Mellpaladines A–C (1–3) and dopargimine (4) are dopamine-derived guanidinealkaloids isolated from a specimen of Palauan Didemnidae tunicate as possible modulators of neuronal receptors. In this study, we isolated the dopargimine derivative 1-carboxydopargimine (5), three additional mellpaladines D–F (6–8), and serotodopalgimine (9), along with a dimer of serotonin, 5,5′-dihydroxy-4,4′-bistryptamine