Effects of Ultraviolet Radiation (UVR) and Other Environmental Stressors on the Development of Intertital Mollusc Embryos
Ozone depletion is a humaninduced global phenomenon that allows increased ultraviolet radiation (UVR) to the Earth's surface. Although UVR is known to be harmful, relatively little is known about how increased UVR impacts natural ecosystems. Ecosystems in New Zealand are particularly at risk, because ozone depletion is much greater here, with levels of biologically harmful UVR up to two times greater than in northern latitudes. In the intertidal environment, potentially negative abiotic stressors are associated with low tide; and organisms inhabiting this environment are particularly vulnerable to UVR. Furthermore, embryos and larvae deposited in this habitat are especially susceptible to these stressors. The aim of this study is to identify the effect of UVR and other environmental stressors on the development of mollusc embryos in New Zealand. Surveys of microhabitats in which egg mass deposition occurs, and what effect this site of deposition has on the survivorship of embryos, revealed that encapsulated embryos of the two common pulmonate limpets Benhamina obliquata and Siphonaria australis are highly vulnerable to the environmental stressors associated with different microhabitats. In particular, egg masses deposited in the sun for both species suffered high mortality. Although, B. obliquata is more susceptible to UVR damage than is S. australis, B. obliquata predominantly deposits egg masses in dry shaded microhabitats while S. australis deposits the majority of its offspring in sunny tidal pools, which surprisingly equated to highest embryonic mortality. Results of manipulative experiments reflected those found in the surveys: egg masses exposed to full spectrum light incurred the greatest embryonic mortality; additionally environmental stressors (e.g. tidal pool conditions and desiccation) synergistically enhanced this mortality. UVR in North America is significantly lower compared to New Zealand; this allowed a unique opportunity to use identical methods to examine the responses of ecologically similar, related species (bubble shell snails in the genus Haminoea), from two regions where UVR naturally differs. Results from surveys and manipulative experiments revealed that the New Zealand species Haminoea zelandiae suffered high embryonic mortality under full spectrum light and this mortality was enhanced by periods of desiccation. The North American species Haminoea vesicula also suffered significant mortality during periods of desiccation, but there were no signs of UVR damage. These results appear to be driven by speciesspecific vulnerability to these stressors and not to ambient UVR intensity in the regions at the time of study. Relative concentrations of the chemical sunscreen compounds, mycosporinelike amino acids (MAAs), varied depending on several factors, but the biggest differences were among species. Analyses revealed that B. obliquata had the highest concentration of MAAs despite suffering high embryonic mortality when exposed to direct sunlight. MAA concentrations in S. australis were intermediate, with H. zelandiae having the lowest concentrations of all three species. MAA concentration for B. obliquata was dependent on stage of development and initial sun exposure at egg mass deposition site, suggesting interactions between MAAs, environmental conditions and embryonic development that need to be further explored. MAA concentrations were higher in S.australis egg masses deposited in spring compared to those deposited in early autumn, which may be driven by a shift in diet or nutrient levels. MAA concentrations did not appear to be correlated with ambient levels of UVR or embryonic survival in S.australis. However, MAA concentrations were related to UV irradiance in both Haminoea species with higher MAA concentrations observed in egg masses initially deposited in the sun compared to those found in the shade. Combined these results suggest: (1) increased UVR due to ozone depletion together with increases in temperatures due to climate change are likely to have strong impacts on the early life stages of these species, unless behavioural and physiological adaptations occur; (2) New Zealand species may be at particularly high risk from UVR damage compared to those from the Northern hemisphere; (3) the role of MAAs as photo-protection in these mollusc species is likely to be species specific, with a variety of abiotic and biotic factors influencing their uptake and sequestration. These experiments in part demonstrate how New Zealand's mollusc species are responding to humaninduced changes in UVR levels.