A new species of hermit crab, Diogenes heteropsammicola (Crustacea, Decapoda, Anomura, Diogenidae), replaces a mutualistic sipunculan in a walking coral symbiosis.

Momoko Igawa,Makoto Kato

doi:10.1371/journal.pone.0184311

Abstract

Symbiont shift is rare in obligate mutualisms because both the partners are reciprocally dependent on and specialized to each other. In the obligate accommodation–transportation mutualism between walking corals and sipunculans, however, an unusual saltatory symbiont shift was discovered. In shallow waters of southern Japan, an undescribed hermit crab species was found living in corallums of solitary scleractinian corals of the genera Heterocyathus and Heteropsammia, replacing the usual sipunculan symbiont. We described the hermit crab as a new species Diogenes heteropsammicola (Decapoda, Anomura, Diogenidae), and explored its association with the walking corals. This hermit crab species obligately inhabits the coiled cavity of the corals, and was easily distinguished from other congeneric species by the exceedingly slender chelipeds and ambulatory legs, and the symmetrical telson. Observations of behavior in aquaria showed that the new hermit crab, like the sipunculan, carries the host coral and prevents the coral from being buried. This is an interesting case in which an organism phylogenetically distant from Sipuncula takes over the symbiotic role in association with a walking coral. The hermit crab species is unique in that its lodging is a living solitary coral that grows with the hermit crab in an accommodation–transportation mutualism.

Highlights

Obligate mutualism is a climax of co-evolution between symbiotic organisms
Rare symbiont shifts often result in reciprocal diversification of the partners, especially in obligate pollination mutualisms between fig and fig wasp [1,2] and between leafflower and leafflower moth [3,4,5]
During our survey of benthic fauna in Oshima Strait, north of Kakeroma Island, Ryukyu Islands, southern Japan, we found that some solitary scleractinian corals of the genera Heterocyathus and Heteropsammia were inhabited by hermit crabs, rather than by sipunculans

Summary

Introduction

Obligate mutualism is a climax of co-evolution between symbiotic organisms Because both partners of an obligate mutualism are reciprocally dependent on and specialized to each other, symbiont shift only rarely occurs. Rare symbiont shifts often result in reciprocal diversification of the partners, especially in obligate pollination mutualisms between fig and fig wasp [1,2] and between leafflower and leafflower moth [3,4,5]. In these cases, host shifts have occurred only between phylogenetically related partners, and saltatory host shift between phylogenetically distant taxa is not known. We report here a novel saltatory host shift of partners in an obligate mutualism.

Methods

Results

Conclusion