HETEROCHRONIC EVOLUTION IN TROPICAL AMERICAN CORBULIDS

Recent laboratory study of modern and Neogene corbulid bivalves reveals changes in ontogenetic patterns of shell growth. These observations build on results from a previous study that identified several patterns of shell growth: including an isometric trajectory and two forms of allometric growth, characterized by extensive shell thickening. Integration of these results with sclerochronologic analyses, stable oxygen-isotope (δ¹⁸O) data, and species level phylogenies of two clades within the Corbulidae suggests two types of heterochrony. These results are particularly intriguing because ontogenetic differences may be related to changes in temperature and/or nutrients in pantropical American oceans during the Neogene, thereby linking heterochrony to environmental change.

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.