Population biology and larval ecology of the sea urchin Centrostephanus rodgersii (Agassiz 1863) in New Zealand under the influence of Global Climate Change

Abstract

The ocean has absorbed about one third of the total anthropogenic emissions of CO2 since 1800 and, consequently, global mean surface ocean pH has decreased of approximately 0.1 pH unit. This variation of atmospheric CO2 is also correlated with decadal increases in average global temperature by 0.13°C since 1956. Atmospheric CO2 levels are rising annually of 0.5%, and, if emissions are not reduced, a further decrease of 0.14-0.41 and 0.30-0.7 pH units by 2100 and 2300 respectively is expected, in a process called "ocean acidification". A concurrent average sea surface temperature increase of 2-4°C is predict by 2100, as another aspect of global climate change defined as "global warming". Temperature and pH are known to affect physiology and early development of marine invertebrates and, in the case of temperature, also geographical distribution.

This thesis examines larval and population biology of the echinoid Centrostephanus rodgersii at the Mokohinau Islands, New Zealand, under global climate change. Its growth, population size and age structure and gametogenetic cycle in this region were studied and compared to populations in mainland Australia and Tasmania. The effects of temperature and pH on early life stages of C. rodgersii were compared with those of Evechinus chloroticus and P. huttoni, two echinoids endemic to New Zealand, to provide the first overview of the responses of sea urchins to global climate change in New Zealand. The effect of these variables on the embryos and larvae and the implications of this on the distribution of these species in New Zealand in warmer and more acidic future scenarios are then discussed. C. rodgersii at the Mokohinau Islands is also compared with a population in Coffs Harbour, New South Wales, to assess the potential of this species for early adaptation to a new environment.

C. rodgersii was found to grow at a slightly faster pace than the populations in Tasmania and population size and age structure suggest that recruitments occurs regularly. The timing of the gametogenetic cycle is similar to a population from Sydney and spawning takes place between late July and September. Thermal tolerance of fertilisation was very broad in all the species considered, and the process was practically unaffected by temperature. The optimal thermal range was similar through the subsequent developmental stages and was ≈16–23°C for C. rodgersii at the Mokohinau Islands, ≈17–24°C for C. rodgersii at Coffs Harbour, ≈12–18°C for E. chloroticus and ≈12–17°C. These thermal tolerances for development of C. rodgersii agree closely with its present day distribution in New Zealand, suggesting the distribution range is largely controlled by temperature dependant recruitment in this species. Fertilisation was slightly (<10%) reduced in all species by pH conditions predicted by 2100 and no buffering effect of temperature was found. A 3°C temperature increase was either neutral or beneficial for subsequent developmental stages in E. chloroticus and C. rodgersii (Mokohinau Islands), it did not affect C. rodgersii (Coffs Harbour) and was neutral or negative for P. huttoni. Lowered pH had a general negative effect, which was always minor, compared to the effect of increased temperature. Larval morphology was affected by lowered pH differently in the three species considered. Pluteus larvae showed either shorter arms (both populations of C. rodgersii and E. chloroticus) or smaller larvae with shorter arms and an altered global morphology (P. huttoni).

This study found that the effects of increased temperature and decreased pH are species-specific. It strengthens the idea that temperature is the main stressor of the early life stages and that it often contradicts the effects of pH. Global organismal responses are, therefore, hard to predict. This study also suggests that embryos and larvae of C. rodgersii in New Zealand are, at present, in suboptimal conditions and that a temperature increase could favour the extension of its geographical range to other areas, despite the negative effects of lowered pH. Further research is needed to detail distribution of C. rodgersii in New Zealand and to clarify potential interactions with E. chloroticus. Physiological measurements on embryos and larvae reared in near future scenarios are also recommended to highlight sublethal effects.