Acidification reduced growth rate but not swimming speed of larval sea urchins.

Swimming behaviors of planktonic larvae impact dispersal and population dynamics of many benthic marine invertebrates. This key ecological function is modulated by larval development dynamics, biomechanics of the resulting morphology, and behavioral choices. Studies on ocean acidification effects on larval stages have yet to address this important interaction between development and swimming under environmentally-relevant flow conditions. Our video motion analysis revealed that pH covering present and future natural variability (pH 8.0, 7.6 and 7.2) did not affect age-specific swimming of larval green urchin Strongylocentrotus droebachiensis in still water nor in shear, despite acidified individuals being significantly smaller in size (reduced growth rate). This maintenance of speed and stability in shear was accompanied by an overall change in size-corrected shape, implying changes in swimming biomechanics. Our observations highlight strong evolutionary pressure to maintain swimming in a varying environment and the plasticity in larval responses to environmental change.