Characterization of the shell proteins in two freshwater snails Pomacea canaliculata and Cipangopaludina chinensis

Uncovering the molecular mechanism of shell formation not only reveals the evolution of molluscs but also lay a foundation for shell-inspired biomaterial synthesis. Shell proteins are the key macromolecules of the organic matrices that guide the calcium carbonate deposition during shell mineralization and have thus been intensively studied. However, previous studies on shell biomineralization have mainly focused on marine species. In this study, we compared the microstructure and shell proteins in the apple snail Pomacea canaliculata which is an alien species that has invaded Asia, and a freshwater snail Cipangopaludina chinensis which is native to China. The results showed that although the shell microstructures were similar in these two snails, the shell matrix in C. chinensis contained more polysaccharides. Moreover, the compositions of shell proteins were quite different. While the shared 12 shell proteins (including PcSP6/CcSP9, Calmodulin-A, and proline-rich protein) were supposed to play key roles in shell formation, the differential proteins were mainly immune components. The presence of chitin in both shell matrices and the chitin-binding domains containing PcSP6/CcSP9 underpinned the relevance of chitin as a major fraction in gastropods. Interestingly, carbonic anhydrase was absent in both snail shells, suggesting that freshwater gastropods might have unique pathways to regulate the calcification process. Our study suggested that shell mineralization might be very different in freshwater and marine molluscs, and therefore, the field should pay more attention to the freshwater species to achieve a more comprehensive insight into biomineralization.