The discovery of a new chemical substance frequently triggers a breakthrough in basic science. Various natural products with extraordinary structures and significant biological activities have been isolated and characterized from marine organisms. These bioactive compounds are candidates for drugs or biological probes for physiological studies. It is still important to search continually for unexpected and unforeseen compounds from nature. To overcome the difficulty of targeted pursuit for such bioactive key compounds, careful observation in the field is extremely important.We describe here the isolation, structure, biological activities, and synthetic studies of two novel marine secondary metabolites : symbioimine and haterumalide. Symbioimine, isolated from the cultured dinoflagellate Symbiodinium sp., inhibited the differentiation of RAW 264 cells into osteoclasts. Thus, symbioimine is an antiresorptive drug candidate for the prevention and treatment of osteoporosis. Haterumalides, 14-membered cytotoxic macrolides from the Okinawan sponge Ireinia sp., shows potent cytotoxicity.