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- Canopy microclimate modification in central and marginal populations of a marine macroalgaPublication . Monteiro, Cátia; Zardi, Gerardo I.; McQuaid, Christopher D.; Serrao, Ester; Pearson, Gareth; Nicastro, KatyThe effects of environmental changes on species distribution are generally studied at large geographical scales. However, aggregations of individuals can significantly moderate the impact of the environment at smaller, organismal scales. We focused on the intertidal macroalga Fucus guiryi and carried out field and laboratory common garden experiments to evaluate how the different individual morphologies and canopy densities typical of central and peripheral populations modify microhabitat conditions and associated levels of stress. We show that F. guiryi canopies significantly alter environmental conditions (i.e., temperature, humidity and light regimes) and mitigate the levels of stress experienced by individuals within the group. Southern algae are more branched and form denser canopies but, unexpectedly, despite these considerable differences, the mitigating effects of northern and southern canopies did not differ significantly. Microhabitat conditions beneath canopies were more stressful at marginal locations, indicating that southern populations are not more effective than northern algae at mitigating the harsher climate at the edge of the species distribution. Our findings highlight the importance of assessing structural changes in aggregating species across their distribution and relating these to local climates to understand the impact of environmental changes at scales relevant to individual organisms.
- Latitudinal incidence of phototrophic shell-degrading endoliths and their effects on mussel bed microclimatesPublication . Lourenço, Carla R.; R Nicastro, Katy; McQuaid, Christopher D.; Sabour, Brahim; Zardi, Gerardo I.Aggregations of organisms commonly benefit their members by mitigating the effects of predators and environmental stresses. Mussel aggregations also form important intertidal matrices that support associated infaunal communities, the structures of which are largely shaped by the conditions within the interstitial spaces. Intertidal mussels are frequently parasitized by phototrophic endoliths that infest the shell and have thermoregulatory effects on both solitary and aggregated mussels by changing shell albedo. A large-scale sampling was carried out May June 2016 along Portuguese and Moroccan shores to investigate a latitudinal gradient of endolithic infestation of the intertidal mussel Mytilus galloprovincialis. Endolithic infestation increased towards lower latitudes most likely as a response of greater light availability. Additionally, artificial beds of either 100% non-infested or infested biomimetic mussels were used to test whether infestation alters the temperature and humidity of the interstitial spaces within beds, and if mussels surrounded by infested mussels experience lower body temperatures than those surrounded by non-infested ones. Conditions within beds of infested mussels were significantly cooler and more humid than in non-infested beds and individuals in the centre of infested mussel beds experienced significantly lower body temperatures. Under a scenario of warming climate, endolithic infestation of mussel beds might thus represent an ecological advantage not only for M. galloprovincialis as a species but also for the associated communities.
- Weather and topography regulate the benefit of a conditionally helpful parasitePublication . Monsinjon, Jonathan R.; McQuaid, Christopher D.; Nicastro, Katy; Seuront, Laurent; Oróstica, Mauricio H.; Zardi, Gerardo I.Heat-induced mass mortalities involving ecosystem engineers may have long-lasting detrimental effects at the community level, eliminating the ecosystem services they provide. Intertidal mussels are ecologically and economically valuable with some populations facing unprecedented heat-induced mass mortalities. Critically, mussels are also frequently infested by endolithic parasites that modify shell albedo, hence reducing overheating and mortality rates under heat stress. Using a biophysical model, we explored the topographical and meteorological conditions under which endolithically driven thermal buffering becomes critical to survival. Based on meteorological data from a global climate analysis, we modelled body temperatures of infested and non-infested mussels over the last decade (2010-2020) at nine sites spread across c. 20 degrees of latitude. We show that thermal buffering is enhanced where and when heat stress is greatest, that is, on sun-exposed surfaces under high solar radiation and high air temperature. These results suggest that new co-evolutionary pathways are likely to open for these symbiotic organisms as climate continues to change, potentially tipping the balance of the relationship from a parasitic to a more mutualistic one. However, endolithically driven reductions in body temperatures can also occur at or below optimal temperatures, thereby reducing the host's metabolic rates and making the interplay of positive and negative effects complex. In parallel, we hindcasted body temperatures using empirical data from nearby weather stations and found that predictions were very similar with those obtained from two global climate reanalyses (i.e. NCEP-DOE Reanalysis 2 and ECMWF Reanalysis v5). This result holds great promise for modelling the distribution of terrestrial ectotherms at ecologically relevant spatiotemporal scales, as it suggests we can reasonably bypass the practical issues associated with weather stations. For intertidal ectotherms, however, the challenge will be incorporating body temperatures over the full tidal cycle.
- Symbiotic endolithic microbes reduce host vulnerability to an unprecedented heatwavePublication . Zardi, Gerardo; Monsinjon, Jonathan R.; Seuront, Laurent; Spilmont, Nicolas; McQuaid, Christopher D.; Nicastro, KatyHeatwaves are increasingly severe and frequent, posing significant threats to ecosystems and human well-being. Characterised by high thermal variability, intertidal communities are particularly vulnerable to heat stress. Microbial endolithic communities that are found in marine calcifying organisms have been shown to induce shell erosion that alters shell surface colour, lowering body temperatures and increasing survival rates. Here, we investigate how the symbiotic relationship between endolithic microbes and the blue intertidal mussel Mytilus edulis mitigates thermal stress during the unprecedented 2022 atmospheric heatwave in the English Channel. Microbial infestation of the shell significantly enhanced mussel survival, particularly higher on the shore where thermal stress was greater. Using data from biomimetic temperature loggers, we predicted the expected thermal buffer and observed differences up to 3.2 °C between individuals with and without symbionts under the known conditions of the heat wave-induced mortality event. The ecological implications extend beyond individual mussels, affecting the reef-building capacity of mussels, with potential cascading effects for local biodiversity, carbon sequestration, and coastal defence. These findings emphasize the importance of understanding small-scale biotic interactions during extreme climate events and provide insights into the dynamic nature of the endolith-mussel symbiosis along a parasitic-mutualistic continuum influenced by abiotic factors.