Date of Graduation

Spring 2021


Master of Science in Biology



Committee Chair

Alicia Mathis


Natural selection should strongly favor characteristics that make prey difficult for predators to distinguish from the background, including both morphological and behavioral crypsis. The Rainbow Darter (Etheostoma caeruleum) is a small, cryptic, benthic fish that inhabits gravel-bottomed streams and is preyed upon by predators such as the benthic Knobfin Sculpin (Cottus immaculatus) and the pelagic Longear Sunfish (Lepomis megalotis). In three experiments, I tested whether the behavior of darters was influenced by the opportunity for crypticity offered by their substrate and by the presence of cues from benthic and pelagic predators. First, darters in the non-breeding season chose substrates that were most similar in reflectance values to their bodies. This choice was expressed as preferential occupation of a darker mixture of dark and light rocks in comparison to a homogeneous mixture of light rocks only. This preference was present regardless of the level of predation risk by the benthic (sculpin) predator. Second, during the breeding season, the behavior of darters was counter to what I hypothesized, with darters showing higher levels of activity on the darker background when predation risk was high. This result could be due to changes in reflectance values from bright breeding coloration leading to the light rocks offering a more cryptic background. Third, in the non-breeding season, darters showed the predicted response of decreasing activity on the darker substrate, particularly when predators were present. Effects appeared stronger to a benthic predator (sculpins) than to a pelagic predator (sunfish). In addition, darters increased vigilance behavior in response to presence of cues from both predators regardless of substrate type and showed less swimming behavior in the predator treatments compared to the control treatment. Therefore, behavior of darters is influenced by the levels of both predation risk and habitat crypticity, which may vary between breeding and non-breeding seasons.


antipredator behavior, benthic, chemical cues, evolution, behavior

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© Sarah White

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