In a Mexican Caribbean Bay, a seagrass-coralline algae mutualism protects the seagrass Thalassia testudinum from green turtle overgrazing. We postulate that the state of the seagrass meadow in this bay depends on the strengths of the interactions among seagrasses, green turtles, and coralline algae. Spatio-temporal changes in coralline algae cover, growth rates under conditions mimicking seagrass presence and absence, and mortality under conditions of a well-developed coralline algal mat and not algal mat developed were performed. The grazing protection mutualism between seagrasses and coralline algae came into existence under a co-occurrence of increasing grazing pressure and rapid population growth