Document: GRE-3-46-18

Maintaining diversity in aquatic systems: The causes and consequences of pigment variation.

GERRISH, G.A.* ^1,2 and C.E.CÁCERES ^1,2

University of Illinois, Champaign-Urbana, IL 61820, USA ¹
Illinois Natural History Survey ²

Abstract:
The dormant eggs of Daphnia (Crustacea: cladocera) are encased in a protective, chitonous casing known as an ephippium. Ephippia are pigmented with melanin, and the degree of pigmentation ranges from transparent to opaque. Variation in color exists within and across populations and species, raising questions about the factors that influence the range of pigmentation found in the environment. Mellors suggested that females bearing the darkly pigment ephippia suffered increased mortality in the water column due to predation by fish, but that increased pigmentation increased the chance of the eggs remaining viable following gut passage of the predator. Other studies show that pigmentation in adult Daphnia is associated with UV-light intensities, suggesting that pigmentation in the ephippia may be in response to UV-light levels as well. Our study focuses on the causes and consequences of pigment variation in pond and lake-dwelling Daphnia. We use both image analysis and melanin extraction to quantify the pigmentation in ephippia. The results of the image analysis indicate that a single population can contain pigmentation intensities ranging from 0.5% - 99.5% dark. On-going studies are designed to address the factors (e.g., temperature, UV exposure, genetic variation) that influence the degree of pigmentation found within and across populations. In addition, using both vertebrates and invertebrates as predators, we examine the degree of resistance to predation conferred by the pigmentation. In preliminary predation trials, the amphipod Gammarus gammarus chose the darkly pigmented ephippia twice as often as it chose the light ephippia. However, in observations, it was rare for an amphipod to devour a dark ephippia on a single visit and common for the light ephippia to be fully consumed on its first encounter. By using both experimentation and observation, we document the biotic and abiotic factors influencing the pigmentation in Daphnia ephippia.

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This abstract is being presented at: 10:30 AM in session:
PLANKTON