Impacts of ocean acidification on survival, growth, and swimming behaviours differ between larval urchins and brittlestars

Ocean acidification (OA) is widely recognized as an increasing threat to marine ecosystems. Many marine invertebrates have dual-phase life cycles in which planktonic larvae connect and sustain otherwise disconnected benthic adult populations. Many planktonic larvae are particularly sensitive to environmental stresses including OA. Here,we compared the developmental dynamics, survivorship, and swimming behaviours of plutei of two ecologically important echinoderm species that naturally experience variability in ambient pH: The purple urchin Strongylocentrotus purpuratus and the infaunal brittlestar Amphiura filiformis. Sensitivity to decreasedpH differed between these two species and between maternal lineages. Larvae of both species experienced increased mortality and reduced growth rate under low pH conditions. However, larval brittlestars appeared more sensitive and experienced over 80% mortality after 7-d exposure to pH 7.7. Larval urchins from one maternal lineage underwent highly synchronized budding (release of blastula-like particles) at low pH. Observed budding temporarily increased numerical density and reduced individual size, leading to differences in growth and mortality rates between the two half-sibling groups and another population. Swimming speeds of larval brittlestars were reduced in decreased pH. In contrast, acidification had either no effect or positive effect on swimming speeds of larval urchins. The observed differences between species may be a reflection of pre-exposure in their natural habitats: larval brittlestars experience a relatively stable in situ pH environment, whereas larval urchins are occasionally exposed to low pH in upwelling regions. Urchins may therefore exhibit short-Term compensatory responses such as budding and increased swimming speed. Natural selection could act upon the significant variations we observed between maternal lineages, resulting in more resilient populations confronting chronic exposure to OA.