Causes and consequences of patchy spatial distribution in male and female fairy shrimp, Eubranchipus bundyi

Abstract

In this thesis, I investigated fairy shrimp (E. bundyi) distribution and some possible effects they might have on the temporary pond community, focusing mainly on behavioral responses to environmental variables. Lab and field experiments, as well as transect data, suggest E. bundyi may be attracted to dim light and seek shade and structure within the water column (vegetation, sticks, roots, the bases of trees, rocks, etc.) when light levels are high. At midday, fairy shrimp seem to cluster among shaded structured regions of the pond; for instance, a thick mass of floating bark casting shade in an otherwise open patch of water may shade hundreds of fairy shrimp during the day, whereas few fairy shrimp are found in such locations at night. As evening sets in, the shade aggregations start to break up, and at night the fairy shrimp are common in deep, open, unstructured regions of pools. Overall, females tend to be less mobile, less attracted to light and deeper in the water column than males. The patchy distribution of fairy shrimp (E. bundyi) may be the result of egg hatching cues, microhabitat preferences in regards to light level, and the cryptic behavior of females. Behavioral differences between the sexes may expose males and females to different predators and food resources. The diel migration of both sexes may be responses to predation and/or UV photodamage. The community effects of fairy shrimp distribution, migration, and the different activity levels of the sexes, however, may be dampened because of abundant food resources and habitat disturbance (drying of the pond) truncating a trend towards a competition and predator oriented community. While no previous study has taken the comprehensive lab/field approach that I describe in this thesis, my results are similar to the few other studies of the effects of light, shade, and gender on other species in habitats considerably different from E. bundyi's. Diel migrations and responses to physical microhabitat parameters may be as widespread among anostracans as it is among cladocerans, and may turn out to be as useful for understanding ecology of temporary ponds as it has been for understanding the ecology of the limnetic zone of lakes (Hutchinson 1967, Wetzel 1983, Wetzel and Likens 1991).