At Mar del Plata beach better be in a good shape: morphometric analyses in fossil and modern Eucallista purpurata (bivalvia) shells as a response to environmental energy

SOL BAYER; FERNANDA SERRA; PAULA ANDREA CRISTINI

Salta

Otro; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2022

Asociación Paleontológica Argentina

In high energy habitats, seasonal storms become a press disturbance affecting sediment quality and stability of benthic assemblages in terms of removing organisms from their life positions. Pleistocene clams of Eucallista purpurata from San Antonio Bay (SAB, San Matias Gulf, Patagonia) evidenced variation in shell size and shape concerning for Holocene (late Holocene, 1000-328 yrs. B.P.) and modern ones. This variation in shells could be related to burial behaviour, increasing its efficiency in higher energy environments interpreted as an open sea habitat, compared to the more protected environment of the Holocene and modern beaches located inside a gulf. This species also lives on the inner shelf of Mar del Plata, which is an open sea environment affected by seasonal storms, providing an excellent opportunity to explore this hypothesis. Therefore, this study aims to corroborate the variability of E. purpurata shell shape in terms of energetic coastal environments. We used geometric morphometric methods on landmark data to study the size and shape shells in both the open sea environment (MDQ) and the protected gulf environment (Holocene and modern SAB). There were significant differences in shell shape (p=0.025) between MDQ samples, although shell size did not show significant differences. Shells from MDQ and SAB samples varied significantly between them in both size and shape. On the other hand, Holocene and modern samples from SAB did not show significant differences either in shell size or shape. MDQ shells were smaller and more elliptical than Holocene and modern ones from SAB. The variation in size and shape of the E. purpurata shells could be related to the phenotypic plasticity of this species as a response to environmental energy. In a more energetic environment, such as an open coast affected by waves and storms as MDQ, more elongated shape shells and smaller sizes lessen the resistance to sediment and allow faster and/or easier burrowing within substrate where stability and/or easier burrowing would be needed. In contrast, rounded shells and larger individuals, as Holocene and modern ones from SAB, experienced higher sediment resistance during burrowing. Then, stability and/or easier burrowing would not be needed in a calmer physical habitat. Therefore, E. purpurata size and shape may reflect in which environmental energy intensity have been living in.