Fiddler crab electroretinograms reveal vast circadian shifts in visual sensitivity and temporal summation in dim light

Abstract

Many animals with compound eyes undergo major optical changes to adjust visual sensitivity from day to night, often under control of a circadian clock. In fiddler crabs, this presents most conspicuously in the huge volume increase of photopigment-packed rhabdoms and the widening of crystalline cone apertures at night. These changes are hypothesised to adjust the light flux to the photoreceptors and to alter optical sensitivity as the eye moves between light- and dark-adapted states. Here, we compare optical sensitivity in fiddler crab eyes (Gelasimus dampieri) during daytime and night via three electroretinogram (ERG) experiments performed on light- and dark-adapted crabs. 1) Light intensity required to elicit a threshold ERG response varied over six orders of magnitude, allowing more sensitive vision for discriminating small contrasts in dim light after dusk. During daytime, the eyes remained relatively insensitive, which would allow effective vision on bright mudflats, even after prolonged dark adaptation. 2) Flicker fusion frequency (FFF) experiments indicated that temporal summation is employed in dim light to increase light-gathering integration times and enhance visual sensitivity during both night and day. 3) ERG responses to flickering lights during 60 mins of dark adaptation increased at a faster rate and greater extent after sunset compared to daytime. However, even brief, dim and intermittent light exposure strongly disrupted dark-adaptation processes. Together, these findings demonstrate effective light adaptation to optimise vision over the large range of light intensities that these animals experience.

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

“…The sensitivity to high rotational velocities in flying insects reflects their need to resolve the high rotational image speeds generated by their flight (O'Carroll et al, 1996), whereas in praying mantis it is probably to spot fast-moving small prey (Nityananda et al, 2015). Fiddler crabs, like humans, move at low speeds and both have a similar critical flicker fusion frequency (32-74 Hz;Brodrick et al, 2022;Layne et al, 1997) and are therefore tuned to low retinal image speeds.…”