The new diterpene methoxy-ent-8(14)-pimarenely-15-one (1) and three known metabolites: ent-8(14)-pimarene-15R,16-diol (2), stigmasterol (3) and β-sitosterol (4), were isolated from the roots of the mangrove plant Ceriops tagal. Their structures and relative stereochemistry were elucidated by means of extensive NMR, IR and MS analysis. Compounds 1, 2, 3 and 4 exhibited significant antifouling activities against cyprid larvae of the barnacle Balanus albicostatus Pilsbry, with EC50 values of 0.32 ± 0.01, 0.04 ± 0.00, 4.05 ± 0.15 and 18.47 ± 0.40 µg/cm2, respectively, whereas their toxicities towards cyprids were very low, with LC50 values all above 10 µg/cm2.

A new atisane-type diterpene, ent-16α-hydroxy-atisane-3,4-lactone (4) and three known diterpenes, ent-16α-hydroxy-atisane-3-one (1), ent-atisane-3β,16α-diol (2), ent-3,4-seco-16α-hydroxyatis- 4(19)-en -3-oic acid (3) were isolated from the bark of the mangrove plant Excoecaria agallocha. Their structures and relative stereochemistry were elucidated by means of extensive NMR and MS analysis. Compound 3 exhibited significant anti-microfouling activity against the adherence of Pseudomonas pseudoalcaligenes, with an EC50 value of 0.54 ± 0.01 ppm.