The spectral and temporal properties of fiddler crab photoreceptors in the context of predator avoidance

The ability to detect and effectively escape from predators is critical to an animal's survival, and the ability to do so effectively depends on its sensory system. Vision is the dominant if not sole sensory system fiddler crabs use for predator avoidance. In this thesis I investigate the photoreceptor properties of two fiddler crab species, Uca vomeris and Uca dampieri, and their escape behaviour under field conditions with the aim to further our knowledge of how these crabs use visual information to organise their escape behaviour. In an attempt to elucidate the spectral sensitivities of the crabs' photoreceptors, intracellular electrophysiological recordings were performed and are presented in Chapter 2. Temporal resolution of their visual system was investigated using both intracellular recordings and electroretinograms (ERGs) and presented in Chapter 3. The following two chapters present the results of behavioural experiments into the effect of the predator's elevation in the crabs' visual field on two stages of the crabs' predator avoidance response: the run home stage (Chapter 4) and the burrow descent stage (Chapter 5). I found that both species of fiddler crabs have a UV-sensitive photopigment, plus one or two photopigments with a peak sensitivity in the blue part of the spectrum (400-500 nm). Their temporal resolution, measured as critical flicker fusion frequency (CFF), was similar to other non-flying animals at around 70 Hz. When measured as integration time and time-to-peak, however, their temporal resolution was very high, comparable even to flying animals. The behavioural experiments revealed that at the run home stage, the crabs perceive objects changing in elevation as most dangerous. Additionally, when far from the burrow they respond earlier to objects appearing low in the visual field, whereas when close to their burrow, such as at the stage of the burrow descent, they are more sensitive to objects that are seen high in the visual field. These investigations into the visual system and into predator avoidance behaviours of fiddler crabs move us closer to establishing them as a model system where we understand the first stage of sensory processing, which in turn will allow us to investigate how further neural processing leads to the functional behavioural output that can be observed in the field