Shallow-water Desmophyllum dianthus (Scleractinia) from Chile: characteristics of the biocoenoses, the bioeroding community, heterotrophic interactions and (paleo)-bathymetric implications

We report an unusually shallow-water occurrence of habitat-forming Desmophyllum dianthus (Esper, 1794) from the Chilean fjord region. Most occurrences of the cosmopolitan D. dianthus are known from the bathyal zone. In the northern Chilean fjord region, however, this coral is reported within the euphotic zone. The upper limit of distribution was found at 7 m water depth and is confined to the lower boundary of the low salinity layer. Large accumulations both as living aggregations and as sediment-formers typically occur from 20 m water depth and beyond. The corals prefer to colonise the undersides of rock ledges with downwardfacing corallites. The motivation of this study is to analyse and discuss the existence of an azooxanthellate coral that generally thrives in aphotic environment but here extends into the photic zone by means of screening of bioerosion patterns. Based on the detailed analysis of scratching and boring traces, we compare the ichnocoenosis found within the Chilean D. dianthus with the established bathymetrically indicative ichnocoenoses from other areas around the world. These indicator ichnocoenoses are widely used to reconstruct relative water depths of depositional settings in the geological past. The study of the bioeroding assemblage from two living D. dianthus collected at 28 m water depth in the Reñihue Fjord, Chile, shows some remarkable patterns that shed light on the complex way in which the coral’s soft tissue expands and retracts at the apical zone of the corallum in response to in vivo infestation of endolithic algae. The role of this heterotypic interaction is discussed.

To visualise the endolithic ichnocoenoses, we applied several methodologies such as the vacuum cast embedding technique combined with scanning electron microscopy, fluorescence microscopy and x-ray analyses. In total, 20 different trace makers are identified. Based on the analysis of the indicator ichnospecies, the endolithic community is indicative for the dysphotic zone. This result is compatible with the sciaphile environment of D. dianthus, living under rock ledges in the photic zone.