A recently discovered species of shell-less hermit crab was found living in a walking coral’s cavity, replacing the usual marine worm symbiont! This new hermit crab species was recently discovered in shallow waters of southern Japan. Instead of a shell on its back the crab was found living in corallums (living skeletons) of solitary walking corals. These corals usually have species of marine worm living inside of them, but the new hermit crab (Diogenes heteropsammicola) replaced the usual worm symbiont.
D. heteropsammicola is the only known hermit crab to partner with a living coral. The hermit crab and the coral have a mutually beneficial relationship: the hermit crab prevents the coral from being buried in sea floor sediments by carrying it and providing transportation, and the coral provides shelter and protection for the hermit crab. In this specialized codependence, it is usually impossible to replace either partner with a different species.
While most hermit crabs have an asymmetrical tail which fits into the coil of their seashell homes, D. heteropsammicola has a symmetrical tail which may have better adapted it to walking corals. This relationship is advantageous for the hermit crab because the living coral can grow alongside the crab so the crab never outgrows the coral like it would with a shell.
Read more about this new partnership via ScienceDaily, or click on the journal citation below to read the full article for free.
Igawa, M. and M. Kato. 2017. A new species of hermit crab, Diogenes heteropsammicola (Crustacea, Decapoda, Anomura, Diogenidae), replaces a mutualistic sipunculan in a walking coral symbiosis. PLOS ONE, 12(9):e0184311. DOI:10.1371/journal.pone.0184311.