Environmental correlates of molluscan predator–prey body size in the northern Gulf of Mexico

The Mississippi River delivers tremendous amounts of freshwater and nutrients to the northern Gulf of Mexico, which results in the explosive growth of phytoplankton populations that are typically nutrient limited. Decomposition of phytoplankton blooms by aerobic bacteria can deplete oxygen concentrations in coastal systems, leading to the establishment of oxygen-limited “dead zones.” Changes in the availability of food and dissolved oxygen, as well as changes in temperature, can have wide-reaching effects on coastal food webs. Here, we investigate how primary productivity, dissolved oxygen, and sea-surface temperature affect the sizes of molluscan predators and prey in the northern Gulf of Mexico using collections of shells preserved in seafloor sediment on the continental shelf. We find that the size of bivalves, and the frequency of predatory drilling by snails, are most affected by dissolved oxygen: prey size increases and drilling predation decreases with decreasing concentrations of dissolved oxygen. Sea-surface temperature is positively associated with the size of both molluscan predators and prey. In contrast, net primary productivity has little direct association with size, and the predator-to-prey size ratio also does not vary consistently with environmental conditions in the northern gulf. Larger bivalves in areas with lower oxygen could reflect reduced pressure from predators and, consequently, greater life spans. Larger predator and prey sizes in warmer waters may reflect more optimal conditions for growth. The shells of recently deceased bivalves, and the associated traces of drilling snails on those shells, can be used to investigate long-standing hypotheses about the roles of environmental variation in body-size evolution through geologic time. Furthermore, future studies comparing these historical data with data from present-day communities may help us understand how coastal food webs are changing in response to various human activities.