A new species of hermit crab has been found to be a hermit no more. Off the coast of Japan, the crustacean has managed to find its forever home, by partnering up with a living coral that grows as the crab ages. This means that the hermit no longer needs to pursue the life-long search for a bigger shell in which to live, and can finally settle down.
The new species of crab in question is known as Diogenes heteropsammicola and described in PLOS ONE. It is the first species ever to have been found to live within the coiled cavity of a "walking coral", which is a free-living species of coral that grows as little lumps on the sea floor. It seems that the crab has become perfectly matched for its unusual house, as while most hermit crabs have an asymmetric body to match the asymmetric shells in which they live, this new species has a perfectly symmetrical body in order to fit in the coral.
Usually, the coral forms a symbiotic relationship with what is called a peanut worm, or more formally, a sipunculan. The partnership is mutually beneficial to both organisms, as the coral provides the worm with protection, while the worm carries the coral around the sea floor. This relationship is thought to have evolved over a long period of time, so that both species are perfectly in tune with each other.
Which makes the fact that the coral has now been found to be buddying up with the hermit crabs even more unusual. Situations in which one half of a symbiotic relationship diversify to include other organisms are rarely heard of, simply because such initial partnerships tend to require such high levels of specialization meaning that switching to include other species will often be difficult, if not impossible.
And yet it is clearly the case that the crabs and coral have managed quite successfully to strike up a deal. The crustacean basically fills the same role as the worm would normally, carrying the coral around the sea floor as it goes about its crabby business, and preventing the coral from being buried in the sediment. But the coral provides a service to the crab that cannot be found elsewhere.
The pairing of the coral and crab is unique among hermit crabs, in that rather than having the never-ending search for bigger shells as the crab gets older, the coral physically grows with the crustacean as it ages. This is clearly a huge advantage to the crab, which can now give up its transient way of life.