Behavior of the Planktonic Coelenterates, <i>Sarsia tubulosa</i>, <i>Phialidium gregarium</i>, and <i>Pleurobrachia pileus</i> in Salinity Discontinuity Layers

Abstract

The behavior of Sarsia tubulosa (M. Sars 1835), Phialidium gregarium (L. Agassiz 1862), and Pleurobrachia pileus (O. F. Müller 1776) was investigated in chambers where salinity discontinuity layers could be established. Sarsia tubulosa and P. pileus aggregated at the layer for salinity discontinuities of as little as 2‰ although P. gregarium showed more variable behavior. This aggregation was shown to be due to an active response rather than passive flotation in the density barrier set up when low salinity water is placed over high salinity water. Sarsia tubulosa also showed increased fishing activity in any column containing a discontinuity layer compared with any homogeneous control column.