LIFE HISTORY RESPONSES TO ANTHROPOGENIC EUTROPHICATION IN MARINE BIVALVES WITH DIFFERENT LARVAL DURATIONS

Eutrophication has occurred naturally in the northern Gulf of Mexico for at least the past 1000 years, resulting in periodic occurrences of coastal hypoxia. Increasing human populations and land use practices in the past century have enhanced the delivery of nutrients to coastal ecosystems, which has further exacerbated hypoxia. Because anthropogenic impacts in the Gulf predate biomonitoring efforts, relatively little is known about how marine organisms have responded to anthropogenic eutrophication (AE). Molluscan death assemblages contain valuable biological information that can be used to investigate species responses to environmental changes, and specifically opportunities to examine shifts in life history. Enhanced primary production caused by AE is expected to result in greater fitness for individuals that produce smaller, more numerous eggs relative to individuals that produce fewer, larger eggs. This is expected to lead to a decrease in mean egg size over time. Additionally, hypoxic conditions associated with AE can decrease predation pressure, which may result in further reductions in egg size. In bivalve mollusks, egg size is correlated with the earliest stage of larval shell growth, (Prodissoconch I, PI). We compared the mean PI size of live populations to that of historical populations preserved in surficial death assemblages for the species Ameritella versicolor in coastal Alabama. We then compared our results with a previous live-dead study of life history in Nuculana acuta (Harnik et al. 2017), to assess the effects of larval duration on live-dead disagreement. We hypothesized that bivalves with longer planktonic larval durations (e.g., A. versicolor) would show more limited live-dead disagreement due to increased gene flow between geographically distant populations that occur in different nutrient conditions. In contrast, species with shorter planktonic larval durations (e.g., N. acuta) may be subject to greater local selection and thus exhibit more pronounced live-dead disagreement in PI size. Statistical analyses revealed significant reductions in PI size between historical and live populations for both species. However, counter to our expectations, A. versicolor exhibited greater live-dead disagreement than N. acuta, despite its longer larval duration. Live-dead studies provide a conservative estimate of human impact on coastal ecosystems and offer a unique window on plastic and adaptive responses to anthropogenic environmental change.