Bivalves hold an important role in marine aquaculture and the identification of growth-related genes in bivalves could contribute to a better understanding of the mechanism governing their growth, which may benefit high-yielding bivalve breeding. Somatostatin receptor (SSTR) is a conserved negative regulator of growth in vertebrates. Although SSTR genes have been identified in invertebrates, their involvement in growth regulation remains unclear. Here, we identified seven SSTRs (PySSTRs) in the Yesso scallop, Patinopecten yessoensis, which is an economically important bivalve cultured in East Asia. Among the three PySSTRs (PySSTR-1, -2, and -3) expressed in adult tissues, PySSTR-1 showed significantly lower expression in fast-growing scallops than in slow-growing scallops. Then, the function of this gene in growth regulation was evaluated in dwarf surf clams (Mulinia lateralis), a potential model bivalve cultured in the lab, via RNA interference (RNAi) through feeding the clams Escherichia coli containing plasmids expressing double-stranded RNAs (dsRNAs) targeting MlSSTR-1. Suppressing the expression of MlSSTR-1, the homolog of PySSTR-1 in M. lateralis, resulted in a significant increase in shell length, shell width, shell height, soft tissue weight, and muscle weight by 20%, 22%, 20%, 79%, and 92%, respectively. A transcriptome analysis indicated that the up-regulated genes after MlSSTR-1 expression inhibition were significantly enriched in the fat digestion and absorption pathway and the insulin pathway. In summary, we systemically identified the SSTR genes in P. yessoensis and revealed the growth-inhibitory role of SSTR-1 in bivalves. This study indicates the conserved function of somatostatin signaling in growth regulation, and ingesting dsRNA-expressing bacteria is a useful way to verify gene function in bivalves. SSTR-1 is a candidate target for gene editing in bivalves to promote growth and could be used in the breeding of fast-growing bivalves.

In terrestrial animals, the somatotropic axis, comprising growth hormone (GH) and insulin-like growth factors (IGFs), is pivotal in regulating growth and development. Marine mollusks play a vital role in the aquaculture industry, and understanding the molecular mechanisms of mollusk growth is of great value to breeding fast-growing and high-yielding varieties. Unlike terrestrial animals, marine mollusks lack a model species for laboratory breeding, leaving many growth-related genes unvalidated. The dwarf surf clam Mulinia lateralis, with its small size, short breeding cycle, and ease of cultivation in laboratory settings, serves as an ideal model for investigating growth regulation. This study is the first systematic identification of genes related to M. lateralis growth, with 195 differentially expressed genes (DEGs) being found between fast- and slow-growing individuals through transcriptome comparison. KEGG analysis revealed significant enrichment of the insulin-like signaling pathway, and the insulin-like peptide (ILP) was the most significantly upregulated. As the insulin signaling pathway is activated by ligand–receptor binding, we further characterized and functionally validated mlILP and its receptor, the insulin receptor-related receptor (mlIRR). RNA interference (RNAi)-mediated knockdown of mlILP or mlIRR resulted in growth retardation, confirming their positive roles in growth regulation. Notably, silencing of these two genes caused significant upregulation of downstream genes, suggesting a compensatory mechanism for maintaining cell homeostasis. Our findings advance the understanding of growth regulation in mollusks and provide candidate genes for scallop breeding aiming at growth improvement.