Pentapora fascialis, a bryozoan under stress: condition on coastal hard bottoms at Port-Cros Island (Port-Cros national Park, France, Mediterranean) and other sites

The health status of Pentapora fascialis, the largest erect calcified bryozoan in the Mediterranean, was assessed in June 2014 in Port-Cros national Park. Respective proportions of live and necrotic parts in 96 colonies sampled in coralligenous bottoms (20-40 m depth) were evaluated from photo-quadrats or directly during diving. About half of assessed colonies exhibited partial mortality higher than 25 % of their surface area. Most necrosis resulted from old mortality events, presumably induced by positive thermal anomalies. The physical cause, either natural or anthropogenic, of recent fragmentation of live parts observed in some colonies remains unidentified. Supplementary data provided by photo archives of this species from Hyères Islands and Marseille area for the period 2000-2016 indicate that the condition of this bryozoan was quite contrasted according to sites, depth, habitat types and history of colonies, whose growth-rate is assumed to vary widely. Unlike old colonies established on natural basal substrates, in most cases strongly altered, younger, variously sized colonies attached on raised substrates (e.g. artificial reefs, sea-fans) were in good condition. This trend may indicate that effects of stressors are enhanced when colonies are in contact with the basal substrate.