Interactive effects of temperature and salinity on early life stages of the sea urchin Heliocidaris crassispina

Marine organisms are currently challenged by multiple and interactive environmental stressors. In the subtropics, warming and intensified precipitation, and hence, reduced salinity, are particularly relevant. Using the sea urchin, Heliocidaris crassispina, we investigated the effects of warming and low salinity on fertilization success and early development. These planktonic developmental stages play significant roles in shaping population dynamics. Gametes were exposed to a temperature gradient (28–43 °C) while being held at two salinities (24 and 32). Fertilization had a higher critical temperature (LT₅₀), the temperature at which 50% individuals reached the designated stage, of 39 °C than that of blastula formation at 31 °C for both salinities, suggesting between-stage variations in sensitivity. The LT₅₀ for blastula formation was very close to present-day recorded maximum sea surface temperature of 31 °C suggesting a small thermal safety factor. Larvae were also reared to the eight-arm stage in one of the four combinations of temperatures (24 and 28 °C) and salinities (24 and 32), which correspond to sea surface temperatures and salinities observed during the urchin’s spawning season. Low salinity and high temperature had interactive effects in reducing larval survivorship. However, amongst larvae that survived the combined stress, warming reduced the negative impact of reduced salinity on arm growth. Unexpected release of blastula-like particles was documented in all treatments except the control (24 °C and salinity 32). Incomplete separations that resulted in conjoined twins, however, were only found at 28 °C. There were significantly different responses in fertilization success and larval growth between maternal lineages. Such intra-specific variations highlight the presence of phenotypic plasticity and could imply the presence of genetic variations in response to thermal and salinity stress. Such plasticity suggests that although purple urchins are experiencing extreme conditions that are stressful at present, they may be able to cope with the future ocean conditions.