Structural specializations of the eye in the vizcacha (Lagostomus maximus maximus)

Vizcachas (Lagostomus maximus maximus, Chinchillidae) are nocturnal rodents living in burrows in many regions of Argentina, Bolivia, and Chile. We hav e studied the eye of the vizcacha using several light and electron microsco pic procedures, with the purpose of understanding the role of vision in the behavior of this species. Our observations demonstrated an avascular, rod- rich retina, with a specialized region spanning through most of the equator of the eye. In this central band, all neural retinal layers exhibited a hi gh cell density, whereas the photoreceptor layer was characterized by the p resence of very long rods. In addition, the central region was associated w ith a distinct pigmentation pattern, including scarce granulation of the pi gment epithelium, low pigmentation of the choroid, and the selective attach ment of suprachoroidal cells to the inner scleral surface. These central mo difications probably form the structural basis of a reflecting tapetum. The eye of the vizcacha received both long and short ciliary vessels, and a sp ecialized cilio-sclero-choroidal vascular network appeared at the equatoria l region. Our findings suggest that the equatorial region of the eye of the vizcacha could be a highly sensitive light detector related to foraging be haviors during crepuscular or nocturnal hours. (C) 1999 Wiley-Liss, Inc.