Mar Biol (2010) 157:1799–1810 DOI 10.1007/s00227-010-1452-2 123 ORIGINAL PAPER Mechanisms of habitat segregation between an invasive (Mytilus galloprovincialis) and an indigenous (Perna perna) mussel: adult growth and mortality Sarah J. Bownes · Christopher D. McQuaid Received: 25 September 2008 / Accepted: 12 April 2010 / Published online: 8 May 2010  Springer-Verlag 2010 Abstract The invasive mussel Mytilus galloprovincialis and the indigenous mussel Perna perna coexist intertidally on the south coast of South Africa through partial vertical habitat segregation: M. galloprovincialis dominates the upper shore and P. perna the lower shore. Recruitment pat- terns can explain the zonation of P. perna, but not the inva- sive species. We examined the role of post-recruitment interactions by measuring spatial and temporal diVerences in adult growth and mortality rates of the two species. Spe- ciWcally, we tested the hypothesis that interspeciWc diVer- ences in growth and mortality reXect adult distribution patterns. The two study locations, Plettenberg Bay and Tsitsikamma, are 70 km apart with two sites (separated by 300–400 m) per location, each divided into three vertical zones. Growth was measured seasonally using diVerent marking methods in 2001 and 2003. Cumulative adult mor- tality was measured through summer in 2003/2004. Both species generally grew more slowly upshore, but they showed diVerent eVects of season. For P. perna, growth was signiWcantly reduced in winter in the low zone, but unaVected by season in the high zone. For M. galloprovin- cialis, growth was either unaVected by season or increased in winter, even in the high zone. Thus, growth of P. perna and M. galloprovincialis was reduced under cool winter and warm summer temperatures, respectively; and while growth was more similar between species in summer, M. galloprovincialis grew much faster than P. perna in winter. Mortality of P. perna increased upshore. For M. galloprovincialis, mortality was not zone-dependent and was signiWcantly greater than for P. perna on the low-shore and (generally) across the shore in Tsitsikamma. Both spe- cies had higher growth and mortality rates in Plettenberg Bay than in Tsitsikamma. Thus, P. perna seems able to maintain spatial dominance on the low-shore and at certain sites because of higher mortality of M. galloprovincialis. We conclude that seasonality in growth of the two species reXects their biogeographic aYnities and that coexistence is possible through pre-recruitment eVects that limit the verti- cal distribution of P. perna and post-recruitment eVects that limit M. galloprovincialis. Introduction Physical stresses and interspeciWc interactions such as com- petition play a signiWcant role in intertidal community dynamics and the coexistence of competing species (Connell 1961; Dayton 1971; Suchanek 1978) but are also important to the success of biological invasions. One of the main concerns with invasive species is that they can replace or reduce the number of native ones, and many studies have shown how invasive species outcompete local residents due to superior competitive abilities and physiological toler- ances (Holway 1999; Byers 2000; SteVani and Branch 2005). However, interactions with the environment and native species make the invasiveness of non-indigenous species unpredictable, and competitive ability can be equally important to the resistance of a community to inva- sion (Sans et al. 2004; Paini et al. 2008). In mussels, growth and mortality are good indicators of competitive ability (Barkai and Branch 1989; Petraitis 1995). Superior growth rates can lead to mortality or reduced Wtness over slower-growing individuals either Communicated by P. Kraufvelin. S. J. Bownes (&) · C. D. McQuaid Department of Zoology and Entomology , Coastal Research Group, Rhodes University, Grahamstown 6140, South Africa e-mail: sarah.bownes@gmail.com