Effects of CO2, pH and temperature on respiration and regeneration in the burrowing brittle stars Hemipholis cordifera and Microphiopholis gracillima

Hemipholis cordifera and Microphiopholis gracillima are burrowing brittlestars that differ in burrow architecture and oxygen obtaining strategies: M. gracillima actively ventilates a gallery while H. cordifera has a simple chamber and instead relies on oxygen being transported from arms exposed to the overlying water using hemoglobin in its water vascular system. To determine the possible effects of near future climate change on both species, in terms of metabolism and regeneration, they were exposed to current (25 °C & 28 °C) and elevated (32 °C) temperatures, as well as normal (8.1) and hypercapnia/lowered pH (7.8 & 7.6) in all combinations for six weeks. Oxygen uptake was measured weekly during this period. As expected, M. gracillima had a higher overall oxygen uptake rate than H. cordifera. Both species had highly variable oxygen uptake and were significantly affected by the week was measured. H. cordifera experienced increased oxygen uptake at the higher temperature (32 °C) and as a result of interactive effects of time and pH 7.6. Both species experienced interactive effects of pH and temperature, but there was no clear pattern. Increased temperature positively affected arm regeneration in H. cordifera, increasing both length and percent recovery. There were no effects on disc regeneration observed in M gracillima, however lower pH decreased the dry weight in both intact and regenerating animals. Calcification, measured as percent inorganic content, was not affected in either species by regeneration, temperature or pH. Despite the minor effects on its physiology, M. gracillima experienced lower percent survivorship than H. cordifera. Temperature had the most effect, with survivorship higher at 28 °C, than at 25 °C or 32 °C. These results indicate that both species are operating near or at their physiological limits and may be unable to cope with future drastic changes.