Calanus Pacificus, a Marine Copepod, Shows Poor Avoidance of Hypoxic Water

Calanus pacificus, a marine copepod that diurnally vertically migrates, is a pillar of the marine food web. It inhabits seasonally hypoxic waters including the California Current and Puget Sound. Climate change may contribute to more extreme hypoxia in these regions, and if C. pacificus cannot sense and move to avoid hypoxic water layers, its populations may suffer. In order to test C. pacificus’s response to hypoxic water, copepods were collected from Central Basin, Puget Sound via net tows. A mix of female and C4/C5 C. pacificus were selected and stored in darkness at 14°C. Using salinity to create density stratification, four fiberglass columns (1 m x 8.9 cm x 8.9 cm) were prepared with hypoxic water (<0.9 mg/L) in the lower half of two columns, and high oxygen water (>10 mg/L) throughout the other two as controls. Copepods were introduced to the tops of the water columns and their activities were recorded on IR camera. Using 2D tracking software, swimming behavior at the time of introduction was compared with behavior recorded after two hours of acclimation. While activity was initially similar in all four columns, after two hours the recordings showed almost no activity below the oxycline in hypoxic columns versus continued activity throughout the same depths in control columns. Overall, C. pacificus tended to swim downward, and they maintained this behavior in all four columns during the experiment despite the lethal hypoxic lower layers in the experimental columns. Visual observation confirmed the mortality of a majority of the copepods in the hypoxic columns immediately after the final video capture. However, a few individuals were recorded hovering above the oxycline, clearly demonstrating avoidance of the hypoxic layer beneath. Unfortunately, if avoidance behavior is as uncommon in natural settings as it was under lab conditions, the future of C. pacificus could be in jeopardy.