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

Abstract

The sex ratio theory proposed by Fisher (1930) that suggests an equal number of males to females has ignited an interest in studying the sex ratio of various organisms. In particular, this study looked at the tertiary sex ratio of two species of crayfish found in Northern Michigan. Native (Orconectes virilis) and invasive species (Orconectes rusticus) of crayfish were collected from Burt Lake in Cheboygan County, Michigan during a two-week period in mid-July. Crayfish were sampled at four different sites along the Burt Lake shoreline and were caught by dip nets. The species, sex, carapace length, and total length were determined for each individual. A total of 163 crayfish were caught and by looking at the total length of each crayfish, only 137 of them were of reproductive age. The 26 crayfish not of reproductive age were not included in the data analysis. The data showed that O. rusticus and O. virilis were present in nearly equal amounts, 47% and 53% respectively. For O. rusticus, 39 females and 26 males were collected for a total of 65 invasive crayfish. For O. virilis, 72 crayfish were collected with 52 females and 20 males. The data were analyzed by chi-squared tests. Based on the statistical analysis, the data collected for O. rusticus did not show significant bias from a normal sex ratio. However, the data for O. virilis was significant. The number of females was substantially higher than the number of males in the population. External factors like differential mortality or differential movement that might be influencing the sex ratio bias of O. virilis will be discussed