The population dynamics and reproductive strategy of the stream snail Oxytrema silicula (Gould) in relation to stream order Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zk51vk467

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  • The population dynamics and reproductive strategies of the stream snail Oxytrema silicula were examined in four coastal streams in Oregon representing a small first order stream, a wider third order stream and a fourth and sixth order stream. Samples were taken in each type of stream bottom for each stream every three months for one year. During each sampling period several characteristics of the population were determined including snail densities for each stream and each of five predetermined weight classes, sex ratio, reproductive activity, fecundity and apparent mortality. In addition, sediment type, water depth and type of cover, if any, were determined for each sampling point. Snail densities were generally highest in the summer and fall following the egg hatch in the late spring. During any sampling period, snail abundance was greater on gravel and cobble substrates than on sand or silt substrates. Only larger sized snails (greater than 9 mm in length) were found on the latter with smaller snails being concentrated in the shallow, rocky areas. Highest densities were always recorded on allochthonous materials in the stream such as leaves, twigs and flowers. On the basis of the CLUSTER and multivariate analyses, it was determined that stream depth was the most important environmental parameter of the ones measured determining the observed distribution of snails in the stream. Subsequent substrate preference experiments supported this conclusion. As stream order increases, several changes occur in the population structure and reproductive output of O. silicula. Mean density, apparent mortality and fecundity decrease with increasing stream width while mean biomass, and the proportion of larger snails increase. Mortality, as measured by numbers of empty shells and dead snails observed, was usually highest following the winter freshets. Dead snails representing all size classes were recorded in both the first and third order streams while dead snails in the fourth and six order streams were usually in the largest size classes. The shell:biomass ratio of the larger weight snails in each stream (those weighing more than 300 mg total fresh weight) increases with increasing stream order. Thus, relatively more of the total weight of larger snails in larger streams is shell. Perhaps this is indirectly related to the increase in the incidence of gonadal parasitism of O. silicula with stream width which may also be a possible selection force maintaining the relatively low densities and fecundity for snails inhabiting larger streams. Preliminary evidence using isolated snails both from Oak Creek (third order) and Salmon Creek (fourth order) suggested no inherent difference in reproductive capabilities between the two snail populations. Furthermore, results of short-term feeding experiments indicated that snail growth and timing of reproductive maturity are directly related to food abundance and perhaps food quality. Other environmental parameters such as temperature and stream flow rates are believed to be important in regulating the growth and reproductive activity of 0. silicula. These results suggest that reproductive strategies and resulting population structure of O. silicula are due to a high level of phenotypic plasticity which is at least partly controlled by environmental factors such as flow rates, temperature and competitive interactions.
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