Exotic herbivores can exert profound impacts on terrestrial communities, but their ecological effects on marine habitats are not sufficiently quantified. The exotic crab Percnon gibbesi, which is rapidly spreading throughout the Mediterranean Sea, grazes almost exclusively on benthic macrophytes, providing an opportunity to study the potential impacts of herbivores in the marine realm. Here, we first quantified the abundance of P. gibbesi in Mallorca (Balearic Islands; Spain) in 2023 and reported average densities of 67 individuals 100 m-2, approximately 33 times greater than those recorded in 2003 on the islands. We then performed a feeding preference experiment using common native and invasive species of macroalgae (Caulerpa cylindracea, Halimeda incrassata, Haliptilon virgatum, Halopteris scoparia, Padina pavonica and Ulva compressa) from the Mediterranean Sea. The per capita grazing rates of P. gibbesi (3.83 ± 1.71 WW g crab -1 day-1), which can ingest almost 75% of their body weight daily, were higher than those recorded for most native herbivorous species in the Mediterranean. The estimated daily grazing rates for P. gibbesi average 23.98 ± 15.45 kg WW macroalgae ha-1 d-1, value that corresponds with 0.2 to 9.1% (average 5.58%) of the total macroalgae production in this area. Our experiment revealed clear preferences of P. gibbesi for three species of macroalgae, which were not explained by the nutritional content. Overall, our results generate great concern and coupled with the large extent of the invasion, indicate that the ecological impacts of this exotic herbivore on Mediterranean marine communities could be substantial and, until now, unreported.

Seagrasses are vital organisms in coastal waters, and the drastic demise of their population in the last decades has worrying implications for marine ecosystems. Spatial models for seagrass meadows provide a mathematical framework to study their dynamical processes and emergent collective behavior. These models are crucial to predict the response of seagrasses to different global warming scenarios, analyze the resilience of existing seagrass distributions, and optimize restoration strategies. In this article, we propose a model that includes interactions among different species based on the clonal growth of seagrasses. We present a theoretical analysis of the model considering the specific case of the seagrass-seaweed interaction between _Cymodocea nodosa_ and _Caulerpa prolifera_. Our simulations successfully reproduce field observations of shoot densities in mixed meadows in the Ebro River Delta in the Mediterranean Sea. Besides, the proposed model allows us to investigate the possible underlying mechanisms that mediate the interaction among the two macrophytes.

Disentangling spatial variation in climate change impacts is a pressing challenge. Here we compared the performance of Posidonia oceanica seagrass populations to temperature, throughout a year-long translocation experiment across 2800 km in the Mediterranean Sea. Transplants in central and warm-edge locations experienced temperatures >29 ºC during summer, representing thermal anomalies >5ºC above long-term maxima for cool-edge populations, 1.5ºC for central and <1ºC for warm-edge populations. At the onset of the experiment, a highly selective herbivory event removed 75% of cool-to-warm transplant biomass but left adjacent central and warm-edge treatments intact. Despite big differences in thermal stress and acute herbivory, cool-edge populations recovered and matched warm-edge populations across all performance metrics. Central populations displayed significantly lower growth and survivorship in response to thermal stress. Our findings reveal that intraspecific variation in thermal performance does not necessarily reflect thermal geography and suggest greater resilience to warming for Posidonia oceanica than previously recognised.