Convergence in biomineralization patterns across animal eggshells

Shelled eggs are key components of animal reproduction on land, evolving independently in distant lineages of terrestrial animals including nematodes, gastropods, annelids, arthropods and chordates. They perform critical functions such as the exchange of gases between embryo and the environment, desiccation avoidance and protection from harmful radiation, microbial infection and mechanical damage. A core mechanism behind eggshell multifunctionality is the incorporation of biominerals (mainly calcium carbonate and calcium phosphate) into the shell. Very little is known about eggshell structure in invertebrates, but some recent pioneering studies have proposed that similar mineralization patterns may have evolved convergently in eggshells of pulmonate gastropods, some insects, and vertebrates. However, because a detailed characterization of the structural and chemical composition of invertebrate eggshells is not available, it has not been possible to test this hypothesis. Here, we use computed tomography, electron microscopy, electron backscatter diffraction analyses, atomic force microscopy, spectroscopy, and histochemistry to characterize and compare microstructure and chemical composition of pulmonate gastropod, insect and vertebrate eggshells. These techniques revealed the universal presence of an organic matrix in mineralized eggshells. However, disparities in the distribution of calcium throughout the shell, crystallographic orientation that appears random in invertebrates (but not vertebrates), and presence of different calcium types including the rare and unstable vaterite highlight divergence whose functional significance should be the subject of future study.