Formation mechanism of iron–catechol complexes in the colored periostracum of <i>Corbicula</i> spp.

The shell color of Corbicula clams, which are globally distributed, is roughly divided into yellowish and blackish depending on the environmental conditions of the sediment. The formation of an iron-L-3,4-dihydroxyphenylalanine (DOPA) complex in a thin organic layer, called the periostracum, on a calcareous layer causes the blackening of the clamshell. However, the iron-DOPA complex formation mechanism is unclear. To reveal how the iron is transported from the aquatic environment to the periostracum, cross-sectional analyses of the shell were conducted using an electron probe microanalyzer and Raman spectroscopy to investigate the distribution and structure of iron in the shell. Iron was only present in the periostracum, excluding deposition, and all iron was in the form of an iron-DOPA complex. Attenuated total reflection infrared spectroscopy and oxygen K-edge X-ray absorption fine structure spectroscopy revealed that the molecular structure of the native periostracum is independent of shell color. These results indicate that dissolved iron-organic complexes diffuse from the aqueous environment to the periostracum, forming iron-DOPA complex through ligand exchange. Because the iron-DOPA complex color depends on the pH, the shell color can serve as a historical indicator of the shell's growth environment.