Ocean warming and acidification pose synergistic limits to the thermal niche of an economically important echinoderm

Abstract It is critical to understand how abiotic factors may interact to constrain the distribution and productivity of marine flora and fauna in order to make robust projections of the impacts of climate change. We evaluated the effects of projected near-future ocean acidification (OA) and warming (OW) on the thermal tolerance of an important living marine resource, the sea urchin Loxechinus albus , a benthic shallow water coastal herbivore inhabiting part of the Pacific coast of South America. After exposing young juveniles for a medium-term period (1-month) to contrasting p CO 2 (~500 and 1400 μatm) and temperature (~15 and 20 °C) levels, critical thermal maximum (CTmax) and minimum (CTmin) as well as thermal tolerance polygons were assessed based on self-righting success as an end point. Transcription of heat shock protein 70 ( HSP70 ), a chaperone protecting cellular proteins from environmental stress, was also measured. Exposure to elevated p CO 2 significantly reduced thermal tolerance by increasing CTmin at both rearing temperatures and decreasing CTmax at 20 °C. There was also a strong synergistic effect of OA × OW on HSP70 transcription levels which were 75-fold higher than in control conditions. If this species is unable to adapt to elevated p CO 2 in the future, the reduction in thermal tolerance and HSP response suggests that near-future warming and OA will disrupt their performance and reduce their distribution with ecological and economic consequences. Given the wider latitudinal range (6 to 56°S) and environmental tolerance of L. albus compared to other members of this region's benthic invertebrate community, OW and OA may cause substantial changes to the coastal fauna along the Chilean coast.