Polarized Light and the Orientation of Two Marine Crustacea*

Naturally occurring polarized light is used for orientation by many arthropods. Various insects and chelicerates as well as crustaceans, employing either their simple or compound eyes, can detect the pattern of linearly polarized light in the blue sky and from it determine geographical direction. Among Crustacea, the semi-terrestrial beach amphipod TaUtrus and the isopod Tylos show this when, on being disturbed, they are able to head directly towards the sea without using other clues (Pardi & Papi, 1953a, b; Pardi, 1954). Like that of the sky, underwater scattered illumination is known to be polarized in a way primarily determined by the sun's position (Waterman, 1954a, 1955; Ivanoff, 1956; Waterman & Westell, 1956). The possibility thus exists that aquatic arthropods, or other underwater forms sensitive to polarization, may use this submarine light pattern as a similar sort of sun compass (Waterman, 1957). It is certain that some aquatic arthropods can in fact perceive polarized light (Verkhovskaya, 1940; Waterman 1950; Baylor & Smith, 1953; Eckert, 1953), but few quantitative data are available. No direct evidence linking this perceptive ability with their normal orientation and migration has yet been found, although an effect of sky polarization observed underwater has been suggested as a possible factor in the diurnal vertical migration of plankton (Verkhovskaya, 1940). This paper describes an experimental approach to the general problem. The major part of the work comprises a quantitative laboratory study of the responses of a marine mysid to plane polarized light, and this is supplemented by an exploratory field project in which a caridean decapod was found capable of swimming straight courses in open water where landmarks were not available.