Sponges filter sea water to eat, exchange gases and excrete waste products. Filtration is an active process involving choanocytes lining chambers. Each choano-cyte has a central flagellum that actively beats to create a water current, surrounded by a collar of cilia that traps food particles such as plankton and bacteria, as well as detritus. A water current containing food enters the sponge through an osculum and is initially filtered through a series of sieve-like pores (diminishing in size), finally ending up at the collar cells. Food particles are actively carried across the cell wall, engulfed by archaeocytes, and are transferred throughout the mesohyl to other cells. Filtered water leaves the sponge via the exhalant canal system. Unlike most multicellular animals, digestion and excretion of waste products occurs within cells, not within any common body cavity. There may be 7000-18 000 cho-anocyte chambers per cubic millimetre of sponge, and each chamber may pump approximately 1200 times its own volume of water per day. Thus, a sponge is capable of pumping around 10 times its body volume each hour, making them the most efficient vacuum cleaners of the sea. Some sponges, particularly those growing on coral reef flats, also have a unique symbiosis with cyanobacteria, providing the sponge with nutrients derived from photosynthesis to supplement those obtained by the sponge from normal filter feeding activities (phototrophy or autotrophy). These extra nutrients greatly augment sponge growth rates and competitive ability in coral reef systems. There are also often huge populations of bacteria and Ar-chaea living within sponge cells and/or within the sponge mesohyl (hence the term 'sponge hotels'), with

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