Desislava Bögner

• Ocean Acidification (OA) is the result of the dissolution of carbon dioxide (CO2) into the ocean with a high contribution from anthropogenic activities. OA impairs physiological processes of exposed organisms and most calcifying phyla are sensitive. Many of them belong to important food webs, play key roles at ecosystem level and have economical relevance. In aquatic environments fertilization is frequently external and gametes are released into the water column in massive external spawning events being threatened by natural and artificially created factors. The comprehension of OA effects on crucial steps of fertilization and development becomes essential for assessing the real impact on population sustainability and species survivorship. In the current study, the effects of CO2-simulated OA on the fertilization success of the sea urchin species Strongylocentrotus droebachiensis were analysed using preindustrial-actual atmospheric CO2 concentrations and levels expected to occur by the end of the century and beyond. The impact of long exposure periods (simulating asynchronous spawning) on fertilization success and the pHi changes of spawned eggs exposed to acidified scenarios before fertilization were assessed. The impact of OA on cytoskeleton in the actin-rich cortex region was examined during the first hours after fertilization. There was a delayed development and an increased polyspermy risk evidenced by the presence of sperm in the perivitelline space under acidified conditions. This might be related to alterations in pHi and actin polymerization of the exposed eggs that could synergically influence the formation of the fertilization envelope and hyaline layer. These structures are required for protecting the eggs from polyspermy. Cytoskeleton disruptions negatively influence the gamete interaction and cellular division, as found in this study. Disruptions on calcium signalling cannot be excluded. Despite being not in the scope of the current study, the relation of the Ca2+signalling pathway with the pHi homeostasis under acidified conditions was discussed.