Visually mediated neighbour recognition in the fiddler crab Uca capricornis

Full description

Mating signals are often directed at numerous senses, and provide information about species identity, gender, receptiveness, individual identity and mate quality. Given the diversity of colourful body patterns in invertebrates, surprisingly few studies have examined the role of these visual signals in mate recognition. This experiment demonstrates that distinct carapace colour patterns in the fiddler crab, Uca capricornis, enable males to discriminate between their female neighbours and unfamiliar females.

The experiment was conducted in the East Point Reserve, Darwin, NT from September to January 2003 and 2004.

The methodology involved identifying male and female U.capricornis neighbours living 10-30cm apart. The female was caught and randomly assigned to one of seven treatments, including replacement with a similar sized female, and various manipulations to alter her appearance. The female was then tethered close to the males burrow and the interaction monitored. The males behaviour was classified as either 'approach' or 'ignore.' The results show that males tended to approach unfamiliar females tethered nearby, whereas they were more likely to ignore their unaltered neighbours tethered in the same position.

Lineage

Statement: Uca capricornis (like other species of fiddler crab) are capable of rapidly muting their carapace colours, for instance in response to handling, although their specific patterns generally remain discernable. However, unlike many species in which dramatic colour changes occur in association with the diurnal or tidal cycle, changes in carapace patterns and their colours in U. capricornis appear to be long-term. The experiment was therefore not hampered by short-term colour changes. Between neap and spring tide, male and female U. capricornis living 10-30cm apart were identified. The female was caught and randomly assigned to one of seven treatments: 1) unaltered control, 2) replacing her with an unfamiliar female of a similar size, 3) dramatically altering her appearance by painting her carapace black; applying a 2mm spot of black paint to 4) a black part of her carapace or 5) on her abdomen as paint controls; 6) painting her carapace with clear nail polish; or 7) replacing her with a freshly painted black wooden bead (1cm). The female was then tethered half way between the male and female burrows (5-15cm from the male). The experiment ended once the male touched the tethered female, which was considered an approach, or after 5 minutes had passed with no approach. We observed both aggressive and courtship behaviour directed at the females but it was often difficult to classify the behaviour as courtship could turn into aggression, probably due to the females lack of response. Consequently, males behaviour was classified as 'approach' or 'ignore'. To examine the effect of position on recognition unaltered neighbours and unfamiliar females were tethered 10cm from the male's burrow in the same direction as the female's burrow or in the opposite direction. All treatments were conducted on the same male in a random order. When the unfamiliar female was presented first, the neighbouring female was sealed in her burrow with a shell during the trial and was given 10 minutes to recover before being tested. Parameters: Manipulation performed and male response (number that approached or ignored).

Notes

Credit
Funded by The Australian Research Council (ARC) Centre for Excellence for Vision Science

Credit
Funded by The Centre of Visual Sciences (CVS, The Australian National University)

Credit
Funded by an Australian National University (ANU) PhD Scholarship

Purpose
To demonstrate that distinct carapace colour patterns in Uca capricornis enable males to discriminate between their female neighbours and unfamiliar females.