THE DIFFERENT EFFECTS OF TWO GLOBAL WARMING CONDITIONS ON TWO SEA URCHIN SPECIES EXPOSED TO ENVIRONMENTAL POLLUTANTS

Abstract

Gradual ocean warming and marine heatwaves represent major threats for marine organisms already facing other anthropogenic-derived hazards, such as chemical contamination in coastal areas. Sea urchin larvae are a common model system in marine ecotoxicological studies to assess the impacts of climate change and pollution. Here I report the combined effects of thermal stress and exposure to three different pollutants of the marine environment (gadolinium, vanadium and phthalates) on embryos of two key Mediterranean sea urchin species with predicted opposite responses to global warming, the temperate Paracentrotus lividus and the sub-tropical Arbacia lixula. These species are among the most abundant echinoids living in shallow rocky reefs of the southwestern Mediterranean and are ecologically important because their herbivorous grazing impacts macroalgae and leads to the formation of barrens habitats. Embryos were exposed to combined treatments of three temperatures (18°C, 21°C, 24°C) and a wide range of concentrations of the three pollutants (from environmentally relevant to cytotoxic). We tested the single and combined effects to thermal stress and pollutants at two functional levels: i) exposure–response relationships, analyzing the effects in terms of abnormal development and mortality; ii) morphological, analyzing impacts of treatments on larval phenotypes and morphometric traits of larval growth and biomineralization. With respect to developmental progression, elevated temperatures at near-future projections (+3°C, 21°C) accelerated development and achievement of the larval stage, while extreme warming at present-day marine heatwave conditions (+6°C, 24°C) breached the thermotolerance threshold of both species with a higher proportion of abnormal larvae. We found a fascinating double side effect of increased temperature combined to pollution: a mild temperature increase (+3°C) reduced the negative effects of pollutants on development with a lower percentage of abnormality and improved skeleton growth, while combined heatwave conditions (+6°C) and pollution resulted in a lower proportion of embryos reaching the advanced larval stages. Our results indicate that the negative effects of pollutants-exposure on P. lividus and A. lixula larval development and biomineralization will be mitigated by near-future ocean warming temperatures, up to a thermotolerance threshold triggering negative synergistic effects. Our data highlight the need for a better understanding of the interactions between the multiple stressors faced by marine species in coastal environments.