A study of sex ratio in native (Orconectes virilis) and invasive (Orconectes rusticus) crayfish species in Burt Lake, Michigan

Abstract

Sex ratio theory has led to numerous studies that both support current ideas and investigate and explain deviations from the expected 1:1 male to female tertiary sex ratio. Crustaceans have been shown to have aberrant sex ratios (Wenner 1972). This study tests the hypothesis that these results apply to crayfish species. In this study native (O. virilis) and invasive (O. rusticus) species were sampled in Burt Lake in Cheboygan County, Michigan to determine the sex ratios of both populations. Crayfish were captured at 4 sites using dip nets and sex and species were visually determined by examination of the genital pore and color indicators. Carapace length and total length were also measured. The chi-squared tests of the data showed a statistically significant deviation from expected 1:1 sex ratios for O. virilis. In this case there was a female bias with 52 female individuals to only 20 male individuals. O. rusticus samples did not differ significantly from the expected sex ratio. This sex ratio bias in O. virilis could be the result of differential movement or mortality as well as interactions between native and invasive crayfish species.