Yuvero M.C., Lomovasky B., Giménez J


Outline shape in bivalves is a key character that provides information on life habits, and is related with endogenous (genetic and physiological) and exogenous (biotic and abiotic) factors. The mussel Mytilus edulis platensis d’ Orbigny, 1842 presents a wide distribution from Rio Grande do Sul (32° 2'S; 52°5'W), southern Brazil to Santa Cruz (50° 1’S; 68° 31’W), Argentina. The aim of this study was to compare the subtidal and intertidal populations of M. edulis platensis through contour morphology; we hypothesize an association between shell morphology and habitat due to habitat-specific constraints. The subtidal samples were obtained on board the ARA Puerto Deseado, at 35 meters deep, in August 2012 at the Litoral Bonaerense (36º 53’ S; 56º 20’ W) (n=37); the intertidal samples were collected in August 2019 in Villa Gesell (37° 15'S; 56° 57'W), Buenos Aires province, Argentina (n=33). Left valves were digitized with a digital camera. We studied the overall shell shape with an Elliptic Fourier Analysis (EFA) of the closed contour, with seven harmonics, and carried out a Principal Component Analysis (PCA). Significant differences between contours were tested with a MANOVA. The PCA allowed us to discriminate two morphotypes through the outline data. The first principal component (PC1) accounted for 75.59% of the variability, while the second principal component (PC2) determined 12.93%. We associate the PC1 to differences in the width; globose shapes representing intertidal individuals and narrow shapes related to the subtidal. Differences along the PC2 were related to the angle of ventral displacement of the umbo, evident in the subtidal specimens. Statistically significant differences between contours were defined (MANOVA: Wilk’s lambda= 0.31, F2, 69= 78.37, P < 0.001). All these results suggest that M.edulis platensis has developed two ecomorphs with distinct morphological responses to physically stressful conditions registered in the intertidal, as opposed to lower physical stress in the subtidal habitats.