PHENOTYPIC TRAJECTORY ANALYSIS REVEALS TEMPORAL DIVERGENCE OF SHELL SHAPE IN BIVALVES FROM ADJACENT MARINE LAKES

Patterns of shifting phenotypes within a population, be they based on genetic drift or plasticity, reflect potentially significant information regarding ongoing and past adaptation. For bivalves, shell morphology from time-averaged death assemblages may serve as a record of all phenotypic variability from the recent past, representing an expected range of variability by which the morphology of the living population can be tested. The goal of this study was to assess morphological variation of live and co-occurring dead specimens of Ctena orbiculata (Lucinidae, Bivalvia) to address 1) whether temporal morphological trajectories differ between populations, and 2) whether morphological disparity differs between live and dead specimens. Specimens were collected from sieved sediments from Moon Rock Pond and Pain Pond, two adjacent marine lakes on San Salvador Island, Bahamas. Geometric morphometrics were used to quantify shape based on internal shell features and outline curves. Results demonstrate that mean shell shape varies among lakes, between live and dead specimens, and for the interaction between lake and life state. A principal component analysis reveals a common shift along PC2 shared by both populations, primarily as a reshaping of the shell margin. PC1 describes the relative length of the anterior adductor muscle scar, which strongly separates live populations from each lake, whereas dead specimens exhibit intermediate morphology in this regard. Thus, phenotypic change within these populations can be described as a series of shared and unique shifts that may be adaptive to the conditions associated with each lake. Surprisingly, dead specimens did not exhibit greater disparity than live specimens, countering the idea that dead specimens represent a time-averaged history of morphological changes. Rather, phenotypic shifts from dead to live appear to be dramatic departures from typical morphology.