Recent PLOS ONE research reveals nefarious partner-swapping practices in a newly discovered hermit crab species. Hermit crabs usually scavenge empty shells on the seabed and use them as mobile homes, but this species, Diogenes heteropsammicola, instead appears to set up residence in the cavity of live “walking” corals. In this impudent practice, they appear to take the place of the coral’s usual live-in partners, marine worms known as sipunculans.
It is not usually possible to replace either partner in these specialized symbiotic relationships, so such flexible living arrangements are rare. Nonetheless, in this case the crab appears adapted to its coral residence: while most hermit crab species have asymmetrical tail segments to fit into the usually right-handed coil of seashells, D. heteropsammicola has a symmetrical tail, making it better-adapted to the coral cavity.
The coral enjoys beneficial relationships with both the sipunculan and the crab – either species can provide transportation for the coral and unearth it from burial in sea floor sediment. Meanwhile, the sipunculan and the crab gain protection within the coral cavity. Unlike seashell residences, the live coral can grow with the crab, negating its usual need to replace its outgrown home.
D. heteropsammicola is the only known hermit crab species to have adopted a living coral as its partner, and the authors speculate that the crab may have become a secondary partner for the coral after the sipunculan-coral reciprocal relationship had already evolved. Thanks to the walking coral, this crab would appear to be a hermit no more!
Image Credit: Igawa and Kato, 2017
Research Article: Igawa M, Kato M (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. https://doi.org/10.1371/journal.pone.0184311