Kaleidoscope Eyes: Microstructure and Optical Performance of Chiton Ocelli

TL;DR

This study investigates the microstructure and optical performance of chiton Acanthopleura granulata lenses, revealing how their polycrystalline aragonite microstructure affects focusing ability in different environments.

Contribution

It provides new insights into the microstructural features of chiton lenses and their impact on optical performance across wet and dry conditions.

Findings

Polycrystalline aragonite lenses contain curved grain boundaries.

Twinning reduces birefringence-induced aberrations.

Variations in grain orientation cause differences in focal length and image quality.

Abstract

The chiton Acanthopleura granulata uses aragonitic lenses embedded in its shell to focus light onto photoreceptors. Because aragonite is biaxially birefringent, the microstructure of the lens greatly impacts the optical performance. In addition, the chiton lives in the intertidal, so lenses experience two environments with different refractive indices: air and water. Using EBSD, we find that the lens is polycrystalline and contains curved grain boundaries. A combination of large, twinned grains and nanotwins ensure that the aragonitic <001> axis is consistent across the lens. However, the orientation of the <001> axis relative to the shell varies between lenses. Ray tracing simulations predict the optical performance of lenses of various microstructures in wet and dry environments. Though twinning helps to limit birefringence-induced aberrations, variations in the orientation of the <001> axis between lenses lead to variations in focal lengths between lenses and cause image doubling in some lenses. As such, the birefringence of aragonite does not help the lens to transmit focused images in both air and water.