The role of the underwater polarized light pattern, in sun compass navigation of the grass shrimp, Palaemonetes vulgaris
1. The sky polarized light pattern is visible underwater within the critical angle of refraction and can be used to position the sun in the sky. The present study determined whether the shrimp, Palaemonetes vulgaris could use this polarization pattern for sun compass orientation. 2. In the laboratory, Palaemonetes oriented parallel to the e-vector of downwelling polarized light regardless of their phototactic state. Outside, they used the sun or blue sky as a cue for orientation in the offshore direction of their home shoreline. In nature, this is an escape response directed away from shoreline predators. Shrimp were disoriented under cloudy skies. 3. The shrimp could orient using blue sky alone from any altitude (above 17°) and from azimuths in the antisolar but not the solar hemisphere. Small circles of blue sky, subtending a 27° angle, were sufficient for orientation. 4. When the alignment of the natural polarization pattern was shifted with polarized filters, the shrimps' orientation shifted similarly. The polarized light pattern was used in conjunction with another blue sky cue, possibly the skylight intensity pattern. Neither cue contained sufficient orientation information to be used alone. Palaemonetes vulgaris was the first aquatic arthropod shown to use the polarization pattern for sun compass orientation. © 1991 Springer-Verlag.
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