The California horn snail (Cerithidea californica (Haldeman, 1840) (Potamididae: Prosobranchia) dominates the heterotrophic biomass of the salt marsh estuaries of California, Baja California, and Baja California Sur. Thus, it is an ecologically important species, functioning as a significant grazer and competitor, and host to more than 19 species of trematode parasites that infect many other species of invertebrates, fish, and birds as second, intermediate, and final hosts. Any changes in the distribution and abundance of this snail would likely have strong effects throughout estuarine food webs. Here I present observational and experimental studies performed both in the laboratory and in nature that advance our knowledge about the distribution and abundance of these snails.

In the first chapter, I show how snail distribution and abundance was restricted by vascular vegetation, bank orientation in channels, and water depth, due to the negative effects of shading on benthic primary production. Vascular vegetation dominates the biomass of the estuaries of California and Baja California, any changes in vegetation cover will subsequently have an effect on the abundance and distribution of snails. In chapter two, I present how crabs negatively affected snails through predation and non-lethal effects that changed the behavior and diminished the growth rates of snails. Predation pressure was especially high on eggs and the smallest size classes of snails, which had been overlooked in previous predation studies. Finally, in chapter three I present results on how low temperature seems likely to set the northern extent of the horn snail's range by reducing snail performance.

Results also suggest a possible tradeoff between growth and reproduction and highlight that although temperature might be important, several other local variables such as predator abundance and parasitism also have a considerable effects on snail abundance and performance. Understanding what sets the distribution and abundance of horn snails at local and regional scales will allow us to better predict the effects of climate change and other anthropogenic effects on these estuarine ecosystems.