The crustose coralline alga, Phymatolithon Foslie, inhibits the overgrowth of seaweeds without relying on herbivores

When surfaces of boulders covered with Phymatolithon Foslie were compared with boulders of bare granite in a grazer removal experiment, the biomass of recruited fleshy algae was significantly lower on the Phymatolithon, being on average less than half that on the granite. The inhibitory effect was not species specific; the species composition and species diversity on the two substrata were similar. S.E.M. studies indicated that the surface of Phytomatolithon is unstable in that epithallial cells slough off frequently. There are also numerous chalky white scales on the crust surface, up to ~1 mm diameter and 10-15 cells thick, that are easily dislodged. We suggest that, although the possibility of chemical inhibition cannot be ruled out, the instability of the surface crust contributes significantly to inhibition of algal settlement on Phytomatolithon. There was no evidence of dieback of Phymatolithon crusts during the 16 months of the experiment.