A 9000 YEARS LONG HISTORY OF TREMATODES PARASITISM IN BRACKISH SETTINGS OF ITALY

The rising global temperature and sea-level have led to concern about the increase of parasites at our latitudes. Ecological studies on parasitism is forcibly restricted to short-time intervals (months/years). The Holocene fossil record can offer a quantitative archive of ecological responses to geological short (102-103 yr) but societally relevant past climate transitions. This study investigates infestation frequency (prevalence) of trematode flatworms attributed to Gymnophallids through the last 9ky in modern and Holocene brackish settings. These parasites typically display a complex lifecycle with three hosts. It is usually in the second host that gymnophallids encyst in a latent stage of metacercaria and induce the active growth of oval pits on the interior of the shell. These pits are preserved in the fossil record, thus providing a proxy for past parasitic dynamics. We focused on the brackish bivalve Abra segmentum due to its abundance in Holocene brackish settings. A sampling campaign was performed in the lagoons of Piallassa Baiona, Saline di Cervia (Emilia Romagna) and Lesina (Apulia). As for Emilia Romagna sites, ~200 specimens have been collected from June to July 2017. In Lesina, ~100 specimens were collected in the winter of 2016 and ~200 specimens in the following summer. Each specimen was examined to detect the presence of sporocyst, cercariae and metacercariae of digenean trematodes. The soft tissues of infected clams have been preserved in ethanol 70% and photographed with a digital camera at different magnification (4x and 10x) for morphological and molecular identification. The shells of all bivalves (modern and fossil) were observed with a stereomicroscope to detect the presence of parasite traces. In modern brackish setting parasite prevalence, when restricted to samples with only metacercariae, shows a low spatial variability. The mean prevalence is comparable among Lesina and Saline di Cervia (21%±5%, and 20%±5%, respectively) and lower in Piallassa Baiona (7%±3%). At seasonal scale, Lesina samples collected in the winter show a reduced prevalence respect to the summer ones, attaining a maximum value of 6%. On the contrary, across Holocene brackish deposits single sample infestation, range from 6% to 70% and display a significant temporal variation along core (that is, outside the 95% confidence bound estimated via randomization). Significantly elevated infestations are recovered proximity to previously documented flooding surfaces. Whereas, non-significantly prevalence estimates (i.e., within 95% confidence bound) all occurred within the sedimentary units bounded by surfaces that testify past floods. In addition, measurements of the pits on the living bivalve shells are smaller and morphologically less pronounced (shallow) than the fossil counterparts. Finally, the average infestation (pits) in the brackish deposits during phases of rapid sea level rise (flooding surfaces), is significantly higher (~57% ± 4%) than the highest prevalence recorded in modern settings (~32% ± 4%). This result hints a possible association between significantly elevated prevalence and centennial scale flooding events and support the link between sea-level rise and increasing parasite activity. The recognition of the link between parasite prevalence and past sea-level rises provides us with an important reference framework for assessing near-future parasite related threats ignited by global warming.