Polyp dropout in a solitary cold-water coral

Scleractinian corals feature both sessile and mobile stages and diverse modes of development. In some cases, development can be reversed. Examples include polyp detachment in response to environmental stress (bail-out or polyp expulsion) and reverse metamorphosis, where juveniles detach from the primary skeleton and revert to the mobile stage. Here, we provide aquaria and field evidence of a new form of reverse development: polyp dropout in the solitary cold-water coral Caryophyllia huinayensis. It features tissue retraction and detachment of an entire adult polyp from the skeleton in the putative absence of a stressor. The dropout polyp remains viable and continues to live for many weeks, albeit in a rather collapsed state lacking a well-developed hydroskeleton. We carried out a long-term (37 months) rearing experiment under constant aquaria conditions and found polyp dropout in four out of 83 individuals. Detachment was accompanied by the extrusion of mesenterial filaments through perforations in the body wall. We believe this resulted in the loss of the hydroskeleton, which prevented the dropouts to subsequently resettle or form a new skeleton. As opposed to other known forms of reverse development, the new form is not accompanied by reversible metamorphosis, abandonment of the colonial way of life, nor is it a survival or asexual reproduction strategy. We found field indications of polyp dropout in Patagonian field populations of C. huinayensis, where 1.4 ± 0.8% (mean ± SD, N = 9322) of the polyps of the natural population showed partial detachment indicative of imminent dropout in the putative absence of external impact. Polyp dropout is the first record of polyp detachment in a solitary CWC with possible repercussions for adult coral mobility, evolution and Stanley’s (2003) ‘naked coral’ hypothesis.