Behaviour of Pleurobrachia pileus to changes in hydrostatic pressure and the possible location of baroreceptors

The responses of Pleurobrachia pileus to changes in pressure have been observed to be depth regulatory in nature. Ctenophores responded to an increase of pressure by active upward swimming, with the oral end in advance, and to a decrease of pressure by passive downward sinking with either the oral or the aboral end in advance. When the aboral end was leading the ciliary beat was reduced and the animals probably sinking under gravity. With the mouth in advance the animals were able to swim upwards through a depth of 32 cm in 18 seconds. When sinking passively they moved through the same depth in 40 seconds. Eventually, they remained orientated with the apical pole touching the bottom and the comb cilia quiescent, until the next pressure increase followed. This behaviour which is distinctly orientated to gravity has been described as “high barokinesis usually combined with negative geotaxis”; (Knight‐Jones & Quasim, 1955). The proportions of animals responding generally increased with increase in pressure magnitudes, and were higher among juveniles than amongst adults. Pleurobrachia are stimulated to swim upwards to the surface by changes in pressure associated with surface waves with heights more than 0.025 M. The nature of responses, to changes of pressure, resulting from surgical interference confirm that the apical sensory organ is concerned with orientation to gravity. It is thus co‐ordinating the responses resulting from changes in hydrostatic pressure. Although some of the pressure receptors may be distinct from the statocysts, the responses are not seen in animals devoid of the apical sensory organ. “Plumose”; cells (Tamm, 1982), or any other types within the cavity of the statocyst may be responsible for the mechanisms of the pressure perception.