The sea sponge: a complicated yet simple animal

Yes, sea sponges are considered animals not plants. But they grow, reproduce and survive much as plants do. They have no central nervous system, digestive system or circulatory system – and no organs!

Sea sponges are one of the world’s simplest multi-cellular living organisms. They grow in all different shapes, sizes, colours and textures. Scientists have identified around 8,500 species, but more than 25,000 are believed to exist in the ocean. A small percentage of sponges are harvested for commercial use (think loofahs). At one point, however, those sponges were overharvested. So today, most household sponges are made of synthetic materials.

Sponges range in size from less than 3 cm to around 2 m (6 feet) in length. They grow in all marine climates, from tropical to polar waters, and survive in all latitudes – from intertidal to the deepest regions of the sea. There are even some freshwater species, too.

About 34 species of sponge thrive off of Canada’s Atlantic coast. They can be found in shallower depths throughout the inter-tidal zone or in water up to eight kilometres deep. Much of the information on the distribution of sponges in Atlantic Canada has been collected by fishery observers on-board commercial fishing vessels. Trawl surveys have also helped identify sponge communities on the Scotian Shelf and along the Grand Banks, Flemish Cap and Labrador Shelf. Visit DFO’s page on Cold-water corals and sponge reefs for further information on sponges in the Maritimes.

Sea sponges have a shell-like or glass outer layer covered with tiny pores (ostia) that travel deep into the soft structure within. These pores allow water to flow in and out of the sponge. In this way, the sponge gathers the food and oxygen it needs to thrive, and releases waste.

Sea sponges populate in three different ways:

• ‘sexual reproduction’: they release sperm into the water to fertilize eggs, which form larvae that swim off in search of places to settle.
• ‘fragmentation’: sponges are known for regenerating from fragments that break off, although this only works if the fragment includes the right type of cells.
• ‘budding’: when conditions deteriorate (e.g. water temperature changes), many freshwater species and a few marine ones produce gemmules (‘survival pods’) of unspecialized cells that remain dormant until conditions improve and then either form completely new sponges or recolonize the skeletons of their parents.

Adult sponges are basically sessile, meaning that they are immobile. But some marine and freshwater species can move very slowly across the sea bed as a result of amoeba-like movements of cells. A few species can contract their whole bodies, and many can close their small mouth-like apertures, or oscula, and the ostia. Juveniles drift or swim freely.

Scientists have only recently begun to look at the function of sponges in terms of how they help with nutrient cycling^{Footnote 1} and how they enhance the diversity of other types of animals. But so far their understanding is limited to a few key species.