A new oroidin dimer, mauritiamine (1), has been isolated from the marine sponge Agelas mauritiana along with the known oroidin (2) and 4,5-dibromopyrrole-2-carbamide (3). Their structures were determined by spectral analysis. Compounds 1 and 2 inhibited larval metamorphosis of the barnacle Balanus amphitrite with ED50 values of 15 and 19 µg/mL, respectively, while 3 promoted larval metamorphosis of the ascidian Ciona savignyi at a concentration of 2.5 µg/mL.