Shell shape and body mass index of two freshwater mussels differs with age and location

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Many taxa exhibit within-species variation in morphology in response to environmental conditions, community structure, and ontogeny. Freshwater mussels are long-lived and relatively immobile, and variation in shell shape has been documented relative to hydrologic and environmental conditions, but patterns and causes of differentiation are not well understood. We measured shell dimensions, mass, and age of two freshwater mussel species: Ellipse (Venustaconcha ellipsiformis) and Mucket (Actinonaias ligamentina), and calculated body mass index (BMI). Dimensionality and correlation of size metrics was reduced using Principal Components Analysis, and components were evaluated across sites for each species. Results were compared to site-specific measures of habitat, link magnitude, and community structure. For Ellipse, two distinct shell forms were observed which varied in prevalence among streams and appeared to be related to age structure. Younger Ellipse tended to be more ovate and had lower BMI whereas larger, older Ellipse were more likely to be elongate with higher BMI. Mucket also displayed two shell forms. Younger Mucket tended to have lower BMI with a more truncated and inflated shape, where larger, older Mucket were more ovate and heavier. Age structure was the most likely explanation for differences in shell morphology among sites, though we discuss relationships to other variables included in this study. These results highlight the prevalence of shell shape variation throughout ontogeny and suggest stream location may be an important consideration for mussel placement during translocation. Optimal relocation or augmentation locations may differ within a system depending on the age and size distribution of mussels and corresponding differences in shell morphology.
Original languageEnglish (US)
Title of host publicationMidwest Fish and Wildlife Conference 2020
StatePublished - 2020


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