Secondary metabolites from the seaweed Gracilaria lemaneiformis and their allelopathic effects on Skeletonema costatum

► Six secondary metabolites were isolated from the seaweed Gracilaria lemaneiformis. ► Their structures were determined and 8-hydroxy-4E,6E-octadien-3-one is new. ► Their allelopathic effects on Skeletonema costatum were measured and two are active.