"A purple Sea Fan (Gorgonia ventalina) graces the top of a coral head covered in the red encrusting sponge Clathria (Microciona) spinose at a depth of 47 feet off the Riviera Maya in the Gulf of Mexico. Although they may look like plants, both sea fans and sponges are colonial marine animals.  Sea Fans are salt water invertebrates and belong to the order Alcyonacea.  Closely related to corals, they are found throughout the tropical and subtropical oceans of the world. They mostly prefer shallow waters with constant current, but have been found several thousand feet deep.  Each gorgonian polyp has eight tentacles which catch the plankton upon which they feed.  To facilitate this ""filter feeding"", the ""fan"" is always oriented across the prevailing current to maximize the water flow and food supplied to the gorgonian. Sponges are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them.  There are 5,000 to 10,000 known species of sponges.  Sponges do not have nervous, digestive or circulatory systems, relying instead on a constant water flow through their bodies to obtain food and oxygen and to remove wastes."