Eye movements and the mechanism of vertical steering in Euphausiid Crustacea

Abstract

1. Euphausiids (Nematoscelis atlantica) move their eyes through 180° in response to the movement of a small light source. In the sea the effect would be for the upper eye to track the direction of the downwelling light (Figs. 1 and 2). The ventral photophores always rotate in synchrony with the eyes and through the same angle. 2. The tail also follows movements of the light, but only when the eye is in particular positions in its orbit. As the eye moves backwards the tail moves up and vice versa (Figs. 3 and 4). The effect of the tail movements on a freely rotating animal is to cause the body to maintain a constant angle to the light (Figs. 5 and 8). 3. The eye movements have a latency of about 80 ms, and maximum angular velocities of about 200°s−1. Tail movements are synchronous with eye movements, but the effect of the rudder action is slow so that corrective body movements may take several seconds to complete (Figs. 5 and 6). 4. Different animals maintain different body angles with respect to the light. It is assumed that these angles represent different phases of diurnal migration. The one parameter that changes when a new course is adopted is the position of the eye in the orbit about which tail movements are initiated. This can be represented as a variable set-point in the feedback loop that controls the action of the tail (Fig. 9). This system of steering is formally similar to the way a helmsman steers a boat with the aid of a compass (Fig. 8). 5. During tail flips this closed-loop navigation system is over-ridden by a programme in which eye and tail move in the same direction, and with different time courses. This is interpreted as a mechanism for ensuring that the eye continues to point upwards during the rapid phase of the movement that results from the tail flip (Fig. 7), and which the closed-loop mechanism would be too slow to deal with.