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Most people have already seen or even used sponges, at least the synthetic ones. The most probable association with the word 'sponge' is that squirty, stinking, yellowish, synthetic thinglet in the drainer which is used to wash dishes, and which can be bought for a bagatelle in the local supermarket. But one should bear in mind that these are the synthetic sponges.
Natural sponges are the skeleton of aquatic animals. They can also be bought, but they're a lot more expensive. For this reason they are mostly used for bathing. On the next trip to the local department store, keep your eyes peeled for very organic looking, brownish-yellow things of soft consistency, resembling a brain with pores1: these are probably natural sponges. They should not be confused with loofah (or luffa) sponges, which are also a natural product, but are made of cellulose fibres (ie, sugar chains) and are obtained from courgette-like2 plants - technically speaking, these should not even be called 'sponge'. The sponge skeleton is mainly made of collagen and/or spongin fibres (ie, protein chains), usually with tiny mineral crystals embedded in it3. True bath lovers know that nothing matches natural sponges: they're softer, they don't get smelly as fast as their synthetic counterparts, they absorb more water and last significantly longer.
Plants or Animals?
Sponges are animals, although it is easy to believe they could be plants. Sponges evolved 650 million years ago4 and have been known and used by humans since ancient times. It is worth noting that the living sponge is not really used, but only its skeleton. The sponges from Kalymnos, a tiny Greek island in the Mediterranean Sea, were famous for their fine quality. Natural sponges were widely used for bathing, painting and as filling material for clothing and helmets. Until the 18th Century people thought sponges were aquatic plants - which is quite understandable, judging by the appearance of these creatures. However, as new definitions were implemented - for example sponges are not self-sufficient and they don't have cell walls - they now belong to the kingdom of animals. Meanwhile there are about 5,000 known species of sponges. Together they form the phylum Porifera5, which is Greek for 'pore bearing' - for obvious reasons.
Sponges are one of the most simple forms of multicellular animals. A sponge is mainly a mass of cells embedded in a porous protein skeleton. The size of sponges ranges from as small as a grain of sand to as big as a watermelon - some sponges are even 2m tall. In all cases their bodies have pores. Since sponges are immobile animals (they attach themselves to the edges of rocks or corals - that's another reason why people thought sponges are aquatic plants) they eat by filtering tiny organisms (eg, bacteria and plankton) from the surrounding water. For that reason sponges are sometimes referred to as 'filter feeders'6. The outer pores, called ostia, serve as an intake socket. They lead to inner pores called oscula, which is where the suction and digestion takes place. There are basically three architectures of sponges: Small simple tube-like sponges (called asconoid sponges), thick tube-like sponges (called synconoid sponges) and big complex sponges (called leuconoid sponges). The tube-like sponges filter water from the inner part of the tube to the outside, whereas the leuconoid has complex channel architectures, with influx and outflux channels and voids comparable to compressor chambers. Such a sponge with a volume of one litre can filter up to 7,000 litres of water in one day.
As mentioned above, the sponge cells themselves are only loosely connected to each other. Any cell can specialize to become one of three different cell types. At a later point the cells can decide to change functions, detach from the skeleton, move somewhere else, and transform into another type of cell. This is also one of the reasons why sponges can regenerate fast. The three cell types are:
Pinacocytes, which work like the skin, cover the exterior and non-functional interior parts of the sponge. These cells protect the inner part of the sponge from predators by releasing toxins - some of the most powerful ever found - and yucky-tasting substances.
Choanocytes, which have one tiny tail called flagellum, are in the inner part of the sponge. The tails are used to pump water in and out of the channels.
Archeocytes, which look like amoebas and are free to diffuse around in the inner part of the organism. The function of these cells is to digest nutrients.
Miscellaneous Stuff about Sponges
Sponges can reproduce both sexually (sponges are hermaphrodites) and asexually, by budding. Furthermore, sponges regenerate easily - a piece of a sponge can develop into a whole new sponge. This is also the reason why some sponges have a life span of 200 years or more.
Although most sponges are sea-water creatures, there are also many freshwater species. Sponges are widespread, being found in deep trenches as well as in shallow waters. Sponges have only a few natural enemies (some reef fish and the Hawskbill turtle) and they provide abode or shelter for many other organisms, like crabs and small fishes. This and the filtering of water from bacteria makes them essential for the ecology of coral-reefs.