The Effects of Different Nutrient Supplements on the Growth and Body Composition of Juvenile Manila clams Ruditapes philippinarum

Shellfish inhabiting mudflats constitute an important component of the mariculture industry in China, with an annual output accounting for approximately one-third of shellfish breeding output. The Manila clam Ruditapes philippinarum, one of the main species of mudflat mollusks cultivated in China, is characterized by a short cultivation cycle and strong adaptability, the culture of which requires low investment and is highly profitable. Moreover, it is a suitable species for artificial high-density cultivation, with an annual output exceeding 3 million tons, accounting for more than 90% of the world's cultured production. R. philippinarum is one of the four major mollusk species traditionally cultured in China that is highly valued for its delicate taste and rich nutrition, and is particularly popular among the general public. The current domestic Manila clam aquaculture industry is based on a mainstream pattern of southern (Fujian Province) breeding and northern (Liaoning and Shandong Province) rearing of seedlings. During the entire Manila clam cultivation process, the intermediate stage of cultivation (cultivating juveniles to suitable sizes for bottom-seeding cultivation) is an important link connecting the two stages of factory-based seed production and bottom seedling cultivation. With the annually increasing market demand for Manila clams, the scale of cultivation scale has also expanded, making the intermediate cultivation stage particularly important. Currently, the southern intermediary stage of Manila clam juvenile cultivation is mainly carried out in natural marine areas, whereas the intermediate cultivation of northern Manila clam is mainly conducted in ponds. Although pond cultivation is conducive to artificial control and management, with juveniles being less affected by predators and more efficiently harvested, in northern regions this type of cultivation has the disadvantages of low temperatures in winter and insufficient food supplies. Consequently, the intermediate cultivation of juvenile shellfish is typically one of the key factors currently constraining the further development of the industry. Studies in this regard have shown that the accumulation of nutrients in juvenile shellfish plays a decisive role in the survival of juveniles for enhanced breeding success, and this requirement is accordingly gaining increasing attention from breeding enterprises and researchers, given that ensuring a sufficient food supply during the intermediate stage of juvenile cultivation is a key factor contributing to the success of the entire cultivation process. In this study, to address the problems of feed shortages or nutritional deficiencies during Manila clam cultivation, we sought to identify effective or alternative feed supplements by assessing the effects of the addition of four nutritionally rich and readily obtainable feed sources, namely, spirulina powder, yeast, plasma protein powder, and soybean peptides. Soybean peptides represent a high-protein source that is obtained by harnessing biological enzymatic technology to degrade large soybean protein molecules to smaller molecular fragments. This process yields a rich array of amino acids, thereby making soybean peptides not only a concentrated source of protein but also a potentially beneficial component for enhancing the growth and immune function of mollusks in aquaculture. Plasma is the liquid fraction of blood that contains a diverse range of proteins, minerals, hormones, and spectrum of essential nutrients. For the purposes of aquaculture, it can be converted to a plasma protein powder and used as a protein supplement, providing a source of essential nutrients for mollusks. Yeast is a nutrient-rich source of microbial proteins, B-vitamins, minerals, and dietary fiber, which can be used as a nutritional supplement that may contribute to enhancing the nutritional value and growth performance of young Manila clams. Spirulina is a high-quality plant-based source of nutrients that is rich in proteins, vitamins, minerals, and essential amino acids, which may have a significant influence on the fat content of Manila clams and also contributes to enhancing immunity. To investigate the effects of soybean peptide, plasma protein powder, and yeast and spirulina powder on the growth, survival, and nutrient accumulation of R. philippinarum juveniles, in this study, we adopted a physiological ecology approach. Specifically, we compared two substrate conditions (with and without sand) for each of the following four supplemental nutrient groups: H1 (soybean peptide), H2 (plasma protein powder), H3 (yeast), and H4 (spirulina powder). As a control group, shellfish were fed solely on microalgae, without nutrient supplementation. The results revealed that among the assessed feed supplements, the survival of juveniles cultivated on a sandy substrate and receiving yeast supplementation was notably higher at (79.60±0.86)%, which was significantly higher than that of juveniles reared in the absence of a sandy substrate (P＜0.05). On the tenth day of observations, a maximum shell length specific growth rate of (1.09±0.09)%/d was recorded in the H4 group cultivated with sand, whereas a maximum wet weight specific growth rate of (3.11±0.62)%/d was recorded for H3 group clams cultivated with sand. And the effect of substratum on growth rate was found to be significant (P＜0.05). Furthermore, the glycogen content of H4 group R. philippinarum juveniles cultivated with sand (79.03±18.60 mg/g) was significantly higher than that in all other groups (P＜0.05), with this difference between the groups with and without sand also being statistically significant (P＜0.05). The carbon content of juveniles in the H3 group with sand (45.23%±0.33%) was significantly higher than that observed in the other groups (P＜0.05), whereas the nitrogen content of juveniles in the H4 group with sand (12.25%±0.22%) was significantly elevated compared with that detected in the other groups (P＜0.05). However, whereas with respect to carbon content, we detected no significant differences between the groups cultivated with and without a sand substratum (P＞0.05), the presence of a substratum did have a significant effect on nitrogen content (P＜0.05). In conclusion, our findings in this study provide evidence to indicate that nutrient supplementation can contribute to a significant alteration in the nutrient content of R. philippinarum juveniles, and that these effects are partially influenced by the presence of sand. The valuable insights gained in this study will contribute to developing systems for the efficient cultivation of healthy R. philippinarum juveniles.