Halocynthia papillosa (Linnaeus, 1767) as an indicator of SCUBA diving impact

The solitary ascidian Halocynthia papillosa (Linnaeus, 1767) is proposed as a good indicator of the deleterious effect of SCUBA diving on the Mediterranean coralligenous communities. A comparative survey of H. papillosa populations at frequented and unfrequented localities was carried out over a two-year period (during 2006 and 2007), before and after a peak diving season in the Sierra Helada Marine Park (SW Western Mediterranean Sea). We observed bigger and more abundant individuals of H. papillosa at undived sites than at frequented dived sites during the period of study. Furthermore, individuals of H. papillosa in the most frequented localities occupied more cryptic positions than in the undived localities. H. papillosa was shown to be very sensitive to the adverse effects of SCUBA diving. This species could represent a reliable bioindicator of diving activity and as such constitute a useful tool for the quick and easy monitoring of impact on coralligenous communities before this damage becomes unmitigatable.