Dietary protein requirement for somatic growth and gonad production in the sea urchin Strongylocentrotus intermedius at different life stages

Abstract

The rise in the demand for sea urchin aquaculture requires the optimization of dietary protein levels to maximize somatic growth and gonad production throughout the aquaculture process. This study aimed to investigate the dietary protein requirement for somatic growth and gonad production of the sea urchin Strongylocentrotus intermedius at different life stages considering protein leaching from diets during seawater immersion. Feeding trials were performed on three sizes of S. intermedius: S, small urchins prior to gonad differentiation; M, medium urchins following gonad differentiation; and L, large urchins close to the biological minimum size. Two feeding trials using diets with different gluten (protein source) levels (experiment I; 5, 10, 15, 20, 30, and 40%: experiment II; 0, 2, 4, 7, and 10%) were conducted for each size group. The diet ingredients were selected to minimize protein leaching, and the protein content of each diet was determined from the average content after immersion in seawater for 24 and 72 h. No significant differences were observed in the specific growth rate (SGR) of body weight in experiment I in all size groups. The SGRs of the body weight of urchins fed 4 and 7% gluten diets were significantly higher than those fed diets containing 0% gluten in experiments SII and MII, and a similar tendency was observed for SGRs for the body weight and test diameter of large urchins. Gonad indices of sea urchins increased with an increase in dietary protein content in experiments MII and LII, and the gonad indices of urchins fed 15 and 20% gluten diets were significantly higher than those fed 5% gluten diet in experiments MI and LI. Broken-line regression analyses on urchins in experiment II estimated the protein requirement for somatic growth to be 2% regardless of the urchin size, which is well below the range of feed protein contents (9–50%) used in previous studies. The analyses on urchins in experiment I estimated the protein requirement for gonad production of medium and large urchins to be 13% and 15%, respectively, which was lower than that previously reported (>20%). These results indicate that previous studies overestimated the protein requirement owing to protein leaching during the immersion of feed in seawater. Our findings indicate that protein content in sea urchin feed diets can be considerably reduced irrespective of their cultivation stage from seed to market size.