Biomineralization,especially shell formation,is a sophisticated process regulated by various matrix proteins.Pinctada fucata chitinase-like protein 1 (Pf-Clp1),which belongs to the GH18 family,was discovered by our group using in-depth proteomic analysis.However,its function is still unclear.In this study,we first obtained the full-length cDNA sequence of Pf-Clp1 by RACE.Realtime polymerase chain reaction results revealed that Pf-Clp1 was highly expressed in the important biomineralization tissues,the mantle edge and the mantle pallial.We expressed and purified recombinant protein rPf-Clp1 in vitro to investigate the function of Pf-Clp1 on CaCO3 crystallization.Scanning electron microscopy imaging and Raman spectroscopy revealed that rPf-Clp1 was able to affect the morphologies of calcite crystal in vitro.Shell notching experiments suggested that Pf-Clp1 might function as a negative regulator during shell formation in vivo.Knockdown of Pf-Clp1 by RNAi led to the overgrowth of aragonite tablets,further confirming its potential negative regulation on biomineralization,especially in the nacreous layer.Our work revealed the potential function of molluscan Clp in shell biomineralization for the first time and unveiled some new understandings toward the molecular mechanism of shell formation.