Emperor angelfish (Pomacanthus imperator) in a barrel sponge. The sponge is the most primitive of the multicellular animals, lacking organs and true body tissues. Some sponges reach 1.5 metres in height and diameter, and are host to a variety of other organisms, including crabs, shrimps and fish. These use the sponge to access the plankton-rich water currents which it generates. Photographed off Bali, Indonesia.