Their varied shapes (tube-like, barrel-like, fan-like, cup-like, branched, and irregular shapes) are structured to provide optimal water flow. This is vital as sponges do not have a circulatory system, respiratory system, digestive system, muscular system, or nervous system as do many other animals. Water circulating through pores allows for gas exchange as well as food filtration. Sponges typically feed on bacteria, algae, and other tiny organisms in water. To a lesser degree, some species have been known to feed on small crustaceans, like krill and shrimp. Since sponges are non-motile, they are typically found attached to rocks or other hard surfaces.