Estimating the patterns of connectivity in marine taxa with planktonic dispersive stages is a challenging but crucial task because of its conservation implications. The red gorgonian Paramuricea clavata is a habitat forming species, characterized by short larval dispersal and high reproductive output, but low recruitment. In the recent past, the species was impacted by mass mortality events caused by increased water temperatures in summer. In the present study, we used 9 microsatellites to investigate the genetic structure and connectivity in the highly threatened populations from the Ligurian Sea (NW Mediterranean). No evidence for a recent bottleneck neither decreased genetic diversity in sites impacted by mass mortality events were found. Significant IBD pattern and high global FST confirmed low larval dispersal capability in the red gorgonian. The maximum dispersal distance was estimated at 20–60 km. Larval exchange between sites separated by hundreds of meters and between different depths was detected at each site, supporting the hypothesis that deeper subpopulations unaffected by surface warming peaks may provide larvae for shallower ones, enabling recovery after climatically induced mortality events.

Background In the ocean, the variability of environmental conditions found along depth gradients exposes populations to contrasting levels of perturbation, which can be reflected in the overall patterns of species genetic diversity. At shallow sites, resource availability may structure large, persistent and well-connected populations with higher levels of diversity. In contrast, the more extreme conditions, such as thermal stress during heat waves, can lead to population bottlenecks and genetic erosion, inverting the natural expectation. Here we examine how genetic diversity varies along depth for a long-lived, important ecosystem-structuring species, the red gorgonian, Paramuricea clavata. Methods We used five polymorphic microsatellite markers to infer differences in genetic diversity and differentiation, and to detect bottleneck signs between shallow and deeper populations across the Atlantic Ocean and the Mediterranean Sea. We further explored the potential relationship between depth and environmental gradients (temperature, ocean currents, productivity and slope) on the observed patterns of diversity by means of generalized linear mixed models. Results An overall pattern of higher genetic diversity was found in the deeper sites of the Atlantic Ocean and the Mediterranean Sea. This pattern was largely explained by bottom temperatures, with a linear pattern of decreasing genetic diversity with increasing thermal stress. Genetic differentiation patterns showed higher gene flow within sites (i.e., shallow vs. deeper populations) than between sites. Recent genetic bottlenecks were found in two populations of shallow depths. Discussion Our results highlight the role of deep refugial populations safeguarding higher and unique genetic diversity for marine structuring species. Theoretical regression modelling demonstrated how thermal stress alone may reduce population sizes and diversity levels of shallow water populations. In fact, the examination of time series on a daily basis showed the upper water masses repeatedly reaching lethal temperatures for P. clavata. Differentiation patterns showed that the deep richer populations are isolated. Gene flow was also inferred across different depths; however, not in sufficient levels to offset the detrimental effects of surface environmental conditions on genetic diversity. The identification of deep isolated areas with high conservation value for the red gorgonian represents an important step in the face of ongoing and future climate changes.

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Summary:Clonal propagation is a common feature of benthic marine organisms. In the present study, we investigated the contribution of clonal reproduction in the red gorgonian Paramuricea clavata. Mediterranean populations of P. clavata were severely affected by mass mortality events caused by increased water temperature in 1999 and 2003. The populations are characterized by slow growth and episodic recruitment, but after the observed mortalities, an unexpectedly high recovery rate was observed in the severely affected populations from the Ligurian Sea, NW Mediterranean. Ten years after the last mortality event, we investigated the contribution of clonal propagation in populations from the Ligurian Sea, where some populations were highly affected by mass mortality events, and from the Atlantic, where mortality was never observed. All individuals were genotyped for nine microsatellite loci. The contribution of clonal reproduction varied from 0% to 13% and did not differ significantly between affected and unaffected populations. We confirm by using genetic markers that clonal propagation in P. clavata is not common, and that the contribution of clones is too low to play an important role in red gorgonian reproduction and cannot contribute to population recovery at sites that have been affected by mass mortality events.Keywords: Paramuricea clavata; gorgonian; clonal reproduction; mortality; microsatellite; climate change. Baja propagación clonal en poblaciones atlánticas y mediterráneas de gorgonia roja Paramuricea clavata (Octocorallia)Resumen: La propagación clonal es una característica común de organismos bentónicos marinos. En el presente estudio, hemos investigado la contribución de la reproducción clonal en la gorgonia roja Paramuricea clavata. Las poblaciones mediterráneas de P. clavata fueron severamente afectadas por eventos de mortalidades masivas en 1999 y 2003, causadas por incrementos de la temperatura del agua. Estas poblaciones están caracterizadas por un crecimiento lento y reclutamientos periódicos, sin embargo, tras las mortalidades observadas, una inesperada elevada tasa de recuperación fue observada en las poblaciones severamente afectadas del Mar de Liguria, NO Mediterráneo. Diez años después del último evento de mortalidad, investigamos la contribución de la propagación clonal en poblaciones del Mar de Liguria, donde algunas poblaciones fueron fuertemente afectadas por eventos de mortalidad masiva, así como del Atlántico, donde estas mortalidades masivas nunca han sido registradas. Todos los individuos fueron genotipados para 9 loci microsatélites. La contribución de la reproducción clonal varió de 0 a 13% y no difirió significativamente entre poblaciones afectadas y no afectadas. Confirmamos, mediante el uso de marcadores genéticos, que la propagación clonal no es habitual en P. clavata y que la contribución de clones es demasiado baja como para jugar un papel importante en la reproducción de la gorgonia roja, siendo insuficiente para la recuperación de lugares afectados por eventos de mortal...