Sex ratio and distribution within a large population of the fiddler crab Uca capricornis

The purpose of this experiment was to determine whether females may be a limited resource within a population of Uca capricornis. The experiment was part of a larger study looking at the relationship between neighbouring males and females in this species. Experiments were conducted in the East Point Reserve, Darwin. Fieldwork was conducted yearly in 18 plots (4mx4m) from November-January in 2002-2006.

All individuals within the plots were caught and carapace widths measured. This allowed the sex ratio in relation to size to be determined. The location of burrows were also recorded to determine whether the males and females were distributed independently of each other. The distance to, and sex of, the nearest resident for each individual was also measured to compare mean distances between male-female neighbours and between male-male neighbours.

Results indicate that an average of 7 crabs per square metre were active on the surface at any time. The sex ratio was strongly male biased, with only 30% of crabs being female. The sex ratio also changed with size; females were significantly rarer in large size classes.