QUANTIFYING ECOPHENOTYPIC VARIATION AND TAPHONOMIC BIAS IN THE MORPHOLOGY OF SUB-FOSSIL CREPIDULA FORNICATA FROM CAPE COD, MASSACHUSETTS, USA

Crepidula fornicata is a gregarious, sedentary, suspension-feeding marine gastropod that lives attached to firm substrates. It is noted for its characteristic slipper-like and highly variable morphology, suggested by initial field observations to vary by environment. Here we test the role of ecophenotypic variation in the morphology of sub-fossil specimens of C. fornicata in Cape Cod. We collected surficial bulk samples from 15 shore locations (five rocky ocean-facing beaches, five sandy ocean-facing beaches, and five muddy/sandy protected harbors) and measured five morphological variables (length, width, height, shelf length, and length along maximum growth) on 898 specimens. Principal components analysis of the ln-transformed data on a var-covar matrix was conducted in PAST. Each group in the PC1/PC2 defined morphospace strongly, but not completely, overlapped. PC1 values were strongly and positively correlated with all morphological variables, and are thus a proxy for overall body size. PC2 values were strongly positively correlated with length along maximum growth and less so negatively correlated with height; therefore PC2 is a size-free shape proxy for shell coiling (+) and relative height (-). The maximum body sizes were similar among the three groups; however the smaller end of the distribution found in sandy beach and harbor samples was not present in rocky beach samples. We interpret this pattern to mean that small individuals were excluded from this environment either while living, or, more likely, via post-mortem destruction. Rocky beach specimens tended to display high coiling/low relative height as opposed to the low coiling/high relative height of the harbor specimens and intermediate morphology sandy beach specimens. We attribute this shift in morphology to increased constraints imposed by nestling between cobbles and boulders in a high energy setting relative to those imposed by growth on relatively flat shell fragments on sandy/muddy substrates in quieter water. Though overlapping in morphology, C. fornicata displays strong morphological variation related to substrate and water energy. Moreover, the body size distribution of rocky beach specimens is subject to a taphonomic filter wherein the smaller specimens are preferentially removed by destructive processes.