Retinal photoreceptors of two subterranean tuco‐tuco species (Rodentia, Ctenomys): Morphology, topography, and spectral sensitivity

Abstract

Traditionally, vision was thought to be useless for animals living in dark underground habitats, but recent studies in a range of subterranean rodent species have shown a large diversity of eye features, from small subcutaneous eyes to normal-sized functional eyes. We analyzed the retinal photoreceptors in the subterranean hystricomorph rodents Ctenomys talarum and Ctenomys magellanicus to elucidate whether adaptation was to their near-lightless burrows or rather to their occasional diurnal surface activity. Both species had normally developed eyes. Overall photoreceptor densities were comparatively low (95,000-150,000/mm(2) in C. magellanicus, 110,000-200,000/mm(2) in C. talarum), and cone proportions were rather high (10-31% and 14-31%, respectively). The majority of cones expressed the middle-to-longwave-sensitive (L) opsin, and a 6-16% minority expressed the shortwave-sensitive (S) opsin. In both species the densities of L and S cones were higher in ventral than in dorsal retina. In both species the tuning-relevant amino acids of the S opsin indicate sensitivity in the near UV rather than the blue/violet range. Photopic spectral electroretinograms were recorded. Unexpectedly, their sensitivity profiles were best fitted by the linear summation of three visual pigment templates with lambda(max) at 370 nm (S pigment, UV), at 510 nm (L pigment), and at 450 nm (an as-yet unexplained mechanism). Avoiding predators and selecting food during the brief aboveground excursions may have exerted pressure to retain robust cone-based vision in Ctenomys. UV tuning of the S cone pigment is shared with a number of other hystricomorphs.

Rhythmic 24 h Variation of Core Body Temperature and Locomotor Activity in a Subterranean Rodent (Ctenomys aff. knighti), the Tuco-Tuco

“…These results reinforce the statement that C. aff knighti displays much less polymorphism in rhythmic patterns than other subterranean rodents [21]. Some morphological features, such as the size of the eye and structure of the retina, also distinguish ctenomyids from other subterranean rodents [52]. Remarkably, tuco-tucos forage aboveground [10], [11] more frequently than other strictly subterranean rodents [53], [47] and it has been shown that these surface excursions ensure photic entrainment of their circadian oscillators [12].…”

Octodon degus (Molina 1782): A Model in Comparative Biology and Biomedicine

“…In the majority of rodent taxa, S-cones have blue sensitivity, depending on the amino acids present at a few key positions. Degus’ UV cones were confirmed by electroretinography (ERG) (Chavez et al 2003), behavior, immunocytochemistry (Jacobs et al 2003), and partial opsin sequences (Schleich et al 2010). Why some rodents have UV versus violet-sensitive cones is not known, but it is speculated to be useful for sexual selection, navigation, foraging, object, and urine detection (Chavez et al 2003; Goldsmith 1994).…”

The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals

“…Many species, for example, possess visual pigments with λ max below 400 nm, and the resultant UV-sensitivity is relatively widespread among invertebrates [6–8], birds, fish, reptiles and amphibians [9,10]. Among mammals, such UV-sensitive visual pigments are relatively rare and have only been described in some rodents [11–18], a mole [19], several marsupials [20–23] and some bats [24–27]. Such animals have lenses that, unlike those of humans, transmit short wavelengths well.…”