Genetic diversity increases with depth in red gorgonian populations of the Mediterranean Sea and the Atlantic Ocean

Temporal genomics help in deciphering neutral and adaptive patterns in the contemporary evolution of kelp populations

The impact of climate change on populations will be contingent upon their contemporary adaptive evolution. In this study, we investigated the contemporary evolution of 4 populations of the cold-water kelp Laminaria digitata by analyzing their spatial and temporal genomic variations using ddRAD-sequencing. These populations were sampled from the center to the southern margin of its north-eastern Atlantic distribution at 2 time points, spanning at least 2 generations. Through genome scans for local adaptation at a single time point, we identified candidate loci that showed clinal variation correlated with changes in sea surface temperature (SST) along latitudinal gradients. This finding suggests that SST may drive the adaptive response of these kelp populations, although factors such as species' demographic history should also be considered. Additionally, we performed a simulation approach to distinguish the effect of selection from genetic drift in allele frequency changes over time. This enabled the detection of loci in the southernmost population that exhibited temporal differentiation beyond what would be expected from genetic drift alone: these are candidate loci which could have evolved under selection over time. In contrast, we did not detect any outlier locus based on temporal differentiation in the population from the North Sea, which also displayed low and decreasing levels of genetic diversity. The diverse evolutionary scenarios observed among populations can be attributed to variations in the prevalence of selection relative to genetic drift across different environments. Therefore, our study highlights the potential of temporal genomics to offer valuable insights into the contemporary evolution of marine foundation species facing climate change.

Investigating the outcomes of a threatened gorgonian in situ transplantation: Survival and microbiome diversity in Paramuricea clavata (Risso, 1827)

Gorgonian octocorals are threatened by global and local stressors that can act synergistically to affect their health. In recent years, mass mortality events triggered by marine heatwaves have caused demographic declines in Mediterranean gorgonian populations that may lead to their collapse. Potential changes in microbiome composition under stressful conditions may further increase the susceptibility of the gorgonian holobiont to disease. Given the low recovery capacity of gorgonians, restoration approaches using transplantation are becoming an increasingly attractive option to counteract their decline. Here, we compared the survival and microbiome diversity of Paramuricea clavata colonies transplanted to sites differing in depth and local environmental conditions. Gorgonians sampled at a greater depth than the transplantation site were more likely to suffer necrosis after 1 year of monitoring. Gorgonian transplantation into environments disturbed by an anthropogenic source of pollution resulted in an imbalance of the microbiome with potential consequences on the success of restoration initiatives.

Global status, impacts, and management of rocky temperate mesophotic ecosystems

The ecology and function of rocky temperate mesophotic ecosystems (TMEs) remain poorly understood globally despite their widespread distribution. They typically occur at 20-150 m (the limit of photosynthesis), and on rocky substratum they support rich benthic communities and mobile fauna. We determined the distribution of rocky TMEs, their conservation status, and their most characteristic biological groups. Rocky TMEs were dominated by algae, turf-invertebrate matrices (<50 m only), sponges, bryozoans, and cnidarians. The community composition of TMEs differed significantly from shallow (0-15 m) subtidal reefs. Data were geographically biased and variable, available only from the North and South Atlantic, Mediterranean, and Temperate Australasia. Degree of protection of rocky TMEs varied considerably across the world. The biggest threats to rocky TMEs were identified changes in temperature, sedimentation rates, nutrient concentrations, and certain fishing types. We propose a conservation framework to inform future rocky TME management and conservation, highlighting the need to recognize the importance of these biologically diverse and functionally important ecosystems.

Not out of the Mediterranean: Atlantic populations of the gorgonian Paramuricea clavata are a separate sister species under further lineage diversification

The accurate delimitation of species boundaries in nonbilaterian marine taxa is notoriously difficult, with consequences for many studies in ecology and evolution. Anthozoans are a diverse group of key structural organisms worldwide, but the lack of reliable morphological characters and informative genetic markers hampers our ability to understand species diversification. We investigated population differentiation and species limits in Atlantic (Iberian Peninsula) and Mediterranean lineages of the octocoral genus Paramuricea previously identified as P. clavata. We used a diverse set of molecular markers (microsatellites, RNA-seq derived single-copy orthologues [SCO] and mt-mutS [mitochondrial barcode]) at 49 locations. Clear segregation of Atlantic and Mediterranean lineages was found with all markers. Species-tree estimations based on SCO strongly supported these two clades as distinct, recently diverged sister species with incomplete lineage sorting, P. cf. grayi and P. clavata, respectively. Furthermore, a second putative (or ongoing) speciation event was detected in the Atlantic between two P. cf. grayi color morphotypes (yellow and purple) using SCO and supported by microsatellites. While segregating P. cf. grayi lineages showed considerable geographic structure, dominating circalittoral communities in southern (yellow) and western (purple) Portugal, their occurrence in sympatry at some localities suggests a degree of reproductive isolation. Overall, our results show that previous molecular and morphological studies have underestimated species diversity in Paramuricea occurring in the Iberian Peninsula, which has important implications for conservation planning. Finally, our findings validate the usefulness of phylotranscriptomics for resolving evolutionary relationships in octocorals.

Exploring the response of a key Mediterranean gorgonian to heat stress across biological and spatial scales

Understanding the factors and processes that shape intra-specific sensitivity to heat stress is fundamental to better predicting the vulnerability of benthic species to climate change. Here, we investigate the response of a habitat-forming Mediterranean octocoral, the red gorgonian Paramuricea clavata (Risso, 1826) to thermal stress at multiple biological and geographical scales. Samples from eleven P. clavata populations inhabiting four localities separated by hundreds to more than 1500 km of coast and with contrasting thermal histories were exposed to a critical temperature threshold (25 °C) in a common garden experiment in aquaria. Ten of the 11 populations lacked thermotolerance to the experimental conditions provided (25 days at 25 °C), with 100% or almost 100% colony mortality by the end of the experiment. Furthermore, we found no significant association between local average thermal regimes nor recent thermal history (i.e., local water temperatures in the 3 months prior to the experiment) and population thermotolerance. Overall, our results suggest that local adaptation and/or acclimation to warmer conditions have a limited role in the response of P. clavata to thermal stress. The study also confirms the sensitivity of this species to warm temperatures across its distributional range and questions its adaptive capacity under ocean warming conditions. However, important inter-individual variation in thermotolerance was found within populations, particularly those exposed to the most severe prior marine heatwaves. These observations suggest that P. clavata could harbor adaptive potential to future warming acting on standing genetic variation (i.e., divergent selection) and/or environmentally-induced phenotypic variation (i.e., intra- and/or intergenerational plasticity).

Weak biodiversity connectivity in the European network of no-take marine protected areas

The need for international cooperation in marine resource management and conservation has been reflected in the increasing number of agreements aiming for effective and well-connected networks of Marine Protected Areas (MPAs). However, the extent to which individual MPAs are connected remains mostly unknown. Here, we use a biophysical model tuned with empirical data on species dispersal ecology to predict connectivity of a vast spectrum of biodiversity in the European network of marine reserves (i.e., no-take MPAs). Our results highlight the correlation between empirical propagule duration data and connectivity potential and show weak network connectivity and strong isolation for major ecological groups, resulting from the lack of direct connectivity corridors between reserves over vast regions. The particularly high isolation predicted for ecosystem structuring species (e.g., corals, sponges, macroalgae and seagrass) might potentially undermine biodiversity conservation efforts if local retention is insufficient and unmanaged populations are at risk. Isolation might also be problematic for populations' persistence in the light of climate change and expected species range shifts. Our findings provide novel insights for management directives, highlighting the location of regions requiring additional marine reserves to function as stepping-stone connectivity corridors.