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

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.

阿拉巴马州素来被认定为水生生物多样性热点区域，其中库萨河（Coosa River）更是栖息着超过80种特有淡水螺类。受人类活动影响，超40%的螺类已区域性灭绝，其中包括肋蜷科（Pleuroceridae）Gyrotoma属物种。该属螺类的壳形与壳饰存在丰富变异，且仅局限于库萨河的特定河段。基于壳形态的多样性估算曾覆盖44个命名类群，而现今公认的Gyrotoma属物种仅为6种。
然而，以定性形态特征作为肋蜷科物种界定的依据，会引发诸多分类与系统学问题。为更精准地估算物种多样性，研究人员采用几何形态测量学（geometric morphometrics）与高斯混合模型（Gaussian mixture models），将单个Gyrotoma属螺壳划分为3个聚类簇。每个聚类簇内的个体壳形均存在显著差异，且对应不同组合的可量化壳部性状；各聚类簇之间未观测到特定的分布模式。尽管可通过统计学方法区分，但每个聚类簇均无法被单独划定为独立的分类单元（taxonomic unit）。