Differential susceptibility to hypoxia aids estuarine invasion

I examined the resistance of 2 competing estuarine mudsnails, Cerithidea californica (native) and Batillaria attramentaria (exotic), to mortality from microbes and associated alteration of the abiotic environment (i.e., hypoxia). B. attramentaria is displacing C. californica from salt marshes of northern California where the 2 species overlap. I first examined the mortality and distribution of the snails during 2 natural experiments in Bolinas salt marsh (Marin Co., California, USA) that exposed the snails to high bacteria levels and low oxygen conditions for prolonged periods (>3 wk). Second, I conducted 2 laboratory experiments designed to quantify the rates and mechanisms of snail mortality related to bacterial activity. In both the field and the lab, B. attramentaria was significantly more resistant to death by low oxygen. Lab experiments demonstrated that indirect effects of bacteria, i.e., low oxygen levels (as opposed to direct infection by bacteria), were responsible for the high mortality rate of C. californica. In the field, C. californica seemed able to mitigate effects of hypoxia by migrating away from sources of high stress, but only when the harmful bacterial activity was highly localized. Ultimately this study illustrates that stress from microbial action differentially affects the 2 competing species and thus contributes to the invader’s documented displacement of the native in marshes where they co-occur. Given the growing incidence of eutrophication and associated anoxic conditions in near-shore aquatic environments worldwide, differential susceptibility to hypoxia/anoxia may increasingly contribute to the outcomes of biological invasions in these habitats.