Climate change is causing an increase in the frequency and intensity of marine heatwaves (MHWs) and mass mortality events (MMEs) of marine organisms are one of their main ecological impacts. Here, we show that during the 2015-2019 period, the Mediterranean Sea has experienced exceptional thermal conditions resulting in the onset of five consecutive years of widespread MMEs across the basin. These MMEs affected thousands of kilometers of coastline from the surface to 45 m, across a range of marine habitats and taxa (50 taxa across 8 phyla). Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. Our findings reveal that the Mediterranean Sea is experiencing an acceleration of the ecological impacts of MHWs which poses an unprecedented threat to its ecosystems' health and functioning.

Following the Mass Mortality Events (MMEs) of the pen shell P. nobilis in Campania region and Sicily, a survey of moribund P. nobilis specimens was also conducted in other Italian regions (Campania, Tuscany, Sardinia, and Apulia) and Spain (Catalunya). Histopathological and molecular examination of 27 specimens of P. nobils revealed different types of pathogens associated with tissue lesions, morbidity and mortality. Presence of Mycobacterium, Vibrio species, Haplosporidium pinnae and Perkinsus sp. were detected, differently distributed into the areas. The Mycobacterium sp., previously reported in Campania and Sicily samples, was observed in all the analyzed areas and individuals, associated to systemic inflammatory lesions. In Spain, H. pinnae was observed in 36% of cases, always associated to the Mycobacterium sp. Molecular study using hsp65 genes and Internal Transcriber Spacer ITS support that a new species of Mycobacteria is infecting P. nobilis, close to M. triplex and belonging to the group of M. simiae complex with M. sherrisi. Presence of Perkinsus spp. resembling P. mediterraneus was observed in 2 out of 13 Italian individuals whose presence should be addressed as potential risk for shellfish aquaculture of the area. Vibrio spp. were also detected in some case. The preliminary results of this study suggest that Mycobacterium sp., Vibrio spp., H. pinnae and Perkinsus sp. cooperate to disease pathogenesis, being Mycobacterium and Haplosporidium most of the time involved. Vigilant inspection of those areas where MME is now starting, along with continuous systematic surveys, are crucial to define the spatiotemporal progress of mortality and the role of every single pathogen in the disease outcome.

Predation on sea urchins, which are important structuring agents in seagrass communities, can be modulated by habitat structural complexity. Here we examine the importance of meadow structure (leaf length and presence of unburied root-rhizome layer) in determining predation pressure on the main invertebrate herbivore of the temperate seagrass Posidonia oceanica (L.) Delile, the sea urchin Paracentrotus lividus (Lamarck). The attributes of seagrass habitat complexity are subject to considerable spatial and temporal variability, thus affecting refuge availability. In the field, exposure of the root-rhizome layer depends on local sedimentary conditions, while changes in the leaf canopy are generally dictated by hydrodynamic regimes, light, nutrient availability as well as grazing activity. We reproduced 5 habitat conditions in the laboratory and 2 in the field by modifying leaf length and burial level of the root-rhizome layer, and we measured fish predatory efficiency under controlled conditions. In the laboratory we focused on juvenile sea urchins (0.2 to 1.2 cm test diameter, TD) while in the field we studied young adults (3 to 5 cm TD). Mortality of juveniles and young adults was significantly lower under long leaves, while the presence of an unburied root-rhizome layer appeared to provide a key refuge for juveniles, independent of leaf length. The presence of refuges thus appears to be a key factor in the interaction between sea urchins and predatory fish in seagrass meadows, and highlights the importance of local structural complexity (e.g. sedimentary regimes, fish herbivory pressure) on the regulation of predator-prey interactions.

In this study, we examine the hypotheses that the size of the seagrass meadow and marine protection influence the abundance of seagrass herbivores and their associated grazing pressure. The effect of meadow size was tested in 9 shallow unprotected meadows of Posidonia oceanica encompassing a wide range of areas. The effect of fishing protection was examined by comparing 9 unprotected meadows (controls) with 3 marine protected areas (MPAs) with at least 20 yr of protection that also varied in meadow size (large > 3 ha, medium 1 ha < x < 2 ha and small <1 ha). At each site, we quantified the abundance, size distribution and grazing pressure of the 2 most important herbivores, the fish Sarpa salpa and the sea urchin Paracentrotus lividus. A strong negative relationship between meadow size and both the abundance and grazing rates of S. salpa was evident in the unprotected meadows, but no effects were detected on either the abundance or grazing rates of P. lividus. Results also showed that populations of S. salpa (mainly large individuals) benefit from fishing protection and tended to concentrate in MPAs (with the exception of Port-Cros), resulting in enhanced grazing pressure. In P. lividus abundances or size distributions did not present any significant difference between protected and unprotected areas, although most MPAs presented low sea urchin densities. While MPAs are important to preserve seagrass ecosystems, results from this study indicate that it is crucial to account for the size of these habitats, particularly when functional aspects of seagrass habitats are so strongly altered by size.

Summary 1.The dominant paradigm of top-down control of ecological communities through direct consumption pathways is giving way to a more nuanced understanding of trophic interactions with the recognition that subtler indirect effects can often play an important role in structuring communities. Direct and indirect trophic and non-trophic processes could well be acting simultaneously within the same food web, and their overall effects may even interact with each other. 2. We studied indirect interactions in a relatively simple trophic system comprising a single producer, two herbivores and a guild of predators. In particular, we assessed whether (i) the principal herbivore fish, Sarpa salpa, is capable of modifying a seagrass trait, canopy height, by grazing and (ii) whether grazing-induced habitat alteration can trigger an environment-mediated modification of the interaction between herbivorous sea urchins Paracentrotus lividus and their predators. 3. We tested these hypotheses with field experiments including fish herbivore-exclusion experiments (to examine the ability of S. salpa to modify seagrass habitats) and predation experiments using tethered sea urchins in a meadow with varying canopy heights (to test whether habitat modification can mediate urchin predation risk). 4. The effect of S. salpa herbivory was highly significant. Canopy height in herbivore-excluded plots was more than 3Á5 times higher than in uncaged control plots. In addition, adult sea urchin predation risk in the most highly grazed plots was 4-5 times higher than plots with higher canopy heights. In contrast, predation risk on juvenile urchins was not influenced by canopy height. 5. Our results show that predation pressure on a key herbivore can be modified both by the environmental context within which it finds itself and by the actions of another herbivore that modifies the plant traits that create this environmental context. These act as two discrete pathways that interact in potentially nonlinear ways, mediating top-down control in these ecosystems. Herbivores, particularly when acting as ecosystem engineers, may have the potential to mediate and increase predation risk, as they substantially modify habitat structure, with consequences for refuge availability, among others. Assessing these indirect interactions is not just important to understand the functioning of the system itself but may have important management and conservation consequences.