The red alga Pyropia yezoensis is traditionally cultivated in Japan, and the surplus production that is not consumed is utilized in other ways. Because this alga contains highly concentrated protein, we investigated the dietary effects of fresh P. yezoensis on the gonadal production and quality of the adult sea urchin Mesocentrotus nudus compared with urchins fed the kelp Saccharina ochotensis, in a 60-day rearing experiment from April to June 2009. The digestibility of P. yezoensis by the sea urchins was 96.2%. The gonads were in the recovery stage at the start and end of feeding. The gonad index of the sea urchins fed P. yezoensis was 20.8, significantly higher than that of the urchins fed S. ochotensis. When the urchins were fed P. yezoensis, the proportion of bright reddish-yellow gonads increased markedly, to 50% in the testes and 90.9% in the ovaries. The content of the sweet amino acid, glycine, in the gonads of urchins fed P. yezoensis was significantly lower than at the start of feeding, and the contents of the bitter amino acids, lysine and valine, were significantly higher in the ovaries than in urchins fed S. ochotensis or at the start of feeding. Despite the increase in gonad production and the improvement in gonad color after the consumption of P. yezoensis, these changes in amino acids indicate a reduction in the quality of the gonad as food. The crude protein content of P. yezoensis used in this study was 36.7%, which would reduce gonadal quality. We infer that the use of low-protein P. yezoensis, which is discolored and unfit for market, can improve the quality of the urchin gonad.
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