Gaining ground - cleaning with mesenterial filaments by the cold-water coral Desmophyllum dianthus?

The solitary cosmopolitan cold-water coral Desmophyllum dianthus (Esper, 1794) (A, scale bar approximately 1 cm) occurs as a deep-water emergent species in Patagonian fjords, Chile, and can form extensive banks (Försterra et al. 2005). This coral functions as a bioengineer by providing a 3D habitat for a diverse benthic community (Försterra et al. 2005). With skeletal growth, the living tissue is retracted by the organism to the upper part of the calyx, leaving the lower skeleton uncovered. Endolithic algae are then able to enter the tissue covered skeleton from below and start to encrust the tissue covered part of the skeleton (Försterra et al. 2005, 2008). They are visible as a greenish coat on the skeleton (A). The algae are able to bore into the skeleton (Försterra et al. 2005) and thus could interfere with the coral’s calcifying process. This process could result in a parasitic behavior (Försterra et al. 2012) rather than the previously hypothesized (Försterra and Häussermann 2008) symbiotic or even mutualistic relationship. This infection by algae may be the primary reason for the peculiar behavior we observed on several specimens both in situ (A) and in our aquaria system (B–D, scale bar approximately 1 cm). After an acclimatization time in our aquaria system, wild collected corals were well adapted and well fed, and secondarily expanded their tissue toward their lower skeleton (about 1 cm in 4 mo, CJ and GMS pers obs), recovering an area that had been without any tissue. Concomitantly, the corals everted their mesenterial filaments between this re-entered outer-skeleton and the covering tissue layer (A, B; C, D: both enlargements of B). During the eversion of the mesenterial filaments the new tissue generally appeared unusually pleated (C, D) in contrast to a less common smooth tissue surface (A). This was not a short-term or rare mechanism: corals maintained this behavior in the aquaria over multiple months. Several tropical coral species are known to eject