Hollow aggregations of moon jellyfish (Aurelia spp.)

Abstract

The relative importance of behavior and currents in forming and maintaining jellyfish aggregations is not completely understood; the objective of this work was to determine how the physical properties of the water column were related to the formation of hollow aggregations of moon jellyfish (Aurelia spp.). Hollow aggregations were observed near the surface by airborne lidar in shallow water (<37 m) when the winds were light (<4.3 m s-1). In this work, a hollow aggregation is defined as a region of few individuals surrounded by high densities in the two dimensions defined by depth and the direction of flight. Hydrographic profiles were available for most of the observations, and the bottom of the aggregation was correlated (R2 = 0.42, P = 8 × 10-4) with the depth of the shallow (<13 m) surface mixed layer despite differences in position and time between the lidar observations and the hydrographic measurements. The size and shape of these aggregations suggests that they are not simply a result of advection by local currents, but of active behaviors. A likely mechanism is that the individuals are swimming in a vertical circle, and this behavior is predicted to enhance mixing at the top of the pycnocline.