Marine animals, such as hawksbill sea turtles, angelfish, sea slugs, some starfish and the larvae of sponge-flies, feed on sponges. Not many marine animals eat sponges, because multicellular organisms contain less nutrients, are tough to eat and produce chemical toxins as their form of defense.
Sponges, scientifically known as poriferans, are one of the simplest marine organisms. They do not have digestive, circulatory or nervous systems. Sponges do not have eyes, ears, mouths, noses or any true organs or tissues. They remain impassive when touched and only have a few cell types. Sponges are hermaphrodites and can produce asexually or sexually. Mainly oceanic organisms, there are more than 15,000 saltwater species and only 150 freshwater species.
Many species of sponge produce toxic substances when attacked by a predator. A fire sponge is potentially harmful when touched, producing a substance that can cause painful burning and itching. Most sponges have needle-like structures, or spicule, made from calcium carbonate, silica or spongin. These spicules deter predators and support the sponge's soft body. Most sponges are filter feeders, feeding on single-celled organisms and bacteria present in the waters in which they live. Others are carnivorous, preying on small crustaceans and other smaller marine animals.