Modest Dietary Protein Requirement for Sea Urchin Gonad Production Demonstrated by Feeding Trials with Consideration of Protein Leaching

This study is aimed at revealing the dietary protein requirement for gonad production in the sea urchin Mesocentrotus nudus, with consideration of protein leaching from diets during seawater immersion. Feeding trials were performed on M. nudus using two diet types with low capacities for protein leaching: starch diets (STG) containing 0–40% gluten in 10%, 5%, and 4% increments (experiments 1–3) and alginate diets (ALG) containing 5–25% gluten in 5% increments (experiment 4). The protein content in the diets was determined after they were immersed in seawater for 24 h or 72 h. The gonad indices of urchins fed STG with 11.4–19.3% protein were significantly higher than those fed STG with 7.8% protein. The protein requirement was estimated to be 12% based on a broken-line regression analysis of urchins fed STG in experiments 1–3. In experiment 4, the gonads became larger as dietary protein content increased from 4.5% to 12.0%, and the sizes were similar to those of urchins fed ALG with 12.0–21.1% protein. The estimated value of 12% was much lower than that reported previously (>20%), indicating that the protein requirement for sea urchin gonad production is modest when the protein-leaching problem is addressed.