SHELL PLASTICITY IN SAXIDOMUS NUTTALLI (BIVALVIA: VENERIDAE) FROM FOSSIL ROCK-BORING AND MODERN SAND-BURROWING HABITS AND ITS IMPLICATIONS FOR MORPHOLOGICAL INTERPRETATION

Morphological characteristics of bivalves are often used in paleoecological interpretations of life habit, but some of these characteristics can be explained either as functional adaptations or as the result of growth conditions experienced in different habitats. In bivalves that mechanically bore into hard substrates, it has been suggested that a more convex shell with longer ligaments facilitates shell opening against resistant substrates and is adaptive for rock-boring. However, these differences in shell morphology could reflect reduced growth rates in rock-boring individuals due to the constraints of burrowing in resistant substrates.

I investigate the morphological differences between modern sand-burrowing Saxidomus nuttalli and endolithic fossil conspecifics from a 45,000 year-old marine terrace deposit near Goleta, Ca. This species is not recorded to bore in the modern day. The shape of the hole surrounding the fossil bivalve and the absence of shell distortion suggest the holes were indeed bored by the bivalves and do not represent preexisting cavities colonized by nestling S. nuttalli. Traditional morphological analyses of fossil rock-boring individuals and modern sediment-burrowing individuals demonstrate that the rock-boring S. nuttalli have greater shell height, breadth, and thickness, and have longer ligaments and shorter siphons than living soft sediment-burrowers. Landmark-based morphological analyses of interior valve features further show that a majority of the shape change from sand-burrowing individuals to endolithic individuals occur as a relative expansion of the shell near the umbo. This suite of features can be explained by slower growth rates in endolithic individuals, but can not be entirely explained as functional adaptations to penetrating hard substrates. Growth constraints induced by resistant substrates may also be responsible for some shape differences observed in other rock-boring species when compared to non-boring relatives and may not actually have a functional significance.