Shell-specific differentiation: how geometric morphometrics can add to knowledge of Macominae species (Tellinidae, Bivalvia)

Abstract

Tellinidae is one of the most diverse families in Brazilian coastal areas, comprising nine subfamilies, one of which is Macominae. These species are known for their morphological similarities, which make their identification difficult. To minimize such difficulties, taxonomists have sought methods or tools, such as the use of geometric morphometric data, to study the continuous characteristics which are often observed subjectively and are not measurable. Almost thirty right valves were chosen from five of the most representative Macominae species: Austromacoma biota Arruda & Domaneschi, 2005 , Austromacoma constricta (Bruguière, 1792 ), Psammotreta brevifrons (Say, 1834 in Say 1830–1834 ), Psammotreta cleryana (d’ Orbigny, 1846 in d’ Orbigny 1834–1847 ), Macoploma tenta (Say, 1834 in Say 1830–1834 ). The geometric morphometric data were based on the fourteen landmarks of each internal valve, performed by TPSDig, and the morphometric and statistical analyses, carried out with MorphoJ, were Procrustes analysis, Procrustes Anova, principal components analysis, discriminant function analysis, and regression analysis. Permutations and Goodall’s F test with bootstrap were performed in the R software package. All the analyses divided the studied species into two groups based on the external shape of their shells: oval-trigonal or elongated. The variation of the external shape of the shell is related to the life habits of the bivalve, with the elongated shells lying inclined in the substrata, and the oval-trigonal shells lying horizontally, facilitating deposit-feeding habits. The pallial sinus shape and position of the adductor muscle scar also helped differentiate species of each group, relating them to the bivalve anatomy. Psammotreta cleryana and P. brevifrons exhibited remarkably similar shapes, and a broader population study is needed to differentiate between these two species.