Mechanisms underlying heterogeneous distribution of moon jellyfish Aurelia aurita s.l. across a sharp pycnocline

摘要

The vertical distribution pattern of moon jellyfish Aurelia aurita s.l. medusae can vary dramatically within the water column, and a sharp pycnocline is one of the major factors that limit vertical distribution. Causes of this limited distribution are likely to be either or both of 2 behavioral responses: a passive response related to buoyancy and/or an active response related to the organism's habitat requirements. However, these causes have yet to be verified. We conducted behavioral and physiological experiments and also performed numerical simulations to elucidate the mechanisms by which a pycnocline may restrict the vertical distribution of jellyfish. Behavioral experiments conducted in 2-layered water tanks showed that (1) salinity discontinuity alone limited the vertical distribution of jellyfish, (2) distribution was restricted to the layer with the same salinity as the jellyfish had physiologically been acclimatized to, and (3) jellyfish did not have any specific salinity or depth preferences. Analyses showed that the jellyfish bodies had a similar density to that of the ambient seawater. When jellyfish were transferred to seawater of a different salinity, they required several hours to acclimatize to the new environment. Our results indicate that jellyfish, when approaching a sharp pycnocline, are subjected to a strong buoyancy force and thus are unable to readily swim across the pycnocline. Numerical simulations also supported the buoyancy hypothesis. Therefore, we conclude that the passive response to the buoyancy force is the primary mechanism underlying the heterogeneous distribution of moon jellyfish across a sharp pycnocline.