Ocean warming will mitigate the effects of acidification on calcifying sea urchin larvae (Heliocidaris tuberculata) from the Australian global warming hot spot

The negative effect of ocean change stressors - warming and acidification - on calcifying invertebrate larvae has emerged as a significant impact of global change. We assessed the arm growth response of the echinopluteus larva of Heliocidaris tuberculata to simultaneous exposure to these stressors in cross-factorial experiments involving ambient and near future temperatures (control − 20 °C; + 4 °C: 24 °C) and pHNIST levels (control: pH 8.1; − 3-7 pH units: pH 7.6-7.8). The more extreme pH treatment, pH 7.4, not a near-future condition, was used to assess tolerance levels. Experiments were designed with respect to present day conditions determined for the habitat of H. tuberculata and future (2100 +) conditions for the southeast Australia global warming hot spot. Across near future warming-acidification treatments (24 °C/pH 7.6-7.8) there was a 5-25% decrease in normal development and, at pH 7.4, this increased to 11-33%. Increased temperature facilitated larval growth across all pH treatments with a 20-50% increase in arm length at + 4 °C across all pH levels. Larval growth was strongly reduced by acidification with a 15-25% decrease in arm length at pH 7.4-7.6 at control temperature. Warming mitigated the effect of pH on growth. Both stressors increased larval abnormality and asymmetry. The stunting effect of decreased pH on larval growth is typical of echinoplutei, indicating that similar mechanisms operate across species. The large proportion of normal and larger larvae in the + 4 °C/pH 7.8 treatments indicate that H. tuberculata may tolerate near-future ocean change and this may be facilitated by acclimatization or adaption.