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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 <i>SSTR</i> genes have been identified in invertebrates, their involvement in growth regulation remains unclear. Here, we identified seven <i>SSTR</i>s (<i>PySSTR</i>s) in the Yesso scallop, <i>Patinopecten yessoensis</i>, which is an economically important bivalve cultured in East Asia. Among the three <i>PySSTR</i>s (<i>PySSTR-1</i>, <i>-2</i>, and <i>-3</i>) expressed in adult tissues, <i>PySSTR-1</i> 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 (<i>Mulinia lateralis</i>), a potential model bivalve cultured in the lab, via RNA interference (RNAi) through feeding the clams <i>Escherichia coli</i> containing plasmids expressing double-stranded RNAs (dsRNAs) targeting <i>MlSSTR-1</i>. Suppressing the expression of <i>MlSSTR-1</i>, the homolog of <i>PySSTR-1</i> in <i>M. lateralis</i>, 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 <i>MlSSTR-1</i> expression inhibition were significantly enriched in the fat digestion and absorption pathway and the insulin pathway. In summary, we systemically identified the <i>SSTR gene</i>s in <i>P. yessoensis</i> and revealed the growth-inhibitory role of <i>SSTR-1</i> 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. <i>SSTR-1</i> is a candidate target for gene editing in bivalves to promote growth and could be used in the breeding of fast-growing bivalves.