Gonad traits in the sea urchin Mesocentrotus nudus fed on the kelp Eisenia bicyclis: Gametogenesis promotion and lack of sea urchin-like taste

Perennial Eisenia bicyclis kelp beds are one of the main fishing grounds for the sea urchin Mesocentrotus nudus, whose gonad is highly valued. However, little is known about the gonad quality of sea urchins fed on this species of kelp. Feeding with two other kelp species, the biennial Saccharina japonica blade or sporophylls of the annual Undaria pinnatifida, is known to improve the gonad quality of M. nudus. The present study investigated the dietary effects of blades of E. bicyclis on gonad quality of M. nudus in comparison with feeding on S. japonica blade or U. pinnatifida sporophylls. The gonad index of urchins fed E. bicyclis (UEB) increased from 8.2 to 16.9, similar to those fed on S. japonica (USJ), which was significantly lower than those fed on sporophylls of U. pinnatifida (UUP). Of UEB, 69% had premature gonads, indicating that UEB gonads were significantly advanced to maturation compared with those of USJ or UUP, most of which were still in the growing stage. The uncoupling of gonad size and advance to maturation is not explained by the high protein content in E. byciclis, suggesting that unknown substances in E. bicyclis can promote gonad maturation. UEB gonads improved color, texture and contents of glutamic acid, alanine, and arginine similar to results obtained with USJ. However, the methionine and valine contents of UEB gonads were below the threshold of detection, explaining the lack of sea urchin-like taste of the gonads. Many unpleasant odors were detected from the gonads of UEB. In conclusion, gonad flavor of UEB is inferior compared with that of USJ or UUP. Further research is required to understand the details of gametogenesis under dietary supplementation with the species-specific compounds present in E. bicyclis.