Hapalocarcinus marsupialis is common, but rarely seen in the flesh. Females of this species encourage certain Pocillopora corals to grow around them, causing galls to be created in the coral within which the females remain trapped. Males are tiny and are able to enter and exit the gall through its narrow opening to mate with the female. The first photo shows a typical gall in the coral, and the following two photos by Jay Lord show the speckled female through the slit in the gall.
The next two shots show just the galls the crabs make in living coral.
Created 12 September 2018
Updated 21 October 2018