Effects of dietary protein levels on the growth, energy budget, and physiological and immunological performance of green, white and purple color morphs of sea cucumber, Apostichopus japonicus

Abstract

Protein is a necessary nutrient for the sea cucumber, Apostichopus japonicus, an important commercial species in China, South Korea and Japan. In this study, the dietary protein requirements of two newly discovered color morphs—white and purple was investigated and compared with the common, green morph. The green, white and purple morphs were fed a diet with five different protein levels (1, 6, 11, 16 and 21%) for 60days with a mixture of soybean meal and fish meal (1:1) as the protein source, and their growth, energy budget, physiological and immunological performance were measured. Results showed that dietary protein levels had a significant effect on the growth performance of A. japonicus (P<0.05). The maximum specific growth rate (SGR) and proportion of energy allocated for growth occurred at dietary protein levels of 6 and 11%, and the food conversion efficiency (FCE) was highest at the 11% protein level, indicating that the optimum dietary protein level for these color morphs is at ~11%. Apparent digestibility coefficients for dry matter (ADMD) and crude protein (ACPD) increased, but the ingestion rate decreased with increasing dietary protein levels, due perhaps to the decreased sea-mud content of the diets. The energy allocated to excretion, and the free ammonia and malonaldehyde content in coelomic fluid increased with increasing dietary protein levels, which can be explained by enhanced protein metabolism. Trypsin activity decreased significantly when dietary protein was >16%, indicating that the protein content was beyond the digestive ability of sea cucumber, and that excess protein in the diet inhibits protein digestion. Catalase (CAT) and glutamic–oxaloacetic transaminase activity was positively and significantly correlated with SGR, indicating that rapid growth was supported by a robust immune system and effective protein metabolism. At a 1% dietary protein level, the purple color morph had significantly higher SGR, FCE, ACPD and CAT activity, and allocated more energy for growth than the green morph, indicating that the purple morph may be better adapted to a low protein diet than the green morph. The white morph had lower SGR, FCE, superoxide dismutase (SOD) and CAT activity, and allocated less energy to growth, but had a higher excretion energy and higher free ammonia content in coelomic fluid than the green or purple morphs, indicating a difference or physiological defect in protein metabolism in the white morph. Statement of relevanceThe results of this study would be invaluable in improving aquaculture technology and management strategies for green, white, and purple color morphs of Apostichopus japonicus.