Pair formation in a coral inhabiting hermit crab.

The hermit crab, Paguritta harmsi, is a sedentary filterfeeder, previously known as an inhabitant of polychaete tubes on the surface of living corals. It is now reported living directly within colonies of the massive coral, Astreopora myriopthalma, growing in 8-10 m of water at Heron Island, Great Barrier Reef, Australia. Small hermit crabs (2-3 mm body length) were found within normal, tissue-containing corallites of the host, while larger individuals occupied the outer portion of tubular pits that extended as much as 98 mm down into the coral skeleton. P. harmsi tended to occur in pairs of adjacent male and female individuals, which may reduce predation risks during mating. Available evidence favours the hypothesis that hermit crabs are quite long lived, invade normal host corallites and remain there while the coral gradually grows outward producing an elongate pit.