The Quantification of Morphological Variation and Development of Morphology-Based Keys to Identify Species of <i>Fusconaia</i> and <i>Pleurobema</i> (Unionidae) in the Green River, Kentucky, USA

Abstract

We quantified morphological variation among genetically identified specimens of <i>Fusconaia flava</i>, <i>F. subrotunda</i>, <i>Pleurobema cordatum</i>, <i>P. plenum</i>, <i>P. sintoxia</i>, and <i>P. rubrum</i> inhabiting the Green River, Kentucky, species with shells that are morphologically similar to each other and thus difficult to identify. Molecular identifications then were compared with phenotype-based identifications by experts, who on average correctly identified 70% of the specimens. Expert identification of the putative species <i>P. rubrum</i> and <i>P. sintoxia</i> resulted in them usually being identified as the latter. Multi-variable decision tree analysis was conducted to determine the best suite of morphological variables for identifying live mussels and shells to species. Cross-validation error rates for these analyses were 12.6% and 4.14% for live mussels and shells, respectively. Both random forest and decision tree analyses showed the most important variables to be the presence/absence of a sulcus and shell shape (trapezoidal, circular, oval, equilateral triangle, or isosceles triangle). Dichotomous keys for identifying shells and live mussels were developed based on key morphological characteristics readily identifiable in the field, including foot color, beak direction, and beak position relative to the anterior margin. However, a definitive identification of these species may still need to rely on molecular methods, especially for endangered species.