Pronounced effects of the basal frond portion of the kelp Saccharina japonica on gonad qualities of the sea urchin Mesocentrotus nudus from a barren

The gonad quality of sea urchin (Mesocentrotus nudus) adults from a barren can be improved by feeding the animals fronds of the kelp Saccharina japonica from May–July. The improvement in taste is expected to depend on the amino acid composition of the kelp fronds, which varies among the frond portions. We investigated the effects of feeding apical, middle and basal frond portions of S. japonica on gonad size, hardness, color (L*a*b*) and taste (free amino acids) of M. nudus from a barren from May–July in a rearing experiment in which we analyzed the protein and total and free amino acid content of each frond portion. Feeding of each frond portion improved gonad size, hardness, color and taste. Feeding of the basal portion, which contained high levels of glutamic acid, alanine, and proline, markedly increased the alanine and glutamic acid content and decreased the content of bitter-tasting amino acids in gonads, implying greatly enhanced sweet and umami taste. Based on a comparison of alanine content with that reported in past studies of M. nudus, the gonad taste would be evaluated at a higher level. The basal portion of S. japonica fronds has the high potential to improve gonad taste.