Novel guanidine alkaloids dopargimine (1) and mellpaladines A-C (2-4) were isolated from a Palauan Didemnidae tunicate. The structures of 1-4 were elucidated on the basis of spectral data along with chemical reactions. A putative biosynthetic building block of 1-4, 4-guanidinobutyric acid (5), and dimeric polysulfur dopamine, as well as lissoclibadins 11 (6a) and 12 (6b) were also isolated. Compounds 1-3 bound to synaptic receptors, and modulated behavioral profiles of mice after intracerebroventricular injection. (C) 2016 Elsevier Ltd. All rights reserved.