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A morphometric hypothesis of taxon diversity in the freshwater snail genus Gyrotoma

Citation

Minton, Russell (2021), A morphometric hypothesis of taxon diversity in the freshwater snail genus Gyrotoma, Dryad, Dataset, https://doi.org/10.5061/dryad.d2547d822

Abstract

Alabama has long been recognized as an aquatic biodiversity hotspot, and the Coosa River, in particular, was home to over 80 endemic freshwater snail species. Due to human activity, over 40% of the snails have been extirpated including the pleurocerid genus Gyrotoma. Gyrotoma species varied in terms of shell shape and sculpture and were restricted to certain reaches of the Coosa River. Diversity estimates based on shell morphology have ranged from 44 nominal taxa to the modernly recognized six Gyrotoma species. However, basing pleurocerid species boundaries on qualitative morphological features poses many taxonomic and systematic issues. In an effort to better estimate species diversity, geometric morphometrics and Gaussian mixture models were used to assign individual Gyrotoma shells to one of three clusters. Individuals in each cluster had significantly different shapes along with different combinations of quantifiable shell traits. No specific distributional patterns were observed between clusters. Though separable statistically, each cluster cannot be assigned to its own taxonomic unit.

Methods

Gyrotoma shells were borrowed from the Carnegie Museum of Natural History and digitally photographed in apertural view. All 612 shells used were at least 1 cm in length, possessed a sutural fissure, and had both the penultimate and the body whorl intact. For lots containing more than 50 shells that met the above criteria, a subset of 50 shells was randomly selected. Three curves on each shell were digitized using tpsDig2; each curve was resampled into a set of evenly-spaced landmarks for a total of 80 landmarks per specimen. The positions of the first and last landmarks on each curve were fixed, and the landmarks between them were allowed to “slide” in order to minimize bending energy. Shape variables were obtained from the landmark data using a generalized Procrustes analysis in the R package geomorph 3.1.2 (Adams et al. 2019).

Curve and outline analyses, however, may fail to accurately capture snail shell morphology, especially when fine-scale features are present. Therefore, five additional shell features were coded as present or absent in the analyzed individuals: horizontal striations on the body whorl; a posterior callus on the columella; a prominent ridge or row of nodules in the middle of the body whorl; posterior extensions of the whorls that form at the sutures; and a pattern of whorl constriction thought to be diagnostic in G. pagoda.

Usage Notes

The TPS file contains the unaligned scaled landmark positions for all specimens. The data need to be processed to convert the three curves using the sliders file into Procrustes-aligned shape variables.