MEPS prepress abstract  -  DOI:

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

Kentaro S. Suzuki*, Yasuo Niida, Takaki Tsubono, Hiroshi Takimoto, Emi Kumakura, Haruto Ishii, Yasuyuki Nogata


ABSTRACT: The vertical distribution pattern of moon jellyfish Aurelia aurita s.l. medusae can vary dramatically within a water column, and a sharp pycnocline is reported to be one of the major factors that limits vertical distribution. Causes of this limited distribution are likely to be either or both of two behavioral responses: a passive response 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 two-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. Moreover, 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 the jellyfish across a sharp pycnocline.