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Sapphire damsels, Pomacentrus pavo, take shelter in the sponge Cribrochalina. Halmahera, Maluku.

Sponges

Indonesian reef sponges vary in size from tiny to huge, from the small patches of color provided by encrusting sponges (family Clionidae) to the meter-high barrel sponges (Xestospongia). All sponges are members of the phylum Porifera, "the hole-bearers," and their porous, "spongy" nature is crucial to their mode of feeding. Sponges are the archetypal filter-feeders, straining plankters from the water through myriad microscopic pores.

A cross-section of a sponge shows a very sophisticated system for moving water. Small intake pores lead to an internal system of tiny canals and chambers lined with cells bearing whip-like processes. Beating constantly, these cells create a current through the sponge that moves its own volume of water every 4-20 seconds. Even a relatively small sponge can circulate as much as 5,000 liters a day. The chimney or barrel shape of many larger sponges helps increase surface area and the water flow through the animal.