Vertical distribution of echinoid larvae in pH stratified water columns

The abundance and distribution of many benthic marine organisms are shaped by the success of their dispersive larval life-history stage. An increasing number of studies have shown that ocean acidification negatively impacts the larval life-history stage, including those of echinoids which are commercially and ecologically important. However, little is known about the behavioral responses of echinoid larvae to different pH levels in the water column. Changes in vertical movement in response to the naturally occurring pH variations caused by biological activities and/or physical conditions could affect dispersal and recruitment. In this study, we quantified the vertical distribution of larval sand dollars, Dendraster excentricus (Echinodermata), in water columns with stratified layers of seawater varying in salinity and pH. When larval sand dollars swimming upwards in ambient seawater (pH_NBS 7.86 ± 0.04) encountered a layer of low pH (pH_NBS 7.54 ± 0.04) seawater, about half of the individuals (53 ± 28%) were aggregated near the transition layer 60 min after the start of the experiment. Preliminary video analysis showed larvae reversed their direction of travel and altered the shape of their helical swimming trajectories, upon encountering the transition layer moving from ambient to low pH water. In contrast, when larval sand dollars swimming upwards in acidified seawater encountered ambient seawater, they continued to swim upward to aggregate near the top of the column. In control water columns with uniform pH, larvae did not change swimming behavior regardless of whether pH was ambient or acidified and whether salinity was uniform or stratified. These results indicate that stratification itself did not strongly affect the vertical distributions of larvae. These observations suggest that echinoid larvae, and perhaps many other types of planktonic larvae, may use behavioral plasticity to reduce exposure to stresses from ocean acidification. The presence and effectiveness of these responses may improve the ability of larvae to cope with stressful, dynamic habitats, and hence may be significant to prediction of potential impacts of global climate change.