Grazing patterns in Siphonaria gigas (Mollusca,Pulmonata) on the rocky Pacific coast of Panama

The pulmonate limpet Siphonaria gigas is the most abundant molluscan grazer in the mid zone on rocky, wave-exposed shores of the Pacific coast of Panama. Erect macroalgae and sessile invertebrates are rare; crustose algae cover ～90% of the rock. The relative abundance of a common blue-green algal crust (Schizothrix calcicola?) is negatively correlated with Siphonaria's abundance. Large-scale removals of the limpet cause rapid increases in percent cover of Schizothrix and concomitant decreases in other crusts, but no changes in the abundance of erect algae or sessile invertebrates. Removing Siphonaria also (1) increases recruitment of crustose algae and barnacles onto new rock and plexiglass substrata, and (2) decreases the abundance of a calcified form of Schizothrix. Harsh conditions during daytime low tides and foraging by fishes at high tide control the microdistribution of most of this region's mobile and sessile benthic organisms. Wave action and substratum heterogeneity modify these constraints: Siphonaria is rare or absent in sheltered areas, especially on homogeneous surfaces, and is most abundant at wave-exposed sites. However, at extremely wave-beaten sites, Siphonaria and other benthic consumers are rare and ineffective. Crustose algae are reduced in abundance and space is dominated by erect macroalgae and/or barnacles. These normally rare species can outcompete crusts only when thermal or desiccation stress and the effects of benthic consumers and fishes are drastically reduced.