DEVELOPING INTERNAL VOLUME ESTIMATES FOR INEQUIVALVED SPECIES: TESTING CONSTRAINTS ON SHELL SHAPE AND DESIRABILITY AS A PREY ITEM IN CORBULID BIVALVES

In bivalves, internal volume represents the space available for an organism’s visceral mass and therefore is a critical variable for estimating organism size in three dimensions, testing for constraints on shell shape, and inferring desirability as a prey item. Corbulid bivalves, with an established phylogenetic framework, a variety of growth patterns, and strong size trends over space and time, represent an ideal clade in which to examine the interplay between growth strategy, ancestry, and the causes of evolution. Many corbulid lineages, however, are strongly inequivalved with significant valve overlap, making volume estimates based on measurement of individual left and right valves difficult to integrate and potentially inaccurate. To obtain more accurate estimates of internal volume, articulated specimens were embedded and processed using two different methods that involved serial grinding and sectioning at 40 micron intervals. These serial sections were photographed and areas were calculated using NIH Image software. Sequential areas were then integrated to calculate an internal volume for each specimen. Calculated internal volumes then were compared (via correlation and ANOVA with post-hoc tests) with a variety of traditional and landmark-based measurements taken on left and right valves to identify strong allometries that might be used as single valve proxies for internal volume. Because large numbers of articulated specimens are rare in the fossil record, such proxies greatly expand the taxonomic and temporal range for corbulid volume estimates. These data were then used to test a range of existing hypotheses in corbulid evolution concerning the source of interspecific allometries within a subclade and the lack of stereotypy in drilling predation on certain corbulids. Internal volume and shell shape, quantified using landmark-based techniques, were compared to test whether external shape changes are correlated with internal volume, a hypothesis previously proposed to explain strong interspecific allometry in the subclade Caryocorbula. Internal volume and shell length were compared with drillhole data to test the hypothesis previously put forth by Anderson et al. that inequivalved corbulids were less desirable prey items.