Utility of Shell-Valve Outlines for Distinguishing among Four Lampsiline Mussel Species (Bivalvia: Unionidae) in the Great Lakes Region

Abstract

Four freshwater mussel species from the tribe Lampsilini found in the Laurentian Great Lakes region—Lampsilis fasciola (Wavy-rayed Lampmussel), Lampsilis cardium (Plain Pocketbook), Ortmanniana ligamentina (Mucket), and Lampsilis siliquoidea (Fatmucket)—have similar and variable shell morphologies that make some specimens difficult to identify in the field. Identification is further confounded by sexual dimorphism in three of the four species. We used landmark-based morphometric analyses of shell shape in conjunction with DNA barcoding to quantify shell-shape differences between the species. We collected specimens (N = 388) from Great Lakes tributaries in Michigan, USA, and Ontario, Canada. We photographed each specimen and made an initial identification in the field. We then took a tissue biopsy or swab from 248 of the specimens, sequenced a fragment of the mitochondrial cytochrome c oxidase subunit 1 (COI) gene, and confirmed identifications by comparing our sequences with sequences for all four species accessioned in GenBank. On the photographs, we digitized 21 two-dimensional landmarks along the shell margin and used multivariate methods to evaluate the correspondence of shell shape to our COI-confirmed species identifications and sex determinations. Principal-components analysis and linear-discriminant analysis of shell shape correctly identified only 77.8% of specimens to species and 72.2% to species and sex. Sex determination was particularly confounded by the similar shapes of female L. fasciola and female L. cardium specimens. This study demonstrates the limitations of using only two-dimensional valve shape in differentiating among some mussel species.