A phlorotanin that is 1,1'-bioxanthrene substituted by 3,5-dihydroxyphenoxy groups at position 6 and 6' and hydroxy groups at positions 2, 2', 4, 4', 7, 7', 9 and 9' respectively. It is isolated from an edible marine brown alga Ecklonia cava and exhibits antioxidant activity.