Can colonizing mussel outcompete indigenous mussel?

The Red Sea mussel Brachidontes variabilis (Krauss) has colonized the Mediterranean Sea following immigration through the Suez Canal. In the Mediterranean, it encountered Mytilaster minimus (Poli) a smaller species with a higher intrinsic rate of population increase. Both inhabit identical intertidal habitats, where they are mixed together in a random spatial pattern, but in different relative densities, depending on wave exposure. At an Israeli locality, where the indigenous species is 170 times commoner than the colonizer, plots of adult B. variabilis at saturation-density were experimentally created in manually cleared patches within the mytilid bed. Growth rate and survivorship of these mussels were not detrimentally affected by this experimental superabundance. Similar plots with monospecific stands of adult M. minimus were experimentally created, and it was found that adult B. variabilis inhibit survivorship and mean shell length of recruits of M. minimus more than these recruits are inhibited by adults of their own species. These results, together with the information on life history, distribution and habitats of the two species, support the hypothesis that the colonizer should outcompete the indigenous species in sheltered localities, and the indigenous species should outcompete the colonizer in exposed localities. However, environmental patchiness in the Mediterranean with regard to the prevalence of wave exposure enables the two species to coexist stably, both locally and regionally, through an extinction-immigration competition equilibrium.