Wikipedia
Pseudospongosorites suberitoides is a species of demosponge belonging to the family Suberitidae. It is currently considered the only species in the genus Pseudospongosorites. It is found in the Caribbean Sea, the Gulf of Mexico and on the Atlantic coast of the United States as far north as North Carolina.
This sponge is usually found encrusting empty (dead) gastropod shells that are occupied by hermit crabs. The sponge grows continuously such that the shell surface is often no longer visible, but the sponge does not block the aperture and so the hermit crab is still able to function normally. It is thought this arrangement is mutually beneficial. The sponge gains increased feeding opportunities and better-oxygenated water due to the feeding activities of the crustacean, and there is also a decreased chance of the sponge being buried in sediment. The hermit crab gets a home which grows in size, so the hermit crab does not need to hunt to find a larger empty gastropod shell quite as soon as it normally would.