Exploring Gradients of Halogens and Zinc in the Surface and Subsurface of Nereis Jaws

The outstanding mechanical properties of impact-bearing tissues, such as Nereis jaws, make their morphology and chemical composition a subject of particular interest. The complex structure of the jaw was recently reported to exhibit molecular gradients that were closely correlated with stiffness and hardness.18 Accordingly, we have explored the spatial distribution and bonding chemistries of Zn and the halogens in the surface structure of the jaws. Using secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM), we found that Cl, Br, and I distributions are enhanced in surface layers of the basal protected portion of the jaw but are shifted to greater depths toward the exposed jaw tip. There are thus two complementary halogen gradients in the jaw: one on the surface that decreases from the base to the tip, coupled to an increasing one in the subsurface layers. The outer surface coating appeared to have granular morphology, in contrast to the anisotropic, fibrous core that dominates the subarchitecture. Using X-ray photoelectron spectroscopy (XPS), we discovered that Zn, I, and Br in the jaws have single chemical environments whereas chlorine is present in two distinct modes (Cl−Zn and Cl−C). Given the inverse relationship between surface exposure and halogen abundance in the jaws, it is unlikely that the halogens contribute directly to mechanical properties such as wear and hardness.