Geotaxis in the Ciliated Protozoon Loxodes

Geotaxis is demonstrated in the ciliated protozoon Loxodes. This behaviour is mediated by a mechanoreceptor which is probably the Müller body, an organelle characteristic of loxodid ciliates. The geotactic response is sensitive to dissolved oxygen tension: in anoxia or at very low O2 tensions the ciliates tend to swim up and at higher O2 tensions they tend to swim down. This behaviour, in conjunction with a kinetic response allows the ciliates to orientate themselves in vertical O2 gradients and to congregate in their optimum environment. In two appendices, models of the behaviour predicting vertical distribution patterns and considerations of the minimum size of a functional statocyst are offered.