Ecological and paleontological implications of trematode-induced morphospace inflation and pallial sinus reduction in bivalve hosts

Digenean trematodes are parasites with a complex life cycle that often
infest shell-bearing mollusks and produce distinct traces on the host
skeleton that are recognizable in the fossil record. Here, three bivalve
species (Transennella conradina, Abra segmentum, and Chamelea gallina)
from Pleistocene and Holocene deposits of Florida and Italy were used to
evaluate the hypothesis that trematode infestation affects shell
morphology. The morphological effects of infestation were evaluated using
geometric morphometrics and the Pallial Sinus Index (PSI = Pallial Sinus
Length / Shell Length). For all three host species: (1) large size classes
possess higher trematode prevalence (i.e., proportion of specimens
possessing trematode-induced pits within a population) and higher
per-specimen frequency of trematode-induced scars when compared to smaller
size classes, suggesting ontogenetic accumulation of parasites; (2)
infested and non-infested specimens significantly differ in shell
landmark-based morphology. Geometric morphometric analyses indicate that
in two out of three species (Transennella conradina, Abra segmentum): (1)
PSI and thin-plate spline analyses suggest significant pallial sinus
reduction in infested specimens relative to non-infested; (2) overall
morphospace range, estimated by sample-standardized principal component
(PC) hypervolume, was inflated with the inclusion of infested specimens.
Consistent with previous studies, results indicate that trematode-induced
morphological changes may influence the burrowing capabilities of the
studied bivalves, affecting their ecological functioning and fitness.
Changes in morphospace induced by trematode parasites hamper species
delineation and confound morphometric and disparity patterns in the fossil
record of infestation-prone species. Excluding fossil specimens with
trematode traces can mitigate those confounding effects. Conversely,
comparative morphometric analyses of infested and non-infested host
specimens may allow us to investigate host responses to parasites over
evolutionary time scales.