Geometric morphometrics of donacids from Brazil reveals a new species, Donax trapesialis n. sp. from Northeastern coast

The genus Donax Linnaeus, 1758 comprises 90 described bivalve species, five of which occur on the Brazilian coast. No taxonomic investigation has been conducted on the Donax species found along the Sergipe coast (Northeastern Brazil). In order to fill some of these gaps in the knowledge about Brazilian donacids, we employed conchological analysis and geometric morphometric approaches to compare the morphology of donacids in Sergipe with D. gemmula Morrison, 1971 from southeastern Brazil, species with which they closely resembled. Our conchological analysis revealed two morphotypes within the Donax genus. The first morphotype is readily identifiable as D. gemmula. The second morphotype exhibits significant morphological differences from all other Donax species. The results of the morphometric analysis statistically supported the findings of the conchological analysis. Therefore, our findings mark the second record of D. gemmula on northeastern Brazilian waters, supporting the hypothesis of a continuous distribution along the Brazilian coast. Additionally, we identified a new species, named Donax trapesialis n. sp., characterized by a consistently rounded posterior ridge, a trapezoidal shape with a wider anterior margin, a concave ventral margin, more ventrally positioned anterior and posterior extreme points, and often triradial purple zones. We provide a dichotomous key to all six Brazilian Donax species, including D. trapesialis n. sp.