Behavioural adaptations of the fiddler crabs Uca vocans borealis (crane) and Uca lactea lactea (De Haan) for coexistence on an intertidal shore

Abstract

The distribution and coexistence of the fiddler crabs U. vocans borealis and U. lactea lactea was investigated in the upper shore of a sandy beach with respect to particle size, water content and total organic carbon content of the sediments. A clear segregation of habitats between the two species was apparent. U. lactea lactea were only found in sediments with significantly lower total organic carbon content. U. lactea lactea had a higher ratio for the size of third maxilliped to body size than U. vocans borealis. There was no statistical difference in the median sediment particle size of the habitats where both species were found on the study shore. U. lactea lactea was larger on shores where U. vocans borealis were absent. Smaller U. vocans borealis individuals occupied sediments with higher water content and finer particles. Smaller individuals of U. vocans borealis also possessed fewer spoon-tipped setae on second maxilliped than their larger conspecifics, which were found mostly on coarser sediments. U. lactea lactea spent significantly more time on the surface than in the burrow during low tide when compared with U. vocans borealis. Both Uca species exhibited similar desiccation resistance. U. lactea lactea tends to keep its frontal region in close contact with sediments to maintain moisture when being exposed to air.

Keywords:

• Behavioural adaptations
• Coexistence
• Fiddler crabs
• Intertidal shore
• Sympatric

Acknowledgements

We would like to thank Azura Tsang for her assistance in the fieldwork. The work described in this article was fully supported by a Strategic Research Grant (No. 7001200) from City University of Hong Kong.