Duration of overgrowth affects survival of encrusting coralline algae

Encrusting algae can withstand being overgrown for relatively long periods of time. Knowledge of the mechanisms operating to reduce loss of biomass and of the temporal scales over which these are efficient is crucial for understanding competitive interactions occurring via overgrowth. In some red encrusting algae, the translocation of metabolites from unshaded to shaded parts of the thallus has been indicated as a possible mechanism for their resistance to overgrowth. This study tested the hypothesis that the efficacy of the connection between shaded and unshaded portions of the thallus in promoting the survival of the encrusting coralline alga Neogoniolithon brassica-florida, when overgrown, would vary according to the duration of the overgrowth. This hypothesis was tested experimentally by manipulating the availability of light and the connection between shaded and unshaded parts of the thalli. Although there was a slightly different response between experimental trials, the connection with unshaded parts enhanced the survival of shaded parts of the thallus, over a period of about 3 mo. In contrast, 8 mo after the start of the experiment, the percentage cover of living N. brassica-florida under artificial shading devices was small, irrespective of the presence or absence of a connection with unshaded parts of the thallus. These results support the model that translocation of nutrients across the thallus could enable encrusting algae to survive being overgrown, but not indefinitely. This study underpins the need to take into account life-history traits of interacting species when interpreting competition occurring via overgrowth.