HETEROCHRONIC EVOLUTION IN TROPICAL AMERICAN CORBULIDS
Our study focused on two genera, Bothrocorbula and Corbula (sensu strictu). We sectioned representatives of the four known species of Bothrocorbula (B. radiatula, B. synarmostes, B. viminea, and B. wilcoxii) and found that each species displays the same initial pattern of growth. B. viminea, however, modifies this morphology by thickening its shell and adding material to the ventral margin, thereby producing a nepioconch. Phylogenetic analysis suggests that this character is apomorphic and therefore heterochronically peramorphic. Furthermore, δ18O profiles, used here to establish ontogenetic age, reveal that B. viminea lived longer than its congeners, suggesting hypermorphosis as the heterochronic process. Further phylogenetic analysis is needed to confirm this hypothesis.
We also compared growth patterns between C. speciosa from the eastern Pacific and C. dietziana from the Gulf of Mexico. Both are extant and thought to be trans-Panamic cognates. Sclerochronologic analysis reveals that their cross-sectional shape is identical, and δ18O analysis suggests that they possess similar longevities. Nevertheless, C. speciosa is nearly twice the size as C. dietziana. These observations suggest proportional giantism or dwarfism may have operated in this clade. A better understanding of the age/size relationship within each species is needed to confirm this hypothesis. Further analyses of corbulid evolution may provide new insights into heterochronic processes related to environmental change.