Intraspecific variation is variation among individuals of the same species. This variation is the variation in which natural selection acts on and fills the role as the starting point for the evolution of life. Intraspecific variation occurs in three forms: genetic, environmental, and gene-by-environment variation. Also, intraspecific variation can occur over both large, regional scales and smaller, local scales. The River Continuum Concept (RCC) helps for the study of intraspecific variation within freshwater lotic systems. One dominant group in these environments that have been study extensively are freshwater snails. Freshwater snails are found all over the world in freshwater habitats, where they play critical roles in these systems. Freshwater snails also vary along these freshwater gradients. Past studies have shown that these intraspecific morphological variations are caused by predation, the stream gradient itself, and in fact other morphological characters. Therefore the objectives of this study is to validate and build on the reasons for morphological differences seen in the literature.
The specific species used in this study is Elimia potosiensis, a freshwater snail found in the streams of east-central Missouri. Five sites along the Meramec River were chosen where 50 snails were collected from each and taken back to the lab for morphological analyses. For all snails, foot area was measured, each snail was then euthanized, and the soft body removed from calcareous shell. Each shell’s size, shape, and density were then quantified. Finally each shell was then crush using a custom apparatus to determine the rushing force in which each shell can withstand.
What I found was that mean shell size and shape both varied among sites with more downstream sites containing larger, more rotund shells than upstream sites. Also, as shells got larger they got more slender. No relationship was seen between mean relative wet body weights among sites. Relative foot area did show a significant relationship between relative wet body weight, and shell size and shape, with heavier snails with larger, more rotund shells exhibit larger foot areas. Mean relative foot area among sites was only found to be significantly different at the most upstream site. Shell density showed not relationship with shell size and shape. Mean shell density decreased among sites as you moved from upstream to downstream. Lastly, larger, more rotund shells exhibited a higher crush resistance. Mean crush resistance among sites showed that downstream shells were harder to crush than that of the upstream shells.
Shell size and shape results were consistent with was has been seen in past literature. As for relative weight body weight, it contradicts the notion that snails bodies in less productive environments will grow slower than that of the continuous growing shell yielding a smaller snail body relative to the shell size. As for foot area, heavier snails with larger, more rotund shells may need the larger foot area to counter being less hydrodynamic. However, once upstream and already slender and more hydrodynamic, dislodgment is less of a factor and the snail foot area is greatly reduced. Finally, shell size and shape, and location along the stream gradient showed to have an effect on the crushing resistance of the shell, whereas shell density was thrown out of the analysis on crush resistance upon closer inspection due to a possible systematic methodology error. Overall, although some clear trends can be observed through this study, more research is needed to parse out how different morphological features are interacting with each other and affecting crush resistance.
|Advisor:||Brunkow, Paul E.|
|Commitee:||Jennings, David H., Williams, Jason B.|
|School:||Southern Illinois University at Edwardsville|
|School Location:||United States -- Illinois|
|Source:||MAI 58/06M(E), Masters Abstracts International|
|Subjects:||Biology, Ecology, Zoology|
|Keywords:||Freshwater snails, Shell crush resistance, Shell morphology, Stream gradient|
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