Ishigadine A, a new canthin-6-one alkaloid from an Okinawan marine sponge Hyrtios sp.

•A new indole alkaloid was isolated from an Okinawan marine sponge Hyrtios sp.•The alkaloid is the third alkaloid with the canthin-6-one skeleton from sponges.•The alkaloid might be generated from l-arginine, l-histidine, and l-tryptophan.•The alkaloid exhibited moderate cytotoxicity against L1210 murine leukemia cells.