Low barnacle fouling on leaves of the mangrove plant Sonneratia apetala and possible anti-barnacle defense strategies

A field survey of barnacle fouling on leaves of 5 co-occurring mangrove trees (Kandelia candel, Bruguiera gymnorrhiza, Rhizophora stylosa, Sonneratia apetala, and Avicennia marina) was conducted in Tongan Bay, southeastern China. The leaves of S. apetala were less frequently and less abundantly fouled by barnacles relative to the other mangrove species, suggesting that S. apetala may possess anti-barnacle mechanisms. The surface wettability and flexibility of leaves of the 5 investigated species were measured. Except for the lower surface of A. marina leaves, the leaf surfaces of all investigated species exhibited low wettability, with water contact angles of 82 degrees to 94 degrees. The leaf flexibility of S. apetala was similar to that of A. marina, and both were much greater than those of the other 3 species. Leaves of all 5 mangrove species were also subjected to chemical extraction. The extracts of K. candel, S. apetala, A. marina, and B. gymnorrhiza showed anti-settlement activities against the barnacle Balanus albicostatus, with median effective concentration (EC50) values respectively of 0.85, 10.13, 37.03, and 50.67 mu g cm(-2). An antifouling compound, identified as oleanolic acid, was isolated from the 2 most active extracts (from K. candel and S. apetala) via bioassay-guided fractionation. Based on our results, we propose a potential multiple defense strategy of S. apetala against barnacle fouling, including low surface wettability, use of an anti-barnacle settlement metabolite of oleanolic acid, and post-settlement detachment of barnacles from its flexible leaves.