Eyes, eye stalks and the visual world of semi-terrestrial crabs

Abstract

1. We used in vivo optical techniques to study compound eye organisation and the extent of eye stalk development in 17 species of semi-terrestrial crabs (Brachyura). 2. ‘Narrow-fronted’ species (Ocypodidae, Mictyridae) which have their eyes close together on elongated, vertically oriented eye stalks have a narrow acute zone for vertical resolving power along the horizon (Figs. 2, 4, 6), while ‘broad-fronted’ species (Grapsidae, Xanthidae, Portunidae) which have their eyes far apart on short eye stalks lack this specialisation (Figs. 3, 4, 6). 3. There is no pronounced acute zone for horizontal resolving power in either ‘type’ of crab (Fig. 5). 4. A quantitative comparison of different species shows that acute zones for vertical resolving power are closely associated with long eye stalks (Fig. 6). 5. In contrast to ‘broad-fronted’ species, ‘narrow-fronted’ species are known to inhabit relatively flat terrains. We show that elongated eye stalks and acute zones for vertical resolving power are specific adaptations to the problems of spatial vision in a flat environment and could enable animals to gain depth and size information monocularily from retinal position and retinal size alone (Figs. 8, 9). 6. We discuss the preconditions of spatial vision in differently structured environments and how they might have shaped the evolution of different designs of visual systems in semi-terrestrial crabs.