Orientation and swimming mechanics by the scyphomedusa Aurelia sp. in shear flow

Abstract

Individual Aurelia sp. medusae were distributed around regions of current shear associated with vertical density discontinuities during three vertically towed camera profiles in the northern Gulf of Mexico. Along shear regions, medusae oriented non-randomly and swam horizontally, forming distinct layers. To identify the mechanisms by which Aurelia maintain horizontal orientation in velocity shear, jellyfish swimming mechanics were studied in laboratory kreisel tanks at three shear rates (0.10, 0.21, and 0.34 s-1) and a no flow control. Medusae counteracted the rotational effect of velocity shear by pulsing asymmetrically. Specifically, medusae held a position against shear flow by maintaining a higher bell margin angle on the side of the medusa in higher flow velocity. Swimming asymmetry increased with shear and, as a result, the ratio between bell angles on opposing flow sides was significantly different from the control at all shear rates. Contractions were initiated on the lower flow side of the bell in all cases and at the highest shear rate, the low flow side of the bell contracted 0.2 s before the high flow side. Laboratory observations confirm that patches of jellyfish at vertical discontinuities may be the result of an active behavioral response to vertical velocity shear. Layers of jellyfish formed via an active behavioral response to shear may improve prey encounter or fertilization success.