Factors delimiting the boundary between vertically contiguous mussel beds of Septifer virgatus (Wiegmann) and Hormomya mutabilis (Gould)

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 of Septifer and the upper limit of Hormomya were investigated by collections of natural mussel clumps and single‐ and mixed‐species transplantation experiments. Newly settled juvenile Septifer (<5 mm shell length) were significantly fewer in the natural and artificial Hormomya clumps than in the Septifer clumps. Both natural and artificial Hormomya clumps accumulated a much greater amount of sediment than did the Septifer clumps. Hormomya clumps inhibited the recruitment of Septifer , presumably through accumulation of sediment, which resulted in setting the lower limit of the Septifer zone. Survivorship of small (5–10 mm) and large (>15 mm) Hormomya was much lower in Hormomya clumps transplanted upwards into the Septifer zone than it was in Hormomya clumps in the Hormomya zone. In mixed‐species clumps in the Septifer zone, however, survivorship of small Hormomya was not significantly different from that in mixed‐species clumps in the Hormomya zone. Septifer had a positive effect on the survival of small Hormomya and increased the upper limit of Hormomya , presumably by providing shelter and thus protecting them from desiccation. The upper limit of Hormomya zone was thus considered to be set by desiccation exceeding the physiological tolerance of the species.