Two sessile filter-feeders of similar body sizes, the goose barnacle Capitulum mitella and mussel Septifer virgatus, occur in patches on a moderately wave-exposed rocky shore of south Japan in the western Pacific. Their pattern of coexistence was investigated through field observations and experimental manipulation, focusing in particular on seasonally variable modes of facilitation under harsh environmental conditions found in the upper intertidal. Field observations and reciprocal-removal experiments showed a high survivorship of C. mitella in both mixed patches (containing C. mitella and S. virgatus) and Capitulum-only patches. In contrast, S. virgatus exhibited a substantially higher survivorship in the mixed than in single situations, indicating that S. virgatus was positively affected by the presence of C. mitella, while the latter was not influenced by the presence of the former in the upper intertidal. An artificial patch transplant experiment demonstrated that C. mitella enhanced S. virgatus survivorship and growth both in summer when thermal stress was severe and in winter when physical disturbance caused by wave action was strong. In contrast, the artificial shade treatment enhanced S. virgatus survivorship only in summer. In summer, thermal conditions (temperature and interstitial humidity within patches) were harsher in the unshaded (S. virgatus-only) than in the mixed (C. mitella + S. virgatus) and shaded treatments (S. virgatus with artificial shade). In contrast, the effect of severe physical condition as evidenced by the extent of shell wear in mussels was more pronounced in the shaded and unshaded treatments than in the mixed treatment in winter. These results suggest that C. mitella had 2 different habitat-modifying functions: (1) amelioration of thermal stress and (2) amelioration of physical stress; the artificial shade performed only the first of these.
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