| Summary: | Under future ocean warming, thermal tolerance of developmental stages may be a key driver of changes in the geographical distributions and abundance of marine invertebrates. Additional stressors such as ocean acidification may influence thermal windows and are therefore important considerations under realistic future climate change scenarios. The effects of reduced seawater pH on the thermal windows of fertilisation, embryology and larval morphology were examined using five echinoderm species; two polar (Sterechinus neumayeri and Odontaster validus), two temperate (Fellaster zelandiae and Patiriella regularis) and one tropical (Arachnoides placenta). Using a thermal heat block, heated and cooled at each end to create a temperature gradient, responses were examined across 12 temperatures ranging from -1.1 –5.7 C (S. neumayeri), -0.5 –10.7 C (O. validus), 5.8 –27 C (F. zelandiae), 6 –27.1 C (P. regularis) and 13.9 –34.8 C (A. placenta) under present day and near future (2100+) ocean acidification conditions to test the hypothesis that a synergistic interaction between temperature and reduced seawater pH would result in a narrowing of thermal windows. Thermal windows for fertilisation were broad and were not influenced by seawater pH. Optimal thermal windows for fertilisation were -1.1 –4.3 C for S. neumayeri, -0.5 –10.1 C for O. validus, 17.7 –25.4 C for F. zelandiae, 15.8 –23.4 C for P. regularis and 18.9 –32.2 C for A. placenta. Embryological development was less thermotolerant, with thermal windows ranging from -1.1 –1.35 C for S. neumayeri, -0.5 –5.6 C for O. validus, 10 –19.7 C for F. zelandiae, 10.1 –21.3 C for P. regularis and 20 –31 C for A. placenta. Although near future pH significantly reduced normal development for S. neumayeri, O. validus, P. regularis, and A. placenta, it did not affect the thermal windows for embryonic development. Thermal windows for larval development ranged from -1.1 –3.8 C for S. neumayeri, -0.5 –7.6 C for O. validus, 10 –25 C for F. zelandiae, 11.9 –23.2 C for P. regularis and 20 –32.2 C for A. placenta. Thermal windows were not influenced by seawater pH, however, larvae of S. neumayeri, F. zelandiae, P. regularis and A. placenta reared in reduced pH treatments were significantly smaller than those reared in ambient pH treatments. Postoral arm symmetry of echinoids was not affected by reduced pH, indicating that size reductions were due to developmental delay as opposed to abnormal development. Bipinnaria larvae of O. validus reared in reduced seawater pH had longer body lengths on average than larvae in ambient pH treatments. Contrary to the overarching hypothesis, effects of reduced seawater pH on thermal windows were additive, with no evidence of a synergism between temperature and pH causing thermal windows to narrow, nor was there an antagonistic effect in which temperature buffered deleterious effects of reduced pH. Results of this study suggest that in terms of fertilisation and development, temperature may be the most important factor influencing species’ latitudinal distributions in future ocean conditions, and that there is little evidence of a synergistic effect of ocean acidification on this thermal response. If this is the case, warming may reduce northern ranges of S. neumayeri and A. placenta, which are both found at the upper limits of their thermal tolerance. Ocean warming may facilitate pole-ward range expansions of F. zelandiae and P. regularis if temperature currently limits their southern distributions. The current range of O. validus does not appear to be controlled by temperature, therefore, ocean warming is unlikely to directly effect latitudinal distribution. In all cases, ocean acidification is not likely to play a synergistic role in future species distributions.
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