Contrasting effects of the sea-urchin Tetrapygus niger and the snail Tegula atra in a rocky intertidal of central Peru

HIDALGO F. J.; FIRSTATER F. N.; LOMOVASKY B. J.; IRIBARNE O. O.

Lima

Simposio; Censor Endterm Symposium; 2008

The patterns of distribution, potential for interspecific competition and grazing effects of the sea urchin Tetrapygus niger and the snail Tegula atra were studied on a low rocky intertidal zone of central Peru. Tegula atra was more abundant on vertical than on horizontal patches of substrata, and was more abundant than T. niger on vertical patches. There were no differences in the abundance of T. niger between surface inclinations. To test whether the abundance of one species was affected by the presence of the other, we conducted a field experiment in which we applied the following treatments: (1) both species left untouched (controls); (2) both species removed; (3) T. niger present, T. atra removed; (4) T. niger removed, T. atra present. The abundance of each species was not affected by the presence of the other; however, T. niger and T. atra had a segregated distribution on the vertical and horizontal patches of substrata, with interspecific nearest neighbors occurring less often than expected under conditions of random occupancy. Individual grazing effects were evaluated with inclusion/exclusion experiments in summer and winter. Tetrapygus niger prevented the colonization of sessile organisms on the crustose algae throughout all the experimental period. Tegula atra reduced the abundance of microalgae and green macroalgae, and increased the settlement of the polychaete Phragmatopoma moerchii during the first month after experiment starting, but these effects were overwhelmed by the rapid increase in cover of the mussel Semimytilus algosus. We argued that dissimilar foraging behavior result in the contrasting grazing effects observed, which in turn allow the coexistence of T. niger and T. atra in the low intertidal. These results reveal the importance of the grazing by sea urchin in structuring the low intertidal community of central Peru.