Comparative optics of prosobranch eyes

Sammanfattning: Functional aspects of the large diversity of eye design in prosobranch molluscs was investigated using morphological and optical methods. Three species from different habitats were investigated. Littorina littorea from Scandinavian intertidal rock habitats, Strombus raninus and S. gigas from tropical sand-flats and Ampularia sp. from heavily vegetated habitats in tropical fresh-water. Littorina and Strombus possess eyes which are among the most highly developed in gastropods: the eye of Strombus being one of the the largest and most complex. The eyes have well developed, near-spherical lenses of short focal length, and minimal spherical aberration. The large vitreous bodies allow for a focused image on the retina. The retinal array contains numerous photoreceptor cells with microvillar rhabdoms which are not optically isolated from each other (unshielded). The eye of Ampularia deviates from that of Strombus by having practically no vitreous body and an ovoid lens which displays severe aberrations. As in Littorina and Strombus the rhabdoms are not optically isolated from each other, but have a somewhat larger diameter. The consequence of unshielded rhabdoms is a lateral spread of light in the retina, which blurs the image and degrades spatial vision. The effect of lateral light-spread is significant in Strombus because of the unusually long and slender rhabdoms, but probably insignificant in the other two species. In Strombus there is thus an apparent mismatch between useful resolution and packing density of the retinal receptors. There is a similar but worse mismatch in Ampularia, because of lens aberrations. In Littorina, the match between optics and retina seems to be reasonable. To investigate possible reasons for the different degrees of correlation between optics and retina, a morphological study of the three species was made to compare the number of retinal receptors to the number of axons in the optic nerve. Fewer axons than receptors would imply convergence (summation) of signals, which would increase the sensitivity of the eye at the cost of resolution, by pooling neighbouring receptors into larger functional units. Strong convergence was found in Ampularia, less in Strombus, and none in Littorina, which one woud expect from the optics.