Light and dark adaptation in the eye of the fiddler crab

: mechanisms to cope with extreme fluctuations in brightness

Abstract

Light, in abundance or absence, can provoke a great variety of photomechanical changes in the compound eyes of crustaceans. The fiddler crab visual system is widely used as a model to study aspects of crustacean vision and neural pathways. Their apposition eyes are adapted for vision on sunny tropical and semi-tropical mudflats, but intermittent and temporary use of underground burrows to evade predators means their eyes are additionally exposed to short but frequent periods of darkness. Their light-adaptation mechanisms have not yet been examined, so my project uses a variety of complimentary approaches to investigate important gaps in our understanding of how their eyes cope with extreme fluctuations in brightness. The West African fiddler crab, Afruca tangeri, is also nocturnally-active, suggesting that their eyes undergo effective dark-adaptation at sunset. Transmission electron microscopy (TEM), light microscopy, and synchrotron X-ray tomography were used to describe ultrastructural changes in the eye of this species between day and night. In living crabs, widening of the deep pseudopupil was measured using an ophthalmoscopic camera. The impacts that adaptation state and time of day have on contrast sensitivity of the eye, were assessed using analysis of behavioural thresholds to visual stimuli. Electroretinogram (ERG) recordings from another fiddler crab, Gelasimus dampieri, provided additional evidence for circadian controls on changes in absolute sensitivity of light- and dark-adapted eyes. Crystalline cones and rhabdoms undergo significant physiological change to effectively adapt the eye to dim light at dusk, however, screening pigment migrations remain immobile across the eye and are not part of the response to changes in light intensity. The eye returns to a light-adapted state at dawn and with bright light exposure. Thereafter, during daylight hours, minimal change occurs in response to dim light and the eye remains anatomically light-adapted, even after several hours in the dark. ERGs revealed that G. dampieri uses temporal summations as an alternative dark-adaptation strategy. This provides an explanation for sensitivity increases measured in both species in periods of dim light during daylight hours, which occurred without apparent anatomical change. Effective dark-adaptation occurs at sunset to allow safe foraging and visual communication after dusk in diminishing light. However, during daylight hours, circadian controls prevent the eye making unnecessary adjustments to the fluctuating light levels frequently experienced as the crab moves between the bright mudflat surface and the dark burrow. This benefits the fiddler crab by ensuring that the visual system remains primed for predator detection in very bright light, immediately after spending time underground, where vision is not required.

Date of Award	21 Jan 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Nicholas W Roberts (Supervisor) & Martin J How (Supervisor)