Lithophaga simplex (Bivalvia:Mytilidae) inhabits only two species of live corals in the Red Sea: Astreopora myriophthalma (Lamarck) and Goniastrea pectinata (Ehrenberg). Larvae from mussels originating from both of these corals were offered a variety of coral substrata, either living corals or bleached skeletons, to assess the degree of specificity with regard to settlement substratum and metamorphosis. An index termed “combined chance of metamorphosis” (CCM) was used to compare the overall success of larval recruitment. Settlement was highest on A. myriophthalma for both populations of larvae, regardless of the coral from which they originated. Metamorphosis was induced only by living corals and was highest on the parent host coral. Overall recruitment success (indicated by the CCM value) was also highest on the parent host coral. It is hypothesized that L. simplex is undergoing speciation in a suggested direction.