Growing eyes on growing shells reveal how chitons scale vision

Distributed visual systems are composed of repeated photoreceptive units that are often assumed to be functionally and structurally identical. In chitons, shell eyes form a complex array embedded in the dorsal shell plates, with new eyes added throughout life. Using synchrotron microCT, we reconstructed all shell-eye lenses in six specimens representing five chiton species and three major lineages. We tested the relationship between lens volume and position on the shell to establish whether eyes are formed at consistent sizes. All individuals showed significant changes in eye size along the growth axis, as indicated by lens volume increasing allometrically towards the valve margins. Log-transformed mixed-effects models revealed that both lens volume and body size contribute to scaling patterns, but growth trajectories differ among species. Lens diameter can more than double from the oldest to the youngest eye, meaning individual eyes within the same individual likely have at least a fourfold difference in sensitivity. Contrary to the expectation of homogeneity in a distributed array of eyes, we show that chiton visual systems include a broad range of eye sizes within a single individual that change over ontogeny. This gradient has implications for visual functions, redundancy and the potential integration of distributed information across the shell.