GENDER DISPARITY IN A POPULATION OF BRANCHIOPLAX WASHINGTONIANA (CRUSTACEA: DECAPODA: BRACHYURA) FROM THE EOCENE OF WASHINGTON, USA

Population studies, common for extant decapods, are difficult to conduct on fossils due to the low probability of fossilization leading to small numbers of individuals preserved in a single event. Only in rare cases are population studies possible with fossil organisms. One such instance is presented here, where the Hoko River Formation (Eocene), composed of multiple turbidity deposits, buried successive populations of the crab Branchioplax washingtoniana Rathbun, 1918. We treat these accumulations here as a single, time averaged, population of over 80 individuals. While at one time the fossils were referred to different genera and species, these taxa were synonymized by Schweitzer (2000). This relatively large sample allowed for collection of morphometric data, particularly linear dimensions, for features of the dorsal carapace. These measurements, when plotted against one another, showed linear trends indicative of a population consisting of multiple age classes, supporting the earlier synonymization. The large number of individuals also allowed for the determination of gender in almost 1/3 of the sample. Of those crabs for which gender could be ascertained all were male, in clear opposition to the 1:1 gender ratio expected for most crab populations. Ultimately, the disparity is best explained by a combination of factors, inferred from study of extant species, rendering males more susceptible to burial by turbidity currents because of their more frequent exposure on the sea floor as compared to females. These include a greater frequency of burrowing in females, related to reproductive behavior. In addition, between reproduction events, males may tend to congregate in deeper water zones where turbidity currents are more likely to occur due to steeper slope topography while females tend to congregate in more shallow-water zones where gentle slopes inhibit turbidity currents.