Lobatus gigas, the queen conch, is a central component of Caribbean cuisine but over-fishing of juveniles has threatened the stability of wild populations. Strombid gastropods, upon reaching sexual maturity, cease growing along the shell length axis and continue growing in width via a flared and thickened shell lip. This morphology serves as a useful indicator of an individual's sexual maturity. Here we examine temporal trends in population demographics, size, and morphology of harvested L. gigas individuals over the last ∼1 ky from San Salvador Island, the Bahamas to quantify the dynamics of human-induced stress on the local queen conch fishery. We collected 284 human-harvested individuals from shell middens at seven localities, measured seven morphological variables, and classified the specimens as either adult or juvenile. We randomly selected 64 of these shells for rapid AMS radiocarbon dating in order to establish three geochronological bins: Lucayan (Pre-European invasion, 1492 CE), Modern (∼102 y), and Global (∼101 y). The proportion of juveniles harvested increased significantly from 47% (Lucayan) to 61% (Modern) to 68% (Global) suggesting increasing pressure on the fishery through time. Patterns in body size and morphology diverge between adults and juveniles and are likely the result of an increase in the proportion of harvested juveniles, the selection of smaller juveniles through time, and possibly changes in fishing methods. This size selective predation did not result in the suppression of adult body size as found in other studies. Geohistorical data, such as these, are vital for providing long term ecological context for addressing anthropogenic ecological degradation and are central to the conservation paleobiology approach.
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.
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