6. Will ocean acidification alter thermal windows of echinoderm fertilisation and early development?

Sam Karelitz (1), Sven Uthicke (2), Miles Lamare (1)*

1 University of Otago, Dunedin, Otago, 9016, New Zealand
2 Australian Institute of Marine Science, Townsville MC, PMB No 3, Queensland 4810, Australia

Background
The thermal windows of developmental stages are key drivers of the geographical distribution and abundance of marine invertebrates. If changes in sea temperature and pH interact to affect species in the future, then ocean acidification may influence developmental thermal windows, and may be important considerations under realistic climate change scenarios.

Methods
We examined the effects of reduced seawater pH/increased pCO2 on the thermal windows of fertilisation and embryology of three echinoid species; one tropical (Arachnoides placenta), one temperate (Fellaster zelandiae), and one polar (Sterechinus neumayeri). Using a thermal block heated and cooled at each end to create a temperature gradient, developmental responses were examined across 12 temperatures (centered around current ambient temperatures during spawning seasons) ranging from -1.1 to 5.7 C (S. neumayeri), 5.8 to 27 C (F. zelandiae), and 14.1 to 35.3 C (A. placenta) under present (≈450 ppm pCO2) and near future (year 2100) ocean pH/pCO2 (≈900 ppm pCO2) conditions.

Findings
The tropical A. placenta had a broader optimum thermal range for successful fertilisation (19 to 32 C) than F. zelandiae (15.8 to 23.4 C) and S. neumayeri (-1.1 to 4.3 C). Neither fertilisation nor the fertilisation window was affected by pH in any of the species. A. placenta and F. zelandiae had broad thermal windows for embryological development (21 to 31C and 10 to 21.4C respectively) compared to S. neumayeri (-1.1 to 1.35C). While pH did slightly reduce normal development of A. placenta, no significant interaction between pH and temperature on embryological development was detected across the thermal window for any of the three species.

Conclusions
The results of this study suggest that in terms of fertilisation and early development, while temperature is an important factor influencing species distribution in future ocean conditions, there was little evidence of an interactive effect of ocean acidification on their specific thermal windows.