The largest animal on many slopes, and among the longest-lived.
A rigid barrel-shaped sponge to about 1.5 m tall and as much across, grey-brown to purple-brown outside with a ridged, warty surface, and a smooth open cavity inside. Water is drawn in through the whole outer surface and expelled from the central opening; a large individual moves thousands of litres a day and filters bacteria and organic particles from it.
Growth is slow and individuals are old — estimates for large barrel sponges in the Caribbean run to centuries, and there is no reason to think Indo-Pacific ones are quicker. The cavity holds gobies, cardinalfish, brittle stars and shrimps, and the outer surface carries crinoids and sponge crabs. It is often used as a scale reference in wide-angle photographs, and it is easily damaged: the wall is brittle rather than spongy and a knee or a tank puts a permanent hole in it. Recorded from about 10 to 50 m.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.