Contrast sensitivity, visual acuity, and the effect of behavioural state on optokinetic gain in fiddler crabs

Most animals rely on visual information for a variety of everyday tasks. The information available to a visual system depends in part on its spatial resolving power and contrast sensitivity. Due to their competing demands for physical space within an eye, these traits cannot simultaneously be improved without increasing overall eye size. The contrast sensitivity function is an integrated measure of visual performance that measures both resolution and contrast sensitivity. Its measurements helps us identify how different species have made a trade-off between contrast sensitivity and spatial resolution. It further allows us to identify the evolutionary drivers of sensory processing and visually mediated behaviour. Here, we measured the contrast sensitivity function of the fiddler crab Gelasimus dampieri using their optokinetic responses to wide-field, moving, sinusoidal intensity gratings of different orientations, spatial frequencies, contrasts, and speeds. We further tested whether the behavioural state of the crabs (i.e. whether crabs are actively walking or not) affects their optokinetic gain and contrast sensitivity. Our results from a group of five crabs suggest a minimum perceived contrast of 6%, and a horizontal and vertical visual acuity of 0.4 cyc/deg and 0.28 cyc/deg respectively in the crabs’ region of maximum optomotor sensitivity. Optokinetic gain increased in moving crabs compared to restrained crabs, adding another example of the importance of naturalistic approaches when studying the performance of animals.