A hermit crab living in association with a mobile scleractinian coral, Heteropsammia cochlea

Abstract

Mobile free-living corals of the scleractinian genus Heteropsammia (Dendrophylliidae) are well known for their association with an endosymbiotic sipunculan worm (Yonge 1975; Hoeksema and Best 1991). This association is considered a mutualism, which allows them to inhabit soft substrata. It starts as the coral settles on a small gastropod shell that is already inhabited by the worm, after which it overgrows the shell and continues to provide shelter for the worm. The worm can protrude from its hole and pull the coral over the sediment, enabling it to migrate over the sediment (Goreau and Yonge 1968). Not much is known about the ecology of these mobile corals as most of their records are from greater depths and based on dry specimens (Hoeksema and Matthews 2015), but from an evolutionary perspective, it is clear that they show various traits that give them a unique position within their family (Arrigoni et al. 2014). On March 3, 2016, a living coral of Heteropsammia cochlea (Spengler, 1781) inhabited by a small hermit crab, Diogenes sp. (Diogenidae), was discovered on the sandy sea floor at 31 m depth in Ikomo Bay (28°05'46. N, 129°14'26. E), south of Kakeromajima Island, Kagoshima, southern Japan (Fig. 1). No sipunculan was found inside this coral (length 1.4 cm, width 1.2 cm, height 1.1 cm) in contrast to 17 other individuals of H. cochlea that were observed at the same site (5–30 m depth). This report represents the first observation of an association of a scleractinian coral with a hermit crab. Further surveys to understand the symbiotic relationships between mobile corals and their episymbionts are urgent as shallow habitats of free-living corals are mostly inside sheltered bays (e.g., Hoeksema and Best 1991; Cairns 2004; Hoeksema and Matthews 2015) and tend to be threatened by human activities, such as land reclamation and other disturbances causing runoff.