Despite often being referred to as a "coral" in the aquarium trade, the blue sponge (Haliclona caerulea or Collospongia auris) is actually a primitive invertebrate filter feeder, not a true coral. It features a striking, vivid cobalt-to-violet hue with a matte, porous texture that grows either in thick encrusting mats or lobed, tubular formations dotted with oscula (exhalent water pores). Most blue sponges kept in aquariums house photosynthetic cyanobacteria in a symbiotic relationship, giving them their brilliant color and allowing them to derive energy from light as well as filter feeding.
