Porifera, commonly called sponges, are an extremely primitive organism, which are among the oldest animal fossils on record. Some sources suggest that they only just fit into the Kingdom, Animalia, as they do not have true organs, nerves or muscle cells.1 They can have organic skeletons, made of collagen fibers and filaments, as well as inorganic skeletons, made of separate, or fused, spicules of Calcium Carbonate, or Silicon Dioxide.
Porifera may be autotrophic, harnessing cyanobacteria in a symbiotic relationship, or heterotrophic, feeding on the particles in the water. Heterotrophic feeding occurs not through a mouth, but through tiny pores in their outer walls, with small flagella ensuring that the water flows in one direction and out through larger openings.1 With that being said, there is even one family of sponges, Cladorhizidae, which are carnivorous, trapping small animals such as shrimps in their spicules. The cells then migrate around the prey and consume it whole!2
Some sponges are also toxic, to avoid predation, compete for space and dig holes in the coral. The latter is a vital process in the recycling of Calcium Carbonate. The excavating, also known as boring or bio-eroding, sponges, release enzymes to dissolve the organic matter of coral. This releases calcium carbonate into the water column to be used by other organisms, such as coral and molluscs. They even use chemicals to attract commensal organisms, such as shrimps and worms.3
You can find a list of all of the groups of Porifera found on The Great Barrier Reef, below.
Class – Next level of classification, grouping animals with similar characteristics
Calcarea – Sponges with Calcium Carbonate Spicules
Demospongia – 90% of all Sponges
Hexactinellida – Glass Sponges