Guanine and its retinal distribution in the tapetum of the bigeye tuna, Thunnus obesus

The eye of the bigeye tuna (Thunnus obesus) contains a retinal tapetum composed of guanine. The total amount of the guanine in one eye of the fish (SL=120 cm) was about 88.6 mg. The mean guanine content of the tapetum was approximately 1.25 mg/cm(2) of the retinal surface. The highest content of guanine (2.15 mg/cm(2)) was observed only in the ventro-temporal part of the retina. To distinguish this area from the rest of the eye, we suggested the: term 'locus tapetalis for it. The visual accommodation system clearly indicated that the visual axis of the fish is upper-forward and the resulting retinal area for acute vision was suggested to be in the ventro-temporal retina. We discussed that the area centralis of the bigeye tuna may have two functions: to guarantee high visual acuity and to allow for high photo-sensitivity in dim Light vision.