Distribution patterns of the invasive mussel Arcuatula senhousia in Mission Bay, San Diego, California: Effects of sediment grain size and byssus thread production on predation

The ability of bivalves to avoid predation by using defensive behaviors such as burrowing and byssus production may vary depending on the sediment characteristics of a given habitat. The varying sediment characteristics that change with water velocity within estuaries may influence the distribution of bivalves and limit them to areas that optimize their ability to defend against predators. We examined the distribution patterns of the invasive Asian date mussel Arcuatula senhousia relative to the sediment grain size distribution in Mission Bay, an urban estuary located in San Diego, California, USA. Mussels and sediment were collected using an Ekman grab at 76 stations sampled by University of San Diego students during October 2015, 2016, 2018, 2019, 2021, and March 2017. In 2020-2021, we also conducted a series of complementary laboratory experiments to evaluate the sediment preferences of A. senhousia and how A. senhousia mortality, burrowing behavior and byssal thread production was impacted by water temperature, sediment grain size, and predation by a native muricid snail (Pteropurpura festiva). A. senhousia was associated with fine-grained sediment, in particular silts (\u3c62.5 µm), that were most abundant in the back of the bay. Mussels also experienced lower mortality, burrowed deeper, and produced more byssus in fine-grained sediment, particularly in warmer water. These results likely indicate that sediment grain size in Mission Bay, in addition to the previously documented gradient in predator abundance, plays an important role in the distribution of mussels. Furthermore, given the strong effect of temperature on A. senhousia mortality and byssus production that we observed, warming of Mission Bay related to climate change could enhance overall mortality for A. senhousia, irrespective of sediment type and byssal conditions, potentially shifting predator-prey dynamics