The discovery of new natural products frequently triggers a breakthrough in basic sciences. This review features marine iminium alkaloids, which control biologically and physiologically intriguing phenomena. Pinnatoxins, potent shellfish poisons purified from the Okinawan bivalve Pinna muricata, activate Ca2+ channels. Norzoanthamine hydrochloride, isolated from the colonial zoanthid Zoanthus sp., suppresses decreases in bone weight and strength in ovariectomized mice. Symbioiminc, an amphoteric iminium metabolite from the dinoflagellate Symbiodinium sp., inhibits osteoclast differentiation. The latter two compounds are good candidates for anti osteoporotic drugs.