Echinoderm sperm swimming and fertilization

In an era of climate change, impacts on the marine environment include
warming and ocean acidification. These effects can be amplified in shallow
coastal regions where conditions often fluctuate widely. This type of
environmental variation is potentially important for many nearshore
species that are broadcast spawners, releasing eggs and sperm into the
water column for fertilization. We conducted two experiments to
investigate: 1) the impact of water temperature on sperm swimming
characteristics and fertilization rate in sand dollars (Dendraster
excentricus) and sea urchins (Mesocentrotus franciscanus) and; 2) the
combined effects of multiple stressors (water temperature and pH) on these
traits in sand dollars. We quantify thermal performance curves showing
that sand dollar fertilization rates, sperm swimming velocities, and sperm
motility display remarkably wide thermal breadths relative to red urchins,
perhaps reflecting the wider range of water temperatures experienced by
sand dollars at our field sites. For sand dollars, both temperature (8,
16, 24°C) and pH (7.1, 7.5, 7.9) affected fertilization but only
temperature influenced sperm swimming velocity and motility. Although
sperm velocities and fertilization were positively correlated, our
fertilization kinetics model dramatically overestimated measured rates and
this discrepancy was most pronounced under extreme temperature and pH
conditions. Our results suggest that environmental stressors like
temperature and pH likely impair aspects of the reproductive process
beyond simple sperm swimming behavior.