On moderately wave-exposed rocky shores in middle Japan, the upper interidal mytilid,Septifer virgatus, and lower intertidal mytilid,Hormomya mutabilis, occur together, forming vertically contiguous mussed beds. Factors limiting the lower distribution limit ofSeptiferand the upper limit ofHormomyawere investigated by collections of natural mussel clumps and single- and mixed-species transplantation experiments. Newly settled juvenileSeptifer(15 mm)Hormomyawas much lower inHormomyaclumps transplanted upwards into theSeptiferzone than it was inHormomyaclumps in theHormomyazone. In mixed-species clumps in theSeptiferzone, however, survivorship of smallHormomyawas not significantly different from that in mixed-species clumps in theHormomyazone.Septiferhad a positive effect on the survival of smallHormomyaand increased the upper limit ofHormomya, presumably by providing shelter and thus protecting them from desiccation. The upper limit ofHormomyazone was thus considered to be set by desiccation exceeding the physiological tolerance of the specie
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