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Guanine and its retinal distribution in the tapetum of the bigeye tuna,thunnus obesus
Authors:Hiroaki Somiya  Shiro Takei  Isamu Mitani
Affiliation:(1) Faculty of Bioresources, Mie University, 1515 Kamihama, 514-8507 Tsu, Mie, Japan;(2) Kanagawa Prefectural Fisheries Experimental Station, Jogashima, Miura, 238-0237 Kanagawa, Japan
Abstract: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/cm2 of the retinal surface. The highest content of guanine (2.15 mg/cm2) 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.
Keywords:Bigeye tuna  retinal tapetum  guanine tapetum  guanine distribution  visual axix
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