INFLUENCE OF TREMATODE PARASITISM ON MORPHOLOGY OF BIVALVE SHELLS: PALEONTOLOGICAL IMPLICATIONS

Digenean trematodes are parasites with a complex life cycle, which involve one or more intermediate invertebrate hosts followed by the infestation of a definitive vertebrate host that consumes an intermediate host. The parasite asexually reproduces within the intermediate host. Once they infest the definitive host, the parasite reaches adulthood and can undergo sexual reproduction. In the case of invertebrate hosts possessing biomineralized skeletons, trematode infestation can produce distinct traces that can be recognized in the fossil record. Here, we study two bivalve species (Transennella conradina and Abra segmentum) in order to evaluate the hypothesis that trematode infestation affects shell morphology.

A total of 745 whole right valves of T. conradina fossils were collected from Leisey Shell Pit 03A located near Ruskin, Hillsborough County, Florida. The quarry contains a wide variety of marine and freshwater taxa and various vertebrates from the Upper Pleistocene Fort Thompson Formation. Both left and right valves of A. segmentum totaling 132 specimens were collected from the present-day Lake Lesina death assemblage near Foggia, Italy. Scaled photomicrographs of the valves were taken with a HAYEAR microscope camera and measured using the ImageJ software. Four pseudo-landmarks and nine morphological landmarks were digitized using the TPSDig software.

For each species, Procrustes transformation followed by a principal component analysis of the Procrustes coordinates was performed in R using package “geomorph”. PC1 vs PC2 plots show separation between infested and uninfested populations along the PC1 axis. PERMANOVA of the Procrustes coordinates was performed in R using package “vegan”. Both T. conradina and A. segmentum show a statistically significant difference (p < 0.0001) between infested and uninfested populations, implying that trematode parasitism may influence the burrowing depth of bivalves.

Consistent with previous studies, these preliminary results suggest that the morphology of mollusk shells may be notably affected by trematode parasites. If trace fossils indicating infestation are present in fossil specimens, the impact of parasites should be considered when interpreting morphometric patterns. The study also suggests that morphological responses to parasitic interactions may be explorable over evolutionary time scales.