallow rock pools of the GBR (Great Barrier Reef).
Large, heterotrophic sponges are found on the reef slope and across the shelf of the GBR. They feed on food particles and waste products filtering down from the coral reef above. These sponges have flexible whips, fingers and fans adapted for coping with high currents. Others have soft tubes, vases and other shapes that are common in silty, turbid water where inhalant and exhalant pores are located on different surfaces to prevent smothering.
Recycling on the Reef: A special group of sponges is responsible for most of the carbonate recycling on the reef. They are called excavating (‘boring’ or bio-eroding) sponges and they are responsible for extensive damage to hard and soft corals. Some species degrade live coral while others burrow into dead coral. Another group, the clionaid sponges, produce acid phosphatase and lysosomal enzymes that dissolve the organic matter of the coral. This produces limestone chips that are released into the sea water through the sponge’s exhalant canal. In this way calcium carbonate is recycled in marine systems for uptake by animals and plants with limestone skeletons, such as corals, molluscs, diatoms and coralline algae.