Population dynamics, spatial dispersion and somatic growth of the sand fiddler crab ilea pugilator

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Abstract
While fiddler crabs, Uca spp., are important constituents of the diets of fishes, birds and mammals, little is known of their population dynamics or somatic growth under natural conditions. A stratified random design was used to sample a population of U. pugilator on a 3.9 ha study area on Shackleford Banks, North Carolina, USA. Samples were taken at approximately monthly intervals between March 1979 and Sept. 1980 to obtain estimates of numbers, biomass and size composition. Carapace widths, sex and reproductive stage were measured, and population biomass estimates were calculated from separated dry-weight/carapace-width regressions for males, ovigerous and non-ovigerous females. Somatic growth was estimated from analysis of size-class modes through time. Numbers of fiddler crabs (.gtoreq. 5 mm) in the study area fluctuated between 1,348,000 and 2,544,000 over the 18 mo. period. Trends in numbers over time were generally parallel for the 2 sexes, but females were consistently more abundant than males. The preponderance of females was most pronounced in size classes from 10-15 mm. Average dry-weight biomass was 4.30 g m-2 for males and 6.12 g m-2 for females. Indices of dispersion revealed that crabs become progressively less aggregated during spring and summer and progressively more so during fall and winter. Ovigerous females were observed from May-Aug. Only 1% of ovigerous females were < 10 mm (mean size = 13.6 mm). Somatic growth varied with season, sex and stage of reproductive maturity. Molting crabs were encountered between May and Nov. Crabs that were 6 mm in carapace width in April had grown to about 13 mm by the following Sept, but during the reproductive season, growth of females appears to diminish or cease temporarily when they attain sexual maturity.