Using morphometrics to identify glochidia from a diverse freshwater mussel community

We measured shell length, hinge length, and height of glochidia from 21 freshwater mussel species occurring in the Sipsey River, Alabama, to test our ability to identify species based on these glochidial morphometrics. Glochidial size and shape differed widely among species; for all 3 dimensions, mean values for the largest species were 5 to 7X greater than for the smallest species. Within-species variation in glochidia size was low for all species with the exception of Pleurobema decisum, which was represented by 2 glochidial morphotypes; variation within each morphotype was similar in magnitude to variation within other species. We were able to classify 72 to 79% of total glochidia (n = 870 or 750, respectively) to the correct species with discriminant function analysis. Percentage of correct classification ranged from 40 to 100% for individual species. Misclassifications were caused by overlap in shell dimensions between some species, but even species with poor classification success were confounded with an average of only 2.2 to 3.0 other species. Unlike previous studies, we found that glochidia of closely related species were not necessarily more similar to each other than to glochidia of more distantly related species. For example, species in the tribe Quadrulini were widely divergent in glochidium size and represented some of both the smallest and largest glochidia in our study. These 3 shell measurements and subsequent application of discriminant function analysis can be useful for identification of unknown glochiclia or for rapidly narrowing the range of potential species identifications to smaller groups of species with similar glochidia.