CT Scans of Live-collected Abra segmentum (Semelidae) Evaluate the use of Gymnophallid Trematode-Bivalve Traces as a Proxy for Prevalence in the Paleontological Context

Digenean trematodes (Platyhelminthes) are parasites which often infest mollusks and can produce distinct traces on their biomineralized skeletons. These traces are readily preserved in the fossil record and provide insights on parasite-host relationships from the paleontological context. In this project, we target the bivalve Abra segmentum (Semelidae) in an attempt to identify and document the species which is responsible for the distinct traces we use as a proxy for trematode prevalence in the fossil record. Specimens were collected live from Lake Lesina (Southern Adriatic Sea, Italy), which is a brackish lagoon located North of Gargano National Park. Specimens were preserved in 70% ethanol prior to analysis. Each specimen will be CT scanned individually for the presence of trematodes. The number, developmental stage, and location of each trematode/trace will be recorded as well as the dimensions of each host bivalve. The size and type of trace holotypes will also be recorded to be evaluated for association with specific trematode species. Trematodes found to be encysted in the mantle cavity and/or traces will be extracted for molecular analysis. Two gene markers will be used (18S rRNA, cox1) to compare against GenBank accessions for trematode taxa known to infest bivalves. We hypothesize that: (1) Larger individuals will possess a greater amount of trematodes, and the number of traces will be positively correlated to the number of trematodes; (2) Trace-forming trematodes will be restricted to one or two parasite species at most in the locality, and will always be found in the form of metacercariae-bearing sporocysts; (3) If multiple traces are present in one individual, they will be clustered most densely around the umbo-anterior muscle scar; (4) Different species of trace-forming trematodes will produce traces of distinct holotypes and/or size classes. We expect this study to help us identify which species is responsible for the types of traces we may encounter in the fossil record and evaluate the accuracy of trematode traces as a proxy for prevalence in such populations.