Abstract
The dorsal shell surface of the New Zealand-endemic chitonid Onithochiton neglectus exhibits ocelli (eyes) associated with sensory organs (aesthetes) penetrating the shell. Each ocellus includes a mineral lens which focusses light onto a retina, potentially able to resolve images. In order to discover the spatial resolution of lenses of O. neglectus, we isolated them, examined their mineralogy with X-ray diffractometry, imaged the constituent crystals with scanning electron microscopy, and mounted them onto a camera to directly observe and record images. Lenses of O. neglectus are aragonitic and therefore birefringent, providing two different focal lengths, possibly one for in-air and one for underwater vision. Chitons may be able to image nearby shapes with visual acuity equivalent to 6/24 (m), a VA(dec) of 0.25. Onithochiton neglectus lenses can focus images in air about as well as a person with low visual acuity, sufficient to discern shapes and shadows in the surrounding environment. Lenses from subtidal O. neglectus were less eroded than those from intertidal environments; chitons in different habitats exhibit different visual acuity. It is unclear why a few lineages of chitons have developed such complex eyes; we speculate that body-orientation and navigation could be among the advantages driving such development.