Why Does Trematode Parasite Prevalence Surge During Sea-Level Rise?

Abstract

Previous work has demonstrated a persistent link between sea level rise and a significant increase in trematode parasite prevalence among fossil bivalves in both shallow marine and estuarine environments, over millennial and centennial time scales, and on multiple continents; however the driving factors of these patterns remain elusive. In order to elucidate such processes, we are investigating the spatial variation of parasite-host interactions, environmental variables, and trace element concentration values as proxies of environmental variables from bulk samples of Recent bivalves. The Lesina lagoon in the Puglia Region of southern Italy is approximately 20 km long, two km wide, and has an average depth of 60 cm. The sediment composition is predominantly sandy silt and the lagoon experiences strong seasonal variation in temperature and salinity, the latter of which occurs along a gradient due to the occurrence of freshwater springs at the eastern end of the lagoon. Additionally, the bivalve fauna of Lesina is comparable to that previously examined in core samples of Holocene lagoonal sediments in the Po plain, and, for these reasons, Lesina should be a suitable modern analog for previously studied ancient bivalves and environments. We collected 12 surficial bulk samples of death assemblages along an east-west transect spanning the western half of the lagoon where the target taxon Abra segmentum is most abundant. Prevalence was calculated as the proportion of individuals within a conspecific sample presenting with trematode-induced pits. Prevalence among Abra segmentum varies between 0.10 and 0.50 among the 5,462 specimens from the five bulk samples analyzed so far. Multiple well-preserved specimens of A. segmentum were selected from each sampling location, embedded in epoxy, and sectioned along the axis of maximum growth. These specimens are being analyzed using LASER Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) to collect high resolution ontogenetic sclerochemical records of a variety of trace element (e.g. Sr, Mn, Mg, and Ba) paleoenvironmental proxies. These results will allow us to compare the relationship between parasite prevalence and variation of temperature and salinity as recorded in historical and sclerochemical records.