A Chiton Uses Aragonite Lenses to Form Images

Abstract

Hundreds of ocelli are embedded in the dorsal shell plates of certain chitons. These ocelli each contain a pigment layer, retina, and lens, but it is unknown whether they provide chitons with spatial vision. It is also unclear whether chiton lenses are made from proteins, like nearly all biological lenses, or from some other material. Electron probe X-ray microanalysis and X-ray diffraction revealed that the chiton Acanthopleura granulata has the first aragonite lenses ever discovered. We found that these lenses allow A. granulata's ocelli to function as small camera eyes with an angular resolution of about 9°-12°. Animals responded to the sudden appearance of black, overhead circles with an angular size of 9°, but not to equivalent, uniform decreases in the downwelling irradiance. Our behavioral estimates of angular resolution were consistent with estimates derived from focal length and receptor spacing within the A. granulata eye. Behavioral trials further indicated that A. granulata's eyes provide the same angular resolution in both air and water. We propose that one of the two refractive indices of the birefringent chiton lens places a focused image on the retina in air, whereas the other does so in water.

Multifunctionality of chiton biomineralized armor with an integrated visual system

“…S7). Since aragonite is a pseudo-uniaxial crystal, the non-normal orientation of the c-axes should generate double refraction, which is consistent with observations that the lenses are birefringent when viewed with polarized light (17,18). In addition, EBSD showed that the lenses have an average grain size of roughly 10 µm, which is approximately an order of magnitude greater than that of the surrounding granular microstructure ( fig.…”

“…Unlike the few other eyes known to contain lenses made of calcium carbonate, such as those of trilobites (19,20), the eyes of chitons are integrated within and dispersed across their entire dorsal shell surface instead of being localized to a particular region of the body. Although the calcitic lenses of brittlestars are also dispersed across their dorsal arm plates (4), it is unclear whether or not they enable spatial vision in a fashion similar to the lenses of chitons (18). Here we show that the chiton A. granulata is able to tailor the local geometry, crystallography, and interfaces of aragonite to achieve a multifunctional armor.…”

“…The authors of a prior behavioral study of A. granulata hypothesized that the chiton may be required to use different polarizations of light to see in different tidal environments (18), In other words, only one of the two refractive indices of the birefringent lens could possibly focus light onto the photoreceptive region of the eye chamber in air, while the other does so in seawater. Although all of our simulated focal points fall within the eye chamber, we do not know the exact position and shape of the chiton retina, so we cannot conclude if birefringence is indeed functional.…”

“…In certain species, the aesthetes include hundreds of lens eyes (16,17) that may be able to spatially resolve objects (18). In stark contrast to the protein-based lenses of most animal eyes, the lenses of chitons, like their shells, are principally composed of aragonite (18).…”

“…In certain species, the aesthetes include hundreds of lens eyes (16,17) that may be able to spatially resolve objects (18). In stark contrast to the protein-based lenses of most animal eyes, the lenses of chitons, like their shells, are principally composed of aragonite (18). Unlike the few other eyes known to contain lenses made of calcium carbonate, such as those of trilobites (19,20), the eyes of chitons are integrated within and dispersed across their entire dorsal shell surface instead of being localized to a particular region of the body.…”