Size distribution, density and disturbance in two Mediterranean gorgonians: Paramuricea clavata and Eunicella singularis

1. Gorgonians are long-lived engineering species and their conservation is crucial to maintaining the biodiversity of marine communities. The slow dynamics displayed by these species make them especially vulnerable when faced with increasing disturbances. 2. The sessile nature and structural role of gorgonians provide several ecological similarities with trees, allowing the application of different approaches developed by forest ecology studies to overcome the constraint of long time-scales to collect valuable dynamic data. Using these approaches, we examined the distribution and demography of two representative Mediterranean gorgonian species, Paramuricea clavata and Eunicella singularis, along a regional spatial scale as well as their response to disturbances. 3. The regular spatial distribution and the upper distribution limit of E. singularis suggest that this species is more tolerant to a wide range of environmental conditions than P. clavata, which exhibited a more asymmetrical spatial distribution and variation in the upper distribution limit on the latitudinal scale. 4. Size distributions of both species showed contrasting population dynamics. The size distribution of E. singularis was characterized by initial stages of populations (0-10 cm) in contrast to the scarcity of this stage displayed by P. clavata. This suggests differences in recruitment between species. Furthermore, only P. clavata populations displayed a strong negative correlation between density and biomass with a slope close to -3/2, indicating a self-thinning mechanism and therefore the existence of a carrying capacity. This result and the trajectories of disturbed populations below the self-thinning line revealed this approach as a useful method to identify the effects of biological or physical disturbance. 5. Synthesis and applications. The approaches used in this study provide insights into management needs in face of the difficulty of having to deal with the population dynamics of very slow-growing threatened species. In particular, gorgonian populations can be used as an indicator of the effects of climatic anomalies on the coralligenous community.