Size At Maturity and Sex Ratio of Arcitalitrus Dorrieni (Hunt, 1925) (Amphipoda, Talitridae) At Two Sites in County Galway, Ireland

Abstract

AbstractAnthropogenic dispersal has led to the introduction of several terrestrial amphipod species to the northern hemisphere. One such species, Arcitalitrus dorrieni (Hunt, 1925), a native of Australasia, is now found in several woodland sites in Ireland and Britain. Aspects of the biology of A. dorrieni were investigated in County Galway over a period of two years and compared between a "typical" mixed deciduous woodland habitat and a more "atypical" coniferous woodland habitat. At both sites, statistically significant differences were found between summer (Aug.-Sep.) size at maturity and winter (Jan.-Apr.) size at maturity, i.e., females were mature at smaller sizes in summer than in winter. Between-site comparisons of August-September data for both years showed that females were mature at a significantly smaller size at the mixed deciduous woodland site. A sex ratio biased in favour of females was recorded on all but one sampling date at each site, though on some dates this bias was not significant. The change in cohort sex ratio at both sites was quite distinct and exhibited the same general pattern. During the recruitment of each cohort, the ratio was biased towards males. As the cohort increased in length, however, the ratio changed to a bias in favour of females, so that by the time a particular cohort entered its breeding period the following year, it was biased firmly in favour of females. Possible reasons for the observed patterns are discussed.