Published online by Cambridge University Press: 07 August 2013
Here we describe habitat use and the grazing effects of the sea urchin Tetrapygus niger and the gastropod Tegula atra in the low intertidal zone of a rocky shore in central Peru (Ancón bay: 11°46′S 77°12′W), where these two species were documented as coexisting with no evidence of habitat segregation between them. Gut content and isotopic analyses revealed differences between their diets: T. atra consumes mainly microalgae while Tetrapygus niger consumed also benthic macroalgae. Individual grazing effects were evaluated with inclusion/exclusion experiments. Tetrapygus niger prevented the colonization of the substrate by sessile organisms, including the dominant mussel Semimytilus algosus, while Tegula atra reduced the abundance of microalgae and green ephemeral macroalgae during early succession, but these effects were overwhelmed by the rapid increase in cover of S. algosus. We suggest that Tetrapygus niger plays a key role in this low intertidal community. Through directly limiting mussels to monopolize the substratum, T. niger can reduce the diversity of mussel associated species; conversely, through controlling primary space holders, T. niger can also benefit other grazers that live on bare substrate by maintaining a suitable area for feeding or living.