Date of Award

Winter 1997

Project Type

Dissertation

Program or Major

Zoology

Degree Name

Doctor of Philosophy

First Advisor

James Haney

Abstract

A mechanism by which light controls diel changes in locomotor activity and surface location of mayfly nymphs (Stenonema modestum Banks), named the Stimulus-based Timing and Activity-Rate (STAR) Model, was tested. Nymph movements were video-recorded in time-lapse from underneath unglazed artificial substrates in a laboratory stream. Light/dark cycles were simulated using computer-controlled halogen lamps. Light increases and decreases were generated to maintain constant rates of relative light change throughout simulated twilight periods. Nymph locomotor activity and position on the substrate were measured in response to rate of light change. Experiments tested whether adaptation light intensity (10$\sp{-4}$ or 10$\sp{-6}$ W cm$\sp{-2}$), time of day (AM or PM), length of the period of light change, or predators, altered nymph responses to light change.

Timing of both heightened nocturnal locomotor activity and leaving the substrate were significantly correlated (R$\sp2$ =.93; p $<$ 0.001 and R$\sp2$ =.71; p $<$ 0.004, respectively) with rate of relative light decrease. Rate of change in light was a correlated with the difference between daytime and nighttime locomotor activity (R$\sp2$ =.38, p $<$ 0.02). The onset of nocturnal locomotor activity was advanced when nymphs were adapted to a low daytime light intensity. Lowered daytime light did not change the time mayflies left the undersides of the substrate. There was no difference in the locomotor activity response between AM and PM experiments, but significantly greater numbers of nymphs left the substrate undersides during simulated twilight in the PM experiments (p $<$ 0.009, F$\sb{1.14} = 9.3.$ The difference between daytime and nighttime locomotor activity diminished during shortened periods of light decrease. When the time intervals over which light was reduced became smaller than the latency period of the response, there was no nocturnal increase in locomotor activity. Nymphs left the substrate undersides regardless of the length of time over which light was reduced. Locomotor activity was greater in the presence of fish odor (Notropis comutus and Rhinichthys cataractae) than in water not containing predators. Locomotor activity was reduced during the daytime in the presence of Paragnetina media stoneflies. Synergistic effects between fish and stoneflies resulted in differences in the timing and locomotor activity of both stoneflies and mayflies.

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