Sea urchin reproductive performance in a changing ocean: examining the impacts of climate change and environmental variability on gametes, fertilisation and early development in sea urchins

Global oceans are currently changing at a faster rate than at any other time over the last 300 million years, exposing marine fauna to multiple stressors and increasing extinction risks. Temperatures are rising and the oceans are becoming more acidic. At the same time, extreme climatic events are becoming more common and particularly in coastal areas, seasonal and diurnal variability is predicted to increase. Phenotypic plasticity, both within and across generations, is likely to play an important role in the survival of species by enabling individuals to cope with rapidly changing environments. In particular this is true for broadcast spawning species whose gametes are exposed to environmental conditions prior to fertilisation. We examined the combined impacts of temperature and acidity under either static or variable conditions on reproductive performance and early larval development in the sea urchin Paracentrotus lividus. Adult urchins were maintained under one of four experimental regimes: today-static (16 °C, ~500 µatm pCO₂), future-static (20 °C, ~1300 µatm pCO₂), today-variable (15-17 °C, ~500-700 µatm pCO₂), or future-variable (19-21 °C, ~1300-1500 µatm pCO₂). Variable regimes reflected tidal cycles with each extreme being reached twice per 24 h period. Following a two-month exposure, eggs and sperm were collected from adults via controlled spawning. Egg sizes and sperm functional parameters were measured, controlled fertilisations were carried out, and early larval survival and development was examined. We found that egg sizes were influenced by both climate and environmental variability whereas sperm parameters (e.g. straight-line velocity) were only influenced by environmental variability. Fertilisation success did not vary across conditions, but a high amount of inter-individual variability was observed within each condition. Larval survival was influenced by climate, and the interaction of climate and variability. Larval development appeared to be influenced by both climate and variability, with individuals from parents maintained in today-variable conditions developing the fastest. Here, we discuss our findings and the implications of our results on selection in a future ocean.