Polarized Light and the Orientation of Two Marine Crustacea

Abstract

1. Quantitative experiments are described, designed to test the hypothesis that underwater polarized light is used for orientation by aquatic animals as that of the blue sky is by terrestrial arthropods. 2. Studied in the laboratory the littoral mysid, mysidium gracile, was found capable of orientation relative to the e vector in a vertical beam of linearly polarized light. 3. Statistically significant preference was shown by this animal for aligning its longitudinal axis at right angles to the plane of polarization. 4. A secondary slight preference was shown for swimming parallel with the plane and the degree of random orientation was often high (4% mean). 5. The littoral prawn, Palaemon northropi, when released in quiet water far enough from the bottom and the shore to prevent visual contact, was found to swim on relatively straight horizontal courses for appreciable distances in forty-one out of sixty-one tests. 6. The mean length of these runs was 24.5 m., the maximum 119 m. ; swimming velocities up to about 1 km./hr. were measured. 7. The sun's position seems to be the only directional clue not controlled in these tests, and the general direction of the runs is significantly correlated with the direction of the sun, being at right anglea to it. 8. It is thus concluded that this animar can use the sun, either directly or through the polarization pattern, as a celestial compass.