![]() ![]() ![]() Sponges are known for regenerating from fragments that are broken off, although this only works if the fragments include the right types of cells. The fertilized eggs form larvae which swim off in search of places to settle. Most species use sexual reproduction, releasing sperm cells into the water to fertilize ova that in some species are released and in others are retained by the "mother". A few species of sponge that live in food-poor environments have become carnivores that prey mainly on small crustaceans. While most of the approximately 5,000 - 10,000 known species feed on bacteria and other food particles in the water, some host photosynthesizing micro-organisms as endosymbionts and these alliances often produce more food and oxygen than they consume. Although there are freshwater species, the great majority are marine (salt water) species, ranging from tidal zones to depths exceeding 8,800 m (5.5 mi). ![]() Many sponges have internal skeletons of spongin and/or spicules of calcium carbonate or silicon dioxide. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where it deposits nutrients, and leaves through a hole called the osculum. Unlike other animals, they lack true tissues and organs, and have no body symmetry. Sponges are similar to other animals in that they are multicellular, heterotrophic, lack cell walls and produce sperm cells. ![]()
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