THE VERTICAL MIGRATION OF THE COPEPOD ACARTIA TONSA UNDER CONTROLLED ILLUMINATION

Abstract

1. The natural diurnal vertical migration of the copepod Acartia tonsa can be reproduced in the laboratory. Animals kept in tall glass cylinders exposed to window light move up at night and down during the day. 2. Incandescent or fluorescent lights can be substituted for daylight without affecting this behavior, provided that the illumination is oblique to the axis of the cylinders. Migration does not depend upon the area of the source, spectral energy distribution, or total intensity of the light. 3. When illuminated obliquely to the axis of the cylinder, A. tonsa sinks downward. This behavior, which is followed by an upward movement in darkness, is believed to account for the normal migration of this form. 4. When the illumination is parallel to the axis of either a vertical or a horizontal tube, A. tonsa swims toward the light. Such highly directional illumination is not likely to be encountered in nature, and this behavior is believed to be a laboratory artifact. 5. Observation of individuals shows that the migration consists of a general trend of scattered movements. No difference was found between the sexes. 6. No evidence was found for a diurnal rhythm other than that caused by the normal alternation of day and night. Migration ceases under constant conditions; it can be reversed by illuminating the animals at night and keeping them in the dark by day; and the normal 24-hour behavior can be compressed into four hours.