Effects of hypoxic layers on the swimming behavior of Lacuna vincta larvae from Friday Harbor, WA, USA

Hypoxia has the potential to influence marine organism function at many stages in the life cycle. Most studies on benthic invertebrates have focused on adult animals, but it is important to consider impacts on larvae, as they will have consequences for survival and recruitment of benthic populations. We sought to determine whether hypoxia alters the swimming behavior of Lacuna vincta larvae, and whether larvae are able to recognize and avoid hypoxic layers. We used video recording to compare larval behavior in stratified water columns with an upper layer with normoxic oxygen levels (~85% saturation, ~7.5 mg/L) and a lower layer with hypoxic oxygen levels (~13% saturation, ~1 mg/L), to behavior in similarly stratified columns where both upper and lower layers were normoxic (~85% saturation, ~7.5 mg/L). The vast majority of L. vincta larvae recorded in the videos were concentrated at the surface in our experiments. Trends among the small fraction of larvae not clustered at the top 20 mm of the columns showed: (1) slower swimming speed along the path of travel in the hypoxic lower layer than the control lower layer, (2) slower downward velocity in the hypoxic columns than the control columns, (3) faster upward velocity in the hypoxic columns than the control columns, (4) the presence of larvae on the very bottom of the control columns versus a near absence of larvae at the bottom of the hypoxic treatment columns, and (5) a slightly higher fraction of time spent in the lower layer in the control columns. The small magnitudes of these differences may be due to the small number of larvae that traveled down below the surface in the columns in all treatments, or may indicate a relatively low sensitivity to hypoxia in this species. As global climate change continues and coastal hypoxia increases in frequency and severity, it is crucial to improve our understanding of the impacts of hypoxia across ontogenetic stages of marine invertebrates, so as to understand the implications for survival, recruitment, and population resilience. Results of this study demonstrate that upswimming tendencies may reduce encounters of L.vincta larvae with hypoxia, that larvae alter swimming behaviors in response to hypoxia, and that the observed behavioral changes appear generally consistent with a weak ability to escape or avoid hypoxia layers