Differential susceptibility to hypoxia aids estuarine invasion

I examined the resistance of 2 competing estuarine mudsnails, Cerithidea ca lifornica (native) and Batillaria attramentaria (exotic), to mortality from microbes and associated alteration of the abiotic environment (i.e., hypox ia). B, attramentaria is displacing C, californica from salt marshes of nor thern California where the 2 species overlap. I first examined the mortalit y and distribution of the snails during 2 natural experiments in Bolinas sa lt marsh (Marin Co., California, USA) that exposed the snails to high bacte ria levels and low oxygen conditions for prolonged periods (>3 wk). Second, I conducted 2 laboratory experiments designed to quantify the rates and me chanisms of snail mortality related to bacterial activity. In both the fiel d and the lab, B, attramentaria was significantly more resistant to death b y low oxygen. Lab experiments demonstrated that indirect effects of bacteri a, i.e., low oxygen levels las opposed to direct infection by bacteria), we re responsible for the high mortality rate of C. californica. In the field, C, californica seemed able to mitigate effects of hypoxia by migrating awa y from sources of high stress, but only when the harmful bacterial activity was highly localized. Ultimately this study illustrates that stress from m icrobial action differentially affects the 2 competing species and thus con tributes to the invader's documented displacement of the native in marshes where they co-occur. Given the growing incidence of eutrophication and asso ciated anoxic conditions in near-shore aquatic environments worldwide, diff erential susceptibility to hypoxia/anoxia may increasingly contribute to th e outcomes of biological invasions in these habitats.