During the course of our survey for vasoconstrictive substances from symbiotic marine microorganisms, we found that a symbiotic alga Symbiodinium sp. (so called zooxanthella) associated with a flatworm produced water soluble vasoconstrictors, named zooxanthellatoxins (ZT). ZT-A isolated as one of the active principles has been determined to be a 62-membered lactone with several characteristic functionalities including a bisepoxide, a sulfate ester, an amide, two conjugated diene, and many allylic alcohols on the basis of its spectral data and chemical degradation experiments. Alkaline hydrolysis of ZT-A afforded a seco-acid and further degraded to give an acid, which was also found in a congener, ZT-B.
