Thermal risks and opportunities for a tropical invasive sea urchin in a fast-warming sea

Ocean warming and bioinvasions act together in many ocean regions to rapidly tropicalize temperate and subtropical marine ecosystems. Tropicalization reshuffles natural communities where tropical species invade and establish in new regions. But will some rapidly warming regions eventually become too warm for some tropical invaders? One place to test this is the southeastern Mediterranean, a global bioinvasion and warming hotspot. This study focuses on the non-indigenous sea urchin Diadema setosum, a recent fast-spreading Mediterranean invader, the Indo Pacific-Red Sea urchin. This research aims to address the present, and future fate of this invasion at the southeastern Mediterranean reef ecosystems given ocean warming, by defining the viable, optimal and lethal temperatures for the Mediterranean D. setosum populations. Specifically, the thermal performance of D. setosum was examined, focusing on metabolic, reproduction, and food assimilation rates as performance proxies. Results show an optimal seawater temperature range of 27-28 °C for metabolic rates, 20-24 °C for gonads growth and maturation as well as food assimilation, while mortality occurred at 36 °C. These results indicate that the urchin can survive even future warming above current peak summer temperatures of 32 °C but will enter the stressful thermal range with further seawater temperature elevation. The species might already physiologically underperform under current summer temperatures in the region, which might affect its population viability. However, warming might increase the probability of spreading to new colder regions. Thus, we expect that the invader will eventually occupy most Mediterranean regions, but fitness might be eroded in the warmest part, the SE Levantine basin.