Phototaxis in Euglena 1: Physiological Basis of Photoreception and Tactic Orientation

Abstract

As an introduction, perhaps a few words concerning the definition of the term “phototaxis” would be in order. At present, the most generally acceptable description of phototaxis defines it as orientation and subsequent oriented movement of freely motile organisms in response to light — towards the light in the case of positive, and away from it in the case of negative phototaxis. While in the past, some authors have distinguished between “topo-phototaxis” and “phobo-phototaxis”, the former term actually means phototaxis proper, while the latter refers to a light-induced nondirectional shock response (positive or negative, respectively, if occurring upon a decrease or an increase of light intensity). Since a nondirectional response is not a true taxis, I have proposed that the term “photophobic response” be used instead of “phobophototaxis” (Diehn, 1969 a, 1970 a), and will adhere to that terminology throughout this presentation.