Recognition of visual stimuli by the warty crab Eriphia verrucosa

Abstract

The aim of the present study is to determine the visual capabilities of the crab Eriphia verrucosa by exposing it to footage simulating a looming stimulus which varies in approach speed, colour, sharpness, and shape. Crabs were harvested from a local shore and allowed to acclimatise for 24 hours in the ambient laboratory conditions. Individuals were then shown footage and the nature of their response and the time taken for it to be initiated were recorded. Behavioural responses were categorised into three main types: a retreat from the stimulus, a squat and a deimatic display. Initial tests indicated that the animals’ sex or presence of injuries did not influence their behaviour; this allowed the data to be pooled when analysing the responses to different stimuli. The crabs showed no statistically significant variation in reaction nature or time as stimulus colour varied. This could be due to Eriphia verrucosa being unable to differentiate between the colours chosen in this experiment (black, red, green and blue). However, it is also possible that the crabs can see the different colours but react in the same manner to them. The results showed that there was a significant association between response type and stimulus speed and shape. It was seen that with faster and more complex, or ‘crab-like’, stimuli, the crabs were more likely to show an aggressive response. This suggests that the animals do obtain information from the velocity, profile and appearance of the stimulus. The results also showed a significant association between response time with stimulus shape, speed, and sharpness. Post hoc analysis showed that the faster, sharper, and more complex a stimulus was, the faster they would respond. This variation in reaction speed would suggest that the animals alter their behaviour based on the amount of information they can resolve from the stimulus. Statistical analysis also showed a that mean response time varied according to the type of behavioural response. This showed that the responses each took a different average time to occur suggesting that the decision-making process to get to those responses may be different. The ability for Eriphia verrucosa to distinguish stimuli based on their clarity (sharpness), profile and speed is essential when inhabiting a shallow water environment. The capabilities of their vision are still not fully understood however there is scope to perform follow-up experiments. By showing a greater variety of stimuli and the harvesting crabs from different locations, a more thorough understanding of their visual capabilities can be developed.