It has been hypothesized that endolithic algae inside the skeleton of cold-water corals may have a mutualistic relationship with the coral host positively affecting coral calcification. This study investigated the effect of endolithic algae on apical septal extion of the cold-water coral Desmophyllum dianthus at Fjord Comau, southern Chile (42.41° to 42.15° S, 72.5° W). The fluorescent staining agent calcein was used to document the linear apical extension of septae for a period of one and a half years between 2006 and 2007. The results showed a severe reduction of extension rates associated with the presence of endolithic algae. Infested individuals grew about half as fast as non-infested polyps with a median value of 1.18 μm day-1 compared to 2.76 μm day-1. Contrary to the initial hypothesis, these results point towards a parasitic relationship between D. dianthus and its endolithic algae potentially impairing coral fitness. However, further data on physiological parameters and other aspects of the calcification process is necessary to confirm these findings.

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