Thesis (M.S.)--Humboldt State University, 2009. / Includes bibliographical references (leaves 39-45). Also available via Humboldt Digital Scholar.

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The goal of this study was to describe the benthic community and the food habits of the round goby Neogobius melanostomus, and to compare these food habits to the available prey observed in southeastern Lake Michigan. Nematodes and chironomid larvae were the most abundant organisms within the benthic samples, composing 22.4% and 17.2% of the total organisms collected respectively. Diet contents were identified, enumerated, and measured volumetrically. Copepods (35.7%) were the most abundant prey consumed, while chironomid larvae accounted for 30.5% of the total prey items. However, by volume, chironomid larvae composed 57.6% of the round goby diet, while zebra mussels Dreissena polymorpha composed 19.3%. The round goby were actively feeding on a variety organisms, but were selecting for certain prey. Diet and benthic community comparisons were also made with previous studies in the Great Lakes, which showed a difference in the dominant prey of the round goby as well as a decline in the abundance of zebra mussels in southeastern Lake Michigan. / Department of Biology

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Social status influences reproductive physiology in many species, and sex change in marine teleost fishes provides an excellent model to understand how an organism can modulate its reproductive system in response to social stimuli. The series of experiments presented in this dissertation has focused on the proximate mechanisms underlying sex change and, in particular, the neuroendocrine factors that might translate social information into physiological changes. The bluebanded goby (Lythrypnus dalli) is a sexually plastic fish, and the dominant female typically changes sex when the male is removed from the social group. The direct physical interactions between the male and the females were found to be the main sensory cues that inhibit sex change. Sex steroids can both modulate and be modulated by behavior, and as a result they have been the most obvious candidates for a key role in the regulation of sex change. Males and females showed similar diurnal patterns for steroid hormones, but females had significantly higher water-borne estrogen levels. Concentrations of estradiol, testosterone and 11-ketotestosterone presented sex and tissue differences in brain, gonad and muscle, and they varied in complex ways in different tissues during sex change. The neurotransmitter serotonin (5-HT) has been suggested to be involved in the inhibition of socially regulated sex change because of its role in the modulation of both reproductive and aggressive behavior. None of the pharmacological manipulations performed in L. dalli to alter serotonergic activity was able to overcome the input from the social environment and affect sex change. Neither monoamine levels nor the area or number of 5-HT immunoreactive neurons were different between males, females and sex changers or between dominant and subordinate females. The results do not support the hypothesis of a serotonergic inhibition on sex change in L. dalli, but show that rapid changes in brain androgen levels might be implicated in inducing behavioral or morphological changes associated with sex reversal. Also, steroids respond to changes in the social environment in different ways in different tissues so local steroid synthesis should receive greater attention, and caution is required when using circulating levels to understand behavioral regulation.

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