Experimental blocking of UV reflectance in Uca coarctata influences male-male aggression

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Discussion on “Experimental blocking of UV reflectance in Uca coarctata influences male-male aggression”

Results from this study reveal a significant effect of UV reflectance in Uca coarctata male-male aggressive interactions. These results support our hypothesis and a recent study by Detto & Backwell (2009) whose research revealed a significant effect of UV reflectance in Uca mjoebergi preferential female sexual selection.

However, the Detto & Blackwell (2009) study measured the influence of ultraviolet cues in both sexual selection and aggressive interactions concluding that UV was not influential in aggressive interactions.

There were several variations between the two studies including the experimental environment and what constituted an “aggressive interaction”. In order to observe the most ‘natural’ behaviours of U. coarctata, specimens were returned to their identified burrows following measurement where behavioural observations were then taken. Our aim with this approach was to account for many of the confounding factors and behavioural adjustments associated with removing an animal from its environment.

Although the results presented here have produced a significant result, they are not offered as conclusive. It is suggested that a greater number of observations, under a variety of tidal conditions (low-tide in the morning, afternoon and evening) could have strengthened the conclusions we can draw from this research. Hydroperiod is a key driver in species structure and distribution causing variation in soil salinity, inundation period and other soil physicochemical properties such as phosphorus, nitrogen and pH (Crase et al., 2013)

Initially, we proposed the collection and analysis of live specimens with a spectrophotometer to determine spectral reflectance range. We were advised the limits of our Ethics (and UQ) approval did not allow for us to collect any specimens so this was omitted from our research. It is proposed that the reflectance range of U. coarctata from the Lota, Queensland population may have been a valuable addition to this research. Spectral reflectance range data may aid the comparability of this research and may have produced insight into the mechanisms behind predator-prey interactions.

Almost all birds have photoreceptor sensitivity reaching into the ultraviolet spectrum (Bennet et al., 1994). An interesting addition to this research would be whether the range sensitivity of both predator and prey overlap at all and if so, how this might affect their behaviour.

As these results seem to support current theories around honest signalling and competitive ability (Berglund et al., 1996) further consideration should be made toward the influence of brachychelous (original) claw compared to leptochelous (regenerated) claw (Crane 1975) on both agonistic and mate choice signalling. This element may have contributed to the interpretation and analysis of our data so it is recommended this be considered in future studies.


Bennett, A., Cuthill, I., & Norris, K. (1994). Sexual selection and the mismeasure of color. The American Naturalist, 144, pp.848-860.

Berglund, A., Bisazza, A. & Pilastro, A. (1996). Armaments and ornaments: an evolutionary explanation of traits of dual utility. Biological Journal of the Linnean Society, 58, 385–399

Crane, J. (1975). Fiddler crabs of the world: Ocypodidae: genus Uca. (Princeton University Press, New Jersey)

Crase, B. et al., 2013. Hydroperiod is the main driver of the spatial pattern of dominance in mangrove communities. Global Ecology and Biogeography, 22, pp. 806-817.

Detto, T., & Backwell, P. (2009). The fiddler crab Uca mjoebergi uses ultraviolet cues in mate choice but not aggressive interactions. Animal Behaviour, 78, pp.407-411.

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