Visually guided eye growth in the squid

Eyes with refractive error have reduced visual acuity and are rarely found in the wild. Vertebrate eyes possess a visually guided emmetropisation process within the retina which detects the sign of defocus, and regulates eye growth to align the retina at the focal plane of the eye's optical components to avoid the development of refractive error, such as myopia, an increasing problem in humans. However, the vertebrate retina is complex, and it is not known which of the many classes of retinal neurons are involved. We investigated whether the camera-type eye of an invertebrate, the squid, displays visually guided emmetropisation, despite squid eyes having a simple photoreceptor-only retina. We exploited inherent longitudinal chromatic aberration (LCA) to create disparate focal lengths within squid eyes. We found that squid raised under orange light had proportionately longer eyes and more myopic refractions than those raised under blue light, and when switched between wavelengths, eye size and refractive status changed appropriately within a few days. This demonstrates that squid eye growth is visually guided, and suggests that the complex retina seen in vertebrates may not be required for emmetropisation.