Goldfish and crucian carp are natural models of anoxia tolerance in the retina.

Vertebrates need oxygen to survive. The central nervous system has an especially high energy demand, so brain and retinal neurons quickly die in anoxia. But fish of the genus Carassius are exceptionally anoxia-tolerant: the crucian carp (C. carassius) can survive months without oxygen in ice-covered ponds, and the common goldfish (C. auratus) can withstand hours of anoxia at room temperature. These fish previously offered insights into anoxia tolerance in the brain, heart, and liver. Here, we advance Carassius spp. as models to study anoxia tolerance in the retina. Electroretinogram and evoked potential recordings show that crucian carp reversibly downregulate their visual systems in anoxia, probably to save ATP. Notably, Carassius suppress their visual systems nearly twice as much as anoxia-tolerant turtles, Trachemys and Chrysemys spp., which are often promoted as the champions of anoxia tolerance. We summarize what is known about anoxia tolerance in the goldfish and crucian carp retinas, including cellular pathways which may protect retinal neurons from excitotoxic cell death. We compare the Carassius retina with two relevant models: natural anoxia tolerance in the turtle brain, and ischemic preconditioning in the rat retina. All three models include mitochondria as oxygen sensors: mitochondria depolarize due to mitochondrial ATP-dependent K ⁺ channels, possibly to trigger neuroprotective second messenger cascades. The Carassius retina is an accessible and inexpensive model, with over 70 fruitful years of history in vision research. As a model for anoxia tolerance, it may provide new insights into diseases of the eye (like diabetes, macular degeneration, and eye stroke).
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