A sea sponge is a multicellular aquatic animal that belongs to the phylum Poriphera. The name of the phylum comes from the fact that these animals are filled with spores that help them circulate water in order to gain nutrients and oxygen and for excretion.
A sea sponge is an animal with a jelly-like structure that is covered in two thin layers of cells. They are full of pores and channels that allow water to move in and out, and they mostly are sessile or fixed into the substrate.
Sea sponges have undifferentiated cells that are capable of migrating between the cell layers and transform into specialized cell types. Because they do not have a circulatory, digestive or nervous system, they rely entirely on a constant flow of water to help them perform all the necessary functions.
The bodies have no defined shape or symmetry, as the cells can migrate through the non-living jelly matter in the middle and occupy other positions in the cell layers that form the exterior. While 5,000 to 10,000 species feed on food particles in the water and bacteria, some either collaborate with photosynthesizing endosymbionts or they are carnivorous, feeding on small crustaceans.
Sponges are distributed worldwide, and they live from the tropics to the polar regions. Many species of sponges live in relatively calm waters, as strong currents and waves could disturb the sediments, which would cover their pores. While most sponges live firmly attached to rocks, there are some species that have a root like structure which allows them to attach on soft sediments. While there are more sponges in temperate areas, they are less diverse, while at the tropics, the variety is greater, but less abundant. This could be linked to the fact that, in tropical waters, there are more predators that feed on sponges.
Although under the traditional system, there were only three sea sponge classes, there are now four admitted classes:
Calcarea or calcareous sponges
Hexactinellida or glass sponges
And the recently added Homoscleromorpha.
The differentiation between classes is mainly correlated to the composition of their skeletons, although recent classification also takes into account phylogenetical evidence.
While sponges themselves are not capable of photosynthesis, some species host photosynthesizing endosymbionts which not only provide them with enough food and oxygen – they also produce an excess which is beneficial to the ecosystem. Most such collaborations between a sea sponge and endosymbionts produce three times more oxygen and organic matter than they can consume.