Sponges are probably the least animal-like of all animals. They are in fact the most primitive of the multicellular animals. They lack all internal organs, a neural system and sensory organs and look a bit like a colony of single celled animals. However, they do have specialized cells that have different roles within the sponge. Sponges cannot move at all and can look like mats or spongy balls. Some species are erect and branch out like plants.
All sponges are filter feeders. Small cells, called choanocytes, synchronously wave their flagella within the sponge and therefore draw seawater through tiny pores into the sponge. They filter out small food particles by other specialized cells before ejecting the seawater through larger openings.
Many sponge species live in Icelandic waters and they are especially conspicuous on hard bottoms 12. However, it is very difficult to identify sponges to species because there can be a great variation within each species and many species are similar as well. To identify the species a specialist is needed to look at the internal structure in a microscope.
Previously, common bathroom sponges were made from certain species of warm water marine sponges. Nowadays plastics have mostly replaced them. However, one should not use Icelandic waters sponges for these purposes as they contain many needle like spicules that can irritate the skin badly.
Recently interest in sponges has increases greatly for two reasons. The first is because sponges can cover large areas on the continental shelf. These increase the habitat diversity on the bottom and this in turn increases species diversity. These sponge areas have been damaged by fishing gear in many areas and therefore there is an on-going project to map and protect the remaining areas.
The other reason is that sponges produce many types of chemicals that are of interest for the biotechnology industry. Animals that cannot move like the sponges have to use other means to defend themselves. The spicules are part of the defence but the sponges need more. They also produce bad tasting or toxic chemicals for defence. Fishermen that accidentally catch large sponges know that they emit a strong smell. This is a smell of chemicals that could possibly be used for medical purposes.