A bioassay-guided fractionation of the antimicrobial crude extract obtained from the marine sponges Monanchora clathrata and Monanchora unguiculata led to the isolation and identification of the new guanidine alkaloid crambescidic acid-671 (1), along with the known alkaloids: crambescidin 826 (2), crambescidin 800 (3), for-miamycalin (4), and crambescidin 359 (5). Their chemical structures were elucidated using 1D and 2D NMR, and HRESIMS spectroscopic data. Compounds 1-4 were isolated from Monanchora clathrata while, compounds 2-5 were isolated from Monanchora unguiculata. Compounds 1-4 showed potent antifungal activity against three Cryptococcus strains with IC50 values of 0.03 mu g/mL to 0.90 mu g/mL, MIC values of 0.16 mu g/mL to 1.00 mu g/mL, and MFC values of 0.16 mu g/mL to 2.00 mu g/mL, comparable to the positive control Amphotericin B. Moreover, compounds 1-4 exhibited promising antibacterial activity against MRSa, E. coli, P. aeruginosa and K. pneumoniae. The structural similarity between compounds 1, 2 and Griseofulvin (common tubulin inhibitor antifungal and antibacterial compound) drove us to study the common pharmacophoric features between them and the ability of our compounds to inhibit the tubulin enzyme.