Hermit crabs live inside armor built by other species, usually a seashell after the death of a mollusk, or in any found object it will fit into. As the crab grows, it needs to find a larger shell to accommodate its body. A newly-discovered species, though, has found a workaround. Diogenes heteropsammicola takes up residence inside a living coral. As the crab grows, the dome of coral grows, too, so they can stay together for life.
But although they may be new to us, these hermit crabs have been living symbiotically with coral for a very long time. The crab’s long, thin tail and spindly arms are well suited to fitting into the coral’s cavity, which is usually inhabited by a marine worm that also shares a symbiotic relationship with the coral. Hermit crab hind ends usually spiral to fit shells, but these crabs are more symmetrical, to fit well in the coral. There are advantages for the coral, too. In the journal article, published in PLOS ONE, researchers describe how the crab “carries the host coral and prevents the coral from being buried.”
This symbiotic relationship is called obligate mutualism, as both species benefit from the arrangement. Read the original research report at PLOS-One.
(Image credit: Momoko Igawa, Makoto Kato)