ABSTRACT
The intracellular conditions required for contraction in the food-gathering tentacle of a marine dinoflagellate Noctiluca miliaris were examined in the isolated tentacle, treated with Triton X-100. The tentacle flexed to its full extent when the pH of the reactivation medium was lowered to 4·0, and extended when it was raised again to its original value. Adenosine triphosphate (ATP) was not needed for the pH-dependent flexion–extension. The flexion was also produced by Ca2+, but in a concentration range more than a thousand times higher than the effective H+ concentration range for producing the flexion. It is concluded that movement of the tentacle in a live specimen of Noctiluca is controlled by membrane potential-regulated cytoplasmic pH.
