Thermal responses of the sea urchin Paracentrotus lividus: evaluating grazing activity in a warming Mediterranean.

Human influence is currently impacting and altering almost all terrestrial and marine ecosystems, increasing the pace of changes and losses in biodiversity. Among these anthropogenic pressures, human-caused climate changes play a pivotal role, particularly in the Mediterranean Sea, a recognized climate change hotspot warming three times faster than the global ocean average. Organisms typically respond to warming by tolerating the changes through genetic adaptation or physiological acclimation, or by shifting their distribution ranges to track optimal environmental conditions. However, thermal responses are highly variable among taxa and even within species’ ranges, especially for ectotherms as most marine organisms. The sea urchin Paracentrotus lividus plays a key ecological role in the Mediterranean sea, being the primary driver of macroalgal forests collapse and state shifts through its grazing activity. Therefore, it appears of a critical importance to understand how its behavioral and physiological traits will respond to future warming scenarios. In this study, we investigated whether P. lividus can plastically acclimate to increasing temperatures, assessing changes in its thermal optima and tolerance limits across different environmental temperatures by analyzing grazing and respiration rates and LT50. Individuals from the same genetic population were sampled from two sites characterized by different thermal regimes. Controlled laboratory exposure to a gradient of 6 temperatures, from 15 to 29°C, allowed to determine whether individuals exposed to different thermal regimes exhibit different responses to warming and whether they can shift their thermal optimum towards higher temperatures. Furthermore, exposure to a thermal ramp enabled the determination of potential shifts in thermal tolerance among the different local populations. Such responses would indicate a capacity for plastic acclimation in P. lividus that could carry important ecological implications for coastal ecosystems in a warming Mediterranean.