Histological atlas of freshwater mussels (Bivalvia: Unionidae) with comparison of Alabama rainbow (Villosa nebulosa), Gulf pigtoe (Fusconaia cerina), and Alabama creekmussel (Strophitus connasaugaensis), from the Mobile River Basin, Alabama
Type of Degreedissertation
DepartmentFisheries and Allied Aquacultures
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The “freshwater mussels” (Mollusca: Bivalvia: Unionoida) are a species rich group of parasitic bivalves comprising an estimated 898 species in 6 families, 2 subfamilies, and 29 genera worldwide. Renown for its biodiversity, Alabama has a large proportion of this total number of species, with 180 species of Unionidae and 2 of Margaritiferidae. The total number of unionid species in Alabama also includes 24 extinct, 26 extirpated, and 74 imperiled unionids, and, because of that, the conservation status of these invertebrates is of interest to the Alabama Department of Conservation and Natural Resources. Historically, our understanding of unionid systematics was based on shell morphology and larval (glochidia) characteristics. More recently, phylogentic relationships of mussels are being revealed following the sequencing of highly conserved genes. However, scant information is available on the soft tissues of these invertebrates, and no comprehensive histological treatment of any species has been published to date. This lack of foundational information is a barrier to advancing our knowledge of mussel biology. This dissertation is a systematic, comparative anatomical study that treats morphological diversity of cells and tissues across three primary lineages (subfamilies) of unionids. I test the hypothesis that freshwater mussel lineages exhibit few morphological differences in their “soft tissues,” which contrasts to the level of morphological diversity exhibited by their shells and larvae. Representative mussel species subjected to anatomical study were selected based upon the taxonomic arrangement of Ortmann (1910) and the combined phylogenetic analysis of Graf and Cummings (2006). The Alabama rainbow (Villosa nebulosa) was selected as the representative of Lampsilinae, Unioninae was represented by the Gulf pigtoe (Fusconaia cerina), and the Alabama creekmussel (Strophitus connasaugaensis) was selected as the representative of Anodontinae. Alabama rainbows were collected from the South Fork of Terrapin Creek during May 2010 (n=36), August 2010 (n=39), and August 2011 (n=5) plus Shoal Creek in May 2011 (n=5). Gulf pigtoes were collected from the Cahaba River in May 2011 (n=10), August 2011 (n=28), and June 2012 (n=7). Alabama creekmussels were collected from Shoal Creek in May 2011 (n=5), January 2012 (n=3), and May 2012 (n=8) as well as from the South Fork of Terrapin Creek in August 2011 (n=7). Select specimens of V. nebulosa (n=45) and S. connasaugaensis (n=10) were sourced from pond and recirculating culture systems of the Alabama Aquatic Biodiversity Center (Marion, Alabama). Mussels were digitally photographed to document gross features and routinely processed for histology following standard protocols before being photographed with a digital camera mounted to a compound microscope. Herein, the null hypothesis that the level of morphological diversity in cells and tissues was qualitatively low across the studied mussel species is accepted. Major and minor anatomical differences between the studied mussels are reported in tables provided herein (totaling 12 distinctive histological features). Some of these differences are relevant to assessing mussel health and propagation efficacy. For example, I speculate that a robust, branched mantle edge with a high surface area may be linked to shell thickness, and, if not fully formed, may cause shell deformities. Workers monitoring mussel health or stream health by studying soft tissue structure should focus on structurally conserved tissues whereas histopathological changes to unionid tissues that exhibit interspecific variation may be less easily comparable.