Emmetropisation in the Camera-Type Eye of the Squid

Abstract

Aim To establish whether the convergently evolved, camera-type eye of the squid can regulate its eye growth when the focal plane is manipulated by exploiting inherent longitudinal chromatic aberration. Methods Sepioteuthis australis squid were separated into two tanks (orange and blue). Each tank was covered with a spectral filter, which restricted wavelengths to either end of the squid spectral sensitivity, and created a longer focal length in the orange tank compared to the shorter focal length in the blue tank. Measurements of relative eye size (Matthiessen’s ratio, MR) and refraction (with infrared photorefraction) were compared between squid from each tank at 45 days post-hatching. The spectral filters were switched, and the effecting of modifying the focal length assessed at 60 days post-hatching. Visual acuity (in cycles per degree, CPD) was assessed behaviourally in octopus, and estimated from the squid crystalline lens MTF. Results At 45 days post-hatching, squid eyes from the orange spectral environment had a larger MR than those from the blue tank (p = 0.006). Relative to infrared light, the squid eyes from the blue tank were more hyperopic than those from orange (p = 0.004). When the filters were exchanged, both MR and refraction values for the blue and orange spectral environments also reversed. There was no significant change in growth of either the lens or the retina alone. Visual acuity is high in the octopus (12.23 ± 1.61 CPD) and the squid (6.56 ± 1.14 CPD) Conclusions Squid eyes possess an emmetropisation mechanism, and are able to regulate their eye growth by adjusting the relative size of the retina to the size of their lens. Refraction values support the idea that this growth attempts to minimise refractive error. As the squid eye has convergently evolved, this suggests that emmetropisation has evolved at least twice within the Animal Kingdom. As the squid retina is comparatively simple, this may suggest that emmetropisation can function without the complexity of the vertebrate retina.