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The effect of grazing by Littorina irrorata on edaphic and epiphytic communities of salt marsh diatoms
Abstract
Samples of natural (ungrazed) and snail-ingested (stomach and fecal) diatom assemblages from a Spartina alterniflora marsh on the southern shore of West Galveston Bay, Texas, were obtained in January, April and August 1992. Littorina irrorata fed predominantly on the exposed sediment in January and April, and on the epiphytic community on S. alterniflora in August. The alternation of the snail's feeding substrates corresponded with the tidal exposure duration of sediment and the development of epiphytic community on Spartina alterniflora culms. A total of 206 diatom taxa, including 25 unidentified species, was recorded in three seasonal samples. The six most dominant species throughout the study period were Achnanthes hauckiana, Amphora coffeaeformis, Denticula subtilis, Navicula tripunctata var. schizonemoides, Nitzschia frustulum and N. punctata. Species composition differed significantly by season and habitat among ungrazed, stomach and fecal assemblages, but species richness did not. The effect of the snail's digestive activity on edaphic and epiphytic diatom communities was demonstrated by comparing stomach assemblages to fecal assemblages as to various parameters (species and cell size composition and percentage of intact cells). Smaller diatom species (cell length < 30 Jim) usually occurred in increased relative abundance in the feces, while larger species decreased in relative abundance. Several characteristics of dominant species in fecal assemblages enhanced survival following digestion by L. irrorata. These characteristics include small size, a streamlined shape, a highly-silicified cell wall, and the possession of a protective external mucilaginous tube. The relative abundance of intact diatom cells (viable or only with cytoplasm-like internal materials) was higher in epiphytic assemblages than in edaphic assemblages. The relative abundance of intact cells was higher in egested (fecal) assemblages than in ingested (stomach). Most of the intact cells in the feces of L. irrorata were of smaller size, and contained relatively greater amounts of digestion-resistant lipid (oil) and proteinaceous material which subsequently becomes available to other marsh heterotrophs.
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Citation
Wu, Tsui-Hui (1994). The effect of grazing by Littorina irrorata on edaphic and epiphytic communities of salt marsh diatoms. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1994 -THESIS -W9596.
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