Interactive effects of near-future temperature increase and ocean acidification on physiology and gonad development in adult Pacific sea urchin,Echinometra sp. A

Increased atmospheric CO₂ will have a twofold impact on future marine ecosystems, increasing global sea surface temperatures and uptake of CO₂ (Ocean Acidification). Many experiments focus on the investigation of one of these stressors, but under realistic future climate predictions, these stressors may have interactive effects on individuals. Here, we investigate the effect of warming and acidification in combination. We test for interactive effects of potential near-future (2100) temperature (+2 to 3 °C) and pCO₂ (~860–940 μAtm) levels on the physiology of the tropical echinoid Echinometra sp. A. The greatest reduction in growth was under simultaneous temperature and pH/pCO₂ stress (marginally significant temperature × pH/pCO₂ interaction). This was mirrored by the physiological data, with highest metabolic activity (measured as respiration and ammonium excretion) occurring at the increased temperature and pCO₂ treatment, although this was not significant for excretion. The perivisceral coelomic fluid pH was ~7.5–7.6, as typical for echinoids, and showed no significant changes between treatments. Indicative of active calcification, internal magnesium and calcium concentrations were reduced compared to the external medium, but were not different between treatments. Gonad weight was lower at the higher temperature, and this difference was more distinct and statistically significant for males. The condition of the gonads assessed by histology declined in increased temperature and low pH treatments. The Echinometra grew in all treatments indicating active calcification of their magnesium calcite tests even as carbonate mineral saturation decreased. Our results indicate that the interactive temperature and pH effects are more important for adult echinoids than individual stressors. Although adult specimens grow and survive in near-future conditions, higher energy demands may influence gonad development and thus population maintenance.