We evaluate the morphological variation of cirri of two intertidal barnacles in response to different wave-exposure regimes and population densities during growth. Length, diameter and number of segments of the third and sixth cirri were determined in Jehlius cirratus and Notochthamalus scabrosus, two dominant sympatric species in the rocky intertidal zone of Chile. Aggregated and isolated individuals of both species were collected in two coastal localities. Reciprocal transplant experiments were undertaken between wave-protected and wave-exposed zones, in addition to experimental modification of density during growth. For both species, individuals inhabiting wave-protected zones had cirri that were longer, thinner, with a greater number of segments, than in those inhabiting wave-exposed zones. The sixth cirri of J. cirratus were longer, thicker and had more segments in specimens growing at high densities, than in individuals growing in isolation. In N. scabrosus, no density dependent effects were observed in cirral structure. Thus, phenotypic responses differed between species, depending on the morphometric or meristic variable of the cirral pair evaluated. Forty-five days after J. cirratus density conditions and wave-exposure regime were modified in the transplant experiments, changes were observed in cirral length, diameter, and number of cirral segments. No modification was observed in the cirral characteristics over time in specimens whose density was artificially reduced during growth. Results indicate that morphological structures associated with filtration and respiration, vary according to density and degree of wave-exposure. We suggest that cirral phenotypic variability can serve as an adaptive mechanism associated with the changing availability and access to food and contributes to explaining the success of these organisms in environments with high spatial-temporal fluctuations in biotic and abiotic factors.