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.

Marine heatwaves on the rise: One of the strongest ever observed mass mortality event in temperate gorgonians.

Gorgonian population after the 2022 mass mortality event (MME) in the Calanques National Park. The year 2022 was marked by a historic gorgonian MME. This study describes the consequences for the red gorgonian (Paramuricea clavata) and red coral (Corallium rubrum) populations in the Calanques National Park (Marseille, France).

Abundance and Diversity of Crypto- and Necto-Benthic Coastal Fish Are Higher in Marine Forests than in Structurally Less Complex Macroalgal Assemblages.

In Mediterranean subtidal rocky reefs, Cystoseira spp. (Phaeophyceae) form dense canopies up to 1 m high. Such habitats, called 'Cystoseira forests', are regressing across the entire Mediterranean Sea due to multiple anthropogenic stressors, as are other large brown algae forests worldwide. Cystoseira forests are being replaced by structurally less complex habitats, but little information is available regarding the potential difference in the structure and composition of fish assemblages between these habitats. To fill this void, we compared necto-benthic (NB) and crypto-benthic (CB) fish assemblage structures between Cystoseira forests and two habitats usually replacing the forests (turf and barren), in two sampling regions (Corsica and Menorca). We sampled NB fish using Underwater Visual Census (UVC) and CB fish using Enclosed Anaesthetic Station Vacuuming (EASV), since UVC is known to underestimate the diversity and density of the 'hard to spot' CB fish. We found that both taxonomic diversity and total density of NB and CB fish were highest in Cystoseira forests and lowest in barrens, while turfs, that could be sampled only at Menorca, showed intermediate values. Conversely, total biomass of NB and CB fish did not differ between habitats because the larger average size of fish in barrens (and turfs) compensated for their lower densities. The NB families Labridae and Serranidae, and the CB families Blenniidae, Cliniidae, Gobiidae, Trypterigiidae and Scorpaenidae, were more abundant in forests. The NB taxa Diplodus spp. and Thalassoma pavo were more abundant in barrens. Our study highlights the importance of using EASV for sampling CB fish, and shows that Cystoseira forests support rich and diversified fish assemblages. This evidence suggests that the ongoing loss of Cystoseira forests may impair coastal fish assemblages and related goods and services to humans, and stresses the need to implement strategies for the successful conservation and/or recovery of marine forests.