Typescript. Includes vita and abstract. Bibliography: Includes bibliographical references (leaves 61-68).

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M.Sc. / The health of aquatic ecosystems can be severely compromised by a variety of pollutants, such as heavy metals, which are related to anthropogenic activities. Increased recognition is given to the use of organisms, especially molluscs, in the biomonitoring of aquatic ecosystems. This promising approach complements the interpretation of the physico-chemical measurements of water quality. The bioaccumulation of pollutants as well as the resultant biological responses (biomarkers) in organisms can be used in assessing the spatial and temporal trends of chronically polluted environments. The aim of this study was to determine if selected freshwater gastropods (Melanoides tuberculata, Physa acuta, Helisoma duryi and Lymnaea columella) can be used as suitable biomonitors or indicators of water quality. Interspecies differences in metal bioaccumulation and biomarker responses were determined in order to select the most suitable biomonitor/indicator organism to be used. The bioaccumulation of metals was related to the biomarker responses of the organisms. This study was divided into an active biomonitoring (ABM) study and a laboratory exposure study. In the first study, the organisms, M. tuberculata and L. columella were chronically (two, four and six week period) exposed to field conditions. Metal bioaccumulation as well as the biomarker techniques, DNA damage, catalase (CAT) activity, reduced glutathione (GSH) content and cellular energy allocation (CEA) were measured in the organisms. These general biomarkers of exposure and effect, on the biochemical and cellular levels of biological organisation can give a rapid and sensitive assessment of organism health. The second study consisted of exposing the gastropods, M. tuberculata, P. acuta, H. duryi and L. columella to sub-lethal zinc and cadmium concentrations. The uptake and depuration kinetics of these metals were determined in M. tuberculata and H. duryi for a six hour and 48 hour period, respectively. The bioaccumulation of Zn and Cd as well as the biomarkers, DNA damage, CAT activity, GSH content and CEA were measured in all the species, after a two week exposure period. / Prof. J.H.J. van Vuren

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Taxonomic studies involving only adult forms of organisms may not necessarily provide complete information about differentiation of species or about the evolutionary relationships between species grouped in higher taxa. The taxonomic importance of embryonic or larval morphology has been limited only by the lack of detailed comparative morphological work on development. Fretter (1967) has shown that larval shell characteristics are reliable for taxonomic identification of some British prosobranchs. Ostergaard (1950) proposed using the structure of egg masses, larval shell type and developmental characteristics to confirm adult taxonomic position. In recent literature reviews of the larval biology of opisthobranch, it has become apparent that a considerable amount of plasticity occurs with respect to patterns for development. Often, closely related species have radically different developmental types. Even different populations of a single species may exhibit different development. Often the ecology of the adults of closely related species is also different. However, it is not possible to make meaningful correlations between the biology of the adults and the larvae unless additional ecological and developmental information is available. There is a clear need for studies of larval development and the ways in which larval characteristics and embryology are correlated with adult biology in marine organisms. Aspects of the larval development and adult ecology of Phyllaplysia taylori Dall, are reported in this study.

Twelve freshwater gastropod families, 39 genera and 70 fossil and Recent species are recognized and systematically treated. Keys to the families, genera and species were constructed and all species are illustrated. The purpose of this study was to synthesize and update the taxonomy, zoogeography, origins and correlations with drainage systems of the entire Recent and fossil freshwater gastropod fauna of Texas.

This study describes the structure and function of the alimentary tracts of B. zonalis and C. californica with particular emphasis on the stomach and style sac region. Both species were found to be generally similar to other styles bearing mesogastropods in that they have little or no development of glandular tissue in the esophageal region and a functional style is present in the stomach. The stomachs of both species are specialized to transport and digest a continuous supply of detritus by means of complex ridges, grooves and ciliary currents. C. californica appears more highly specialized for feeding on fine, highly organic detrital material than B. zonalis due to slightly more complex stomach morphology. A positive correlation is postulated between the length of the crystalline style, relative to shell height, and the percentage of organic matter in the detritus ingested.

Ferrissia rivularis was selected for research because of its local accessibility and because essentially nothing was known of its reproduction and development. Reproductive and developmental biology is first described herein from the natural environment. This is then compared with the effects created by experimental exposure to a selected herbicide, maleic hydrazide, which might be a potential threat to certain invertebrates in the natural environment.

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Recent specimens of the genus Diodora from the Smithsonian Institution Mollusc Collection were examined for the neontological portion of this study. Species included were D. cayenensis, D. listeri, D. sayi, and D. tanneri. Fifty eight measurements were made on each specimen and recorded with latitude and water depth at each collection site. Fifty specimens were selected as reference specimens, photographed, their perimeters digitized, and ornamentation and foramen shape evaluated qualitatively. Principal Components Analysis shows that subtidal specimens are generally flatter, have a less constricted foramen and a greater number of ribs than intertidal specimens. Specimens from higher latitudes have a less constricted foramen and an increased number of ribs. In the subtidal sample, larger specimens characterize higher latitudes. Analysis performed on all specimens of D. cayenensis, reveals 1.) a decrease in mean shell size and relative shell height in individuals collected from below the intertidal zone, 2.) shells from the province north of Cape Hatteras, N.C. have significantly more ribs than those of the same species from further south, and 3.) Gulf specimens are taller and relatively heavier for their size than those found elsewhere. More than 500 fossil Diodorids from Smithsonian Institution collections were measured and analyzed as were the recent specimens. Shell morphology varies latitudinally like those of recent subtidal Diodorids: higher latitude specimens have more ribs and a less constricted foramen. Geologic age has little or no correlation to form. / M. S.