ORIENTED LIGHT REACTIONS OF THE ARROW WORM SAGITTA CRASSA TOKIOKA

¹ Ushimado Marine Laboratory, Kashino, Ushimado, Okayama 701-43 Japan

The arrow worm Sagitta crassa shows positive phototaxis in a horizontal light beam by repeatedly swimming briefly upward with the body axis inclined toward the illuminated side, and then by passively sinking. Phototactic behavior is less evident in vertical beams. When a horizontal light beam is reduced after a period of illumination, the arrow worms have a fast goal seeking response, steering themselves toward the reduced light source in less than 0.3 s and swimming straight to this target at rates greater than 14 cm/s. When two light targets are presented immediately after a reduction in light intensity, the arrow worms choose the brighter one, indicating that the response may be a type of telotaxis. The two kinds of phototactic swimming may be explained by slow taxis by changes in orientation, and target-aiming behavior by two sequential processes: an arousal increase followed by an activity release.