Sponges are probably the least animal-like of all animals. They are the most primitive of the multicellular animals. They lack all internal organs, including sensory organs and do not have any kind of neural system. They can almost better be described as a colony of single celled animals. However, they do have specialized cells that play specialized roles within the sponge.

Sponges are entirely immobile; all known species live affixed to some type of surface. There are a great many species and they grow in a variety of forms. Sponge shape can be classified into several rough groups: spherical, plate shaped, branching, conical, cylindrical, and plaque- or mat-like. What’s more, even within a single species, there can be great variation in the shape of individual colonies.

Almost all sponges are filter feeders, though ther are a few groups that feed by trapping tiny animals that come in contact with their sticky surface. Sponges have intricate networks of channels through their tissue. These channels are lined with small cells called choanocytes that synchronously wave their flagella to draw seawater through pores into the sponge. Small food particles are filtered from the water by other specialized cells before the seawater is ejected through larger openings.

Many sponge species are found in Icelandic waters and they are especially conspicuous on hard bottom such as on the hydrothermal chimneys. However, it is very difficult to identify sponges to species because of the great variation within each species and because many species are very similar. To identify the species a specialist is needed to look at the internal structure in a microscope.

Bathroom sponges were originally made from warm water sponge species. However, this is inadvisable with Icelandic cold water sponges as they contain small needle-like spicules that can badly irritate the skin. Recently interest in sponges has increased greatly for two reasons. First, large areas on the continental shelf can be covered by sponges. These increase the habitat diversity on the bottom and this in turn increases species diversity. These sponge areas have been damaged by bottom trawling in many areas, therefore, there is an on-going project to map and protect the remaining areas.

Secondly, sponges produce many types of biochemicals 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. Many of these substances have been shown to be useful when synthesized in a lab or concentrated by medical or chemical researchers. Some have been shown to be useful as medicine.