Two juvenile Pagurus impressus in hermit crab sponges, Dog Island, Florida. A larger Pagurus impressus from Dog Island, Florida in the opening of a bright orange hermit crab sponge. Several empty sponges, each of which originally contained a hermit crab that has since switched into another shelter, rest nearby. Hermit crabs are a successful group of decapod crustaceans that typically live in empty snail shells. To see a small rounded colorful sponge moving about on the ocean floor for the first time is an unusual experience, and it often takes several minutes before one realizes that there is a hermit crab inside, moving the sponge about from place to place.
Most sponges are sessile.
They grow on a hard substrate and remain there for the duration of their life, a fact that partially explained why early naturalists often viewed sponges more like plants than animals. Hermit crab sponges, however, typically grow on snail shells, although some can grow on other substrates such as other mollusk shells or wharf pilings. The snail shell can either be empty, a living shell still occupied by the living snail, or a dead shell occupied by a hermit crab. The sponge begins its development by thinly encrusting the shell. As it grows the shell becomes increasingly surrounded and covered by the sponge.