Sponges are as un-animal-like as animals can be. They don’t move around or make noises. They don’t have sense organs or heads or brains. They were thought to be plants until 1825 when they were finally racognised to be simple multicelled animals. They are also common and widespread with about 8000 species worldwide. They are found in all oceans in the benthos and are important members of these ecosystems. There is also a group of freshwater sponges but the majority are marine.
Sponges are filter-feeders. They take in water through pores to tiny canals and filter out food particles. These pores give the phylum its name: Porifera. The outside of the sponge is covered in ectodermal cells which protect the sponge. The inside of the sponge contains endodermal cells resembling flagellate protozoans which use their flagellae to pick up food particles. Because they have only two cell layers, they are classed as Parazoa and are considered to be more primitive than the Metazoa, which have three cell layers.
As filter feeders, sponges are important for capturing and recycling nutrients in benthic ecosystems. As animals and plants die in the plankton, their bodies sink and the nutrients could be lost but for filter feeders capturing them and recycling them. Sponges are particularly important in coral reef communities for nutrient recycling and water purification. In other marine ecosystems, sponges replace the corals as the main structural organisms around which the rest of the benthic communities live and grow. Many fish hide and find shelter among sponges.
Sponges also form symbiotic relationships with bacteria and algae. The bacteria may remove harmful ammonia products from the water and convert it to nitrogen gas. The algae gain shelter in the safety of the hard, inedible sponge body and give these sponges their bright colours. Some hermit crabs attach sponges to their shells for camouflage. Many sponges practice chemical warfare, releasing toxins around them to kill off neighbours, giving themselves more growing room.
Sponges are classified by their spicules, which form the hard skeletal base of the sponge. Class Calcarea are sponges that make their spicules out of calcium carbonate crystals and includes the simplest sponge species. Class Hexactinellida are the glass sponges which have skeletons made of six rayed silicon-based spicules. Class Demospongia are sponges whose spicules are made of spongin (a form of keratin) and silicon. The last class is the Sclerospongia, the members of which use all three substances, silicon, calcium and spongin, to make their skeletons.
Sponges can reproduce by asexual budding or by sexual reproduction of gametes that are released into the water. The sponge larvae swim around for a short period and then find a place to settle and develop into the sedentary adult form. Most adult sponges don’t move but in aquaria, some species have been known to move slowly over the bottom, looking presumably for a better site to live.
Sponges are aerobic but have no specialised respiratory organs. Instead, the circulation of water through the sponge channels allows exchange of oxygen from the water and carbon dioxide from the cells.
Sponges evolved half a billion years ago wtih the first multicellular animals. They are not complicated and have changed little over the years but they are obviously quite successful at what they do and are an important part of benthic and coral reef communities.
Meglitsch, P. 1972. Invertebrate Zoology. Oxford Univ. Press