MORPHOLOGICAL VARIATION IN CARIBBEAN CHIONE REVISITED: THE SIGNIFICANCE OF A UNIQUE MORPHOTYPE FROM THE SOUTHEASTERN BAHAMAS

The bivalve genus Chione is a familiar sight to invertebrate paleontologists and neontologists of the Caribbean, as species of the genus are frequently constituents of nearshore benthic communities. Recent work by Roopnarine and Vermeij split the common species Chione cancellata into two, reviving the name Chione elevata to represent specimens from the southeastern coast of the United States and the Bahamas. In this study, we examine the geographic nature of variation within and between these two species, and their relationship to a third, morphologically distinct Chione population from the southeastern Bahamas.

We examined subfossil samples of Chione collected during fieldwork and previous investigations, as well as museum specimens. Collectively, the specimens represent 49 localities in the Caribbean and the Bahamas, and 23 additional localities from northern South America, Mexico, and the southeastern coast of the United States. Individuals were digitally imaged, and morphological data were collected from eleven landmark points corresponding to key shell features and comparable to those used in previous investigations of venerids. Multivariate analyses and Procrustes superimposition were used to compare Chione morphology among localities. Several size-standardization procedures were employed to examine the effects of allometric growth on morphological differences.

Results demonstrate that Chione cancellata, C. elevata, and the southeastern Bahamas morphotype can all be morphologically distinguished using the landmarks selected. The differences between C. cancellata and C. elevata are most striking when they are compared without size-standardization; all size-standardization methods decrease the apparent morphological dissimilarity between these species. However, specimens from the southeastern Bahamas are morphologically distinct from both described species, regardless of size-standardization protocol. These results raise several interesting issues regarding species-level designations for Chione in the Caribbean, including the reliability of morphometric landmarks for describing species-level differences, the geographic context of intra- and interspecific morphological variation, and the nature of biogeographic boundaries in the western Atlantic and Caribbean.