Chemical versus mechanical inhibition of fouling in the red alga Dilsea carnosa

In this study, we used field experiments in natural populations to test whether the low degree of fouling found naturally on the red alga Dilsea carnosa (Schmidel) O. Kuntze (1893) is due to chemical inhibition by antifouling metabolites. Extracts with concentrations volumetrically equivalent to whole algal tissue were incorporated into stable gels, which served as settlement substrata for potential fouling organisms. The gels were placed in the field during several time periods, covering all seasons. We also investigated the fouling intensity on living D. carnosa plants to be able to compare fouling on algae with fouling on gels. The extracts inhibited recruitment of a few fouling organisms, but they were not effective against the dominant bryozoan fouling species Electra pilosa and Membranipora membranacea. Furthermore, a relatively high number of these species occasionally recruited onto D, carnosa plants, Hence, chemical inhibition of fouling could not explain the low degree of fouling of bryozoan species found naturally on D, carnosa. Instead, based on field observations showing that D. carnosa is able to shed its epidermis, we hypothesised that the low degree of fouling is primarily due to cuticle peeling, whereby the alga sloughs off the outermost cell layer in order to remove associated fouling organisms. This hypothesis was tested in a field survey in which D. carnosa plants were marked and surveyed for about 5 mo. The results show that individual algae that experienced a radical drop in their fouling cover had distinct traces of cuticle peeling. Hence, this study suggests that the dominant fouling organisms are not chemically inhibited by D. carnosa. Instead, the low degree of fouling found naturally on D, carnosa is probably a consequence of a mechanical defence, whereby the alga sloughs off the outermost cell layer in order to remove associated epibiota.