The use of celestial cues in the offshore escape response of the shrimp,palaemonetes vulgaris

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Abstract
The involvement of celestial cues in y‐axis orientation was studied for the shrimp Palaemonetes vulgaris. Underwater orientation was monitored with a video system in an outside circular arena which excluded all but celestial cues. Shrimp were collected from three shorelines having different offshore directions. The shrimp had an escape response in which they oriented in a direction similar to the offshore direction of their home shoreline at different times of day when (1) the sky was clear, (2) the sun was apparent through a cloud filled sky, and (3) the sun was obscured leaving blue sky. If the sun was diffused and repositioned with a mirror, the shrimp reoriented relative to the new position of the sun. Under completely cloudy skies, unimodal orientation ceased. These results indicate that the shrimp's offshore escape response is mediated by sun compass orientation. The blue sky contains orientation information for this y‐axis orientation, but the sun is the stronger cue.