A barrel of an animal, up to 1.5 m (5 ft) across, brown and ridged, standing open-topped on a reef slope. It is hard and brittle rather than soft, and it is one of the few animals on a reef that a diver will mistake for furniture.
It is a pump. Water is drawn in through millions of pores across the surface by beating flagella, stripped of bacteria and organic particles, and pushed out of the opening at the top - and a large sponge processes many times its own volume every hour, thousands of liters in a day. On a reef with enough of them, the whole water column is filtered repeatedly.
Most of what it eats is too small to see. Sponges of this family live on bacteria and dissolved organic matter and then release the carbon they capture as a rain of shed cells, which other animals eat - the sponge loop, a pathway that only got its name in 2013 and turned out to be one of the main ways a reef recycles what would otherwise be lost.
They grow slowly and live for centuries; large individuals on Caribbean reefs have been aged at over two thousand years, which makes the barrel sponge one of the oldest living things a diver will ever swim past.
- large barrel or vase with a wide open top, brown to red-brown
- surface ridged and grooved into a rough network, hard and brittle to touch
- usually standing alone on a slope, sometimes with a fish sheltering inside
- up to 1.5 m across
