Chemical composition of the gastric shield of a bivalve, Zyrphea crispata, and of the teeth of the gizzard of a gastropod opisthobranch, Aplysia punctata

The chemical composition of the gastric shield of the bivalve Zyrphea crispata is very similar to that of the teeth of the gizzard of the gastropod opisthobranch Aplysia punctata. Both structures are indeed mainly made up of proteins and chitin. Their protein fractions, although heterogeneous, have almost the same amino acid composition. The proportion of free chitin is remarkably high, amounting to 74–78% of the total chitin. These cuticular structures certainly play a similar function in the gastric digestion, according to the presence of some hydrolases mainly laminarinase, the activity of which is much higher than in the crystalline style. Despite their morphological dissimilarity and specialization in two distinct classes of mollusca, the gastric shield and the teeth of the gizzard thus appear to be homologous structures on the basis of their chemical composition.