PHYLOGENETIC AND ONTOGENETIC VARIATION IN LUICIND BIVALVES: TESTING WHETHER MORPHOMETRICS CAN TRACE THE FOSSIL RECORD OF ENDOSYMBIOSIS

Bacterial endosymbiosis is widespread in the Bivalvia, although the inferred timing of evolutionary origin, mode of endosymbiont transmission, and degree of nutritional dependence on endosymbionts varies greatly. Lucinid bivalves are one of the geologically oldest extant bivalve clades (Silurian – Recent) to possess endosymbionts and endosymbiosis is universal among its living members. The ability to track chemosymbiosis in fossil taxa would aid in determining whether endosymbiosis arose in this clade as a basal synapomorphy, through multiple evolutionary origins, or via differential extinction of non-symbiotic lineages. To date, isotopic proxies for symbiosis using shell carbonate have not been successfully calibrated. A morphologic approach using anatomical features preserved on lucinid shells and directly or indirectly associated with endosymbiosis could provide an alternative proxy. One such feature for lucinids is an elongated anterior adductor scar that is partially detached from the pallial line.

In this study, geometric morphometric techniques were used to quantify morphologic variation in lucinids. Pronounced morphologic separation occurs among six genera from the Neogene and Recent of the Gulf of Mexico, western Atlantic, and the Bahamas using landmarks that describe the orientation and length of the anterior adductor muscle scar and its detachment from the pallial line. About 42% of morphologic variation among the 330 specimens examined relates to shifts of the anterior adductor muscle scar from a posteroventral to a more anterodorsal position. This trend corresponds with valve shape (circular vs. ovate outline). About 27% of morphologic variation describes the depth of the channel between the anterior adductor scar and the pallial line. For genera examined, variation along this axis may correspond with the degree of nutritional dependence of lucinids on endosymbionts. Where growth series were available, ontogenetic variability in the anterior adductor scar also is apparent. In Lucina and Codakia, the depth of the anterior channel increases with valve size; in Stewartia, the position of the anterior muscle scar shifts anterodorsally. Morphometric approaches have great potential for determining proxies of endosymbiosis in extinct clades.