Factors causing brown-colored gonads of the sea urchin Strongylocentrotus nudus in northern Honshu, Japan

In July 2001, to clarify the factors causing brown-colored gonads in the sea urchin Strongylocentrotus nudus, we analyzed the body size, age, gonad size, and grade of gonad color of sea urchins in three Eisenia bicyclis beds and two crustose coralline beds in Miyagi Prefecture, Japan. Brown-colored gonads were analyzed by HPLC and the potential causative pigments described. Gonad indices (gonad wet weight / body wet weight × 100) of S. nudus in Eisenia beds were significantly higher than those in crustose coralline habitats. In Eisenia beds, brown-colored gonads were found in sea urchins with a test diameter > 7 cm, which corresponded to age > 7 years, and was associated with a decrease in gonad indices. Some urchins, however, had brown-colored gonads despite their high gonad indices and young ages (< 7 years). In crustose coralline beds, brown colorization was similarly observed in sea urchins > 7 years old and in those < 7 years old with low gonad indices of < 5%. These findings suggest that the brown colorization is correlated to aging and/or gonad size as determined by food availability. HPLC analysis of acetone extracts showed that two peaks at a retention time of around 16 min in brown gonads were more intense than in bright yellow ones. The main causative pigment of brown colorization is likely to be contained in water- and fat-insoluble residues.