Two Decapod Crustaceans, and , Stabilize Their Gaze Using Achromatic Visual Cues, but Not the Angle of Linearly Polarized Light.

Gaze stabilization is important to animals because it allows them to visually differentiate between their own motion relative to their environment and the motion of objects within their environment. Animals can struggle to stabilize their gaze in environments that have a high amount of visual noise. In shallow aquatic environments, such as tidal creeks, the motion of the water's surface can create dynamic spatiotemporal fluctuations in illumination referred to as "caustic flicker." This type of visual noise can create false-motion cues. To overcome this obstacle, shallow-dwelling aquatic animals may use color or polarized cues to stabilize their gaze rather than achromatic cues. Tidal creeks are often spectrally narrow due to light absorption by suspended particles such as algae, making color vision unreliable. Instead of using achromatic or chromatic cues, we hypothesize that crabs in tidal creeks stabilize their gaze using cues that vary in their angle of linear polarization (AoLP). To ask whether crabs from tidal creeks may use AoLP cues in gaze stabilization, we investigated polarization sensitivity in the Atlantic mud crab, , and the green porcelain crab, . Using optomotor behavioral assays, we found that both porcelain and mud crabs use achromatic cues for gaze stabilization, but neither use AoLP cues across a range of light conditions that varied in spectral width, spectral peak, and total irradiance. Our findings are further evidence that although animals may theoretically benefit from using AoLP cues for gaze stabilization in visually noisy aquatic habitats, decapod crustaceans from tidal creeks seem to rely on achromatic cues for this task.