Retinal structure and visual acuity in a polyprotodont marsupial, the fat-tailed dunnart (Sminthopsis crassicaudata)

The visual system of the fat-tailed dunnart (Sminthopsis crassicaudata), a small polyprotodont marsupial, has been examined both anatomically and beha viourally. The ganglion cell layer was examined in cresyl-violet stained wh olemounts and found to contain a mean of 81,400 ganglion cells (SD +/- 3,36 0); the identification of ganglion cells was supported by a correspondence to optic axon counts. Ganglion cells were distributed as a mid-temporally s ituated area centralis, embedded in a pronounced visual streak. Localised i mplants of horseradish peroxidase into retinal wholemounts revealed both A- type and B-type horizontal cells. Sections of the outer retina showed it to be rod-dominated, with a rod-to-cone ratio of 40:1 at the area centralis; cones were found to contain oil droplets but double cones were not a promin ent feature. The retinal pigment epithelium consisted of squamous cells, Vi sual acuity, estimated from counts of peak ganglion cell density (8,300/mm( 2), SD +/- 1,180) and measurements of posterior nodal distance (2.9 mm), wa s found to be 2.30 cycles per degree. The value was close to that of 2.36 c ycles per degree estimated by behavioural tests using a Mitchell jumping st and; values were similar at low, intermediate and high light levels. Our fi ndings are discussed in relation to the lifestyle of the dunnart.