MOTZ, A.M.; WOLCOTT, T.G.*; WOLCOTT, D.L.; HINES, A.H.: Sperm Limitation and Operational Sex Ratio in Blue Crabs.

To understand whether the intense fishery on male blue crabs (Callinectes sapidus) is leading to sperm limitation and reduced reproductive potential in Chesapeake Bay populations, we are investigating whether the number of males, or their adequacy as mates, is diminished. A long-term S.E.R.C. trawl data set is being analyzed for evidence that the male:female ratio has changed historically. We have developed biotelemetry systems for free-ranging crabs to measure latencies to pairing and mating for each gender, as well as to reveal where these activities occur and thus where operational sex ratio should be measured. We are collecting crabs engaged in pre- or post-copulatory mate guarding (“doublers”) from the field. Many small males are mating, even though they could be displaced if large ones were present. A large proportion of the males in the Bay is severely sperm-depleted. Large males caught as “doublers” often possess and deliver no more seminal resources than do small ones, even though we would expect them to deliver much more if they were “fully-recharged”. The most-depleted males are not found as “doublers” and apparently are not mating. This indicates that the operational sex ratio is lower than suggested by the numerical ratio of adult males to pubertal females. Even if a proportion of the males that escape the fishery is not mating, the remaining number seems adequate to provide almost all females with at least some sperm during their single brief opportunity for mating. The concern is whether it is enough to fertilize their potential egg output; many of those males are small or have partially-depleted sperm stores.