Crocodiles and alligators have perched at the water's edge waiting for the perfect dining opportunity to pass for millions of years. Explaining how the animals also spend much of their time immersed in water with only their eyes and nostrils protruding above the surface, Nicolas Nagloo from The University of Western Australia adds that this posture provides them with a unique perspective on the world. ‘The water surface makes up the majority of the bottom of the visual field and the visual horizon occurs along the riverbanks where the crocodile sees best’, he says. However, while the visual worlds of saltwater and freshwater crocodiles are essentially identical above the water's surface, the light conditions are significantly different when the animals are submerged: ‘In freshwater habitats, there is an abundance of long wavelength [red] light… In contrast, saltwater habitats transmit a broader range of wavelengths, providing a greater amount of short wavelength [blue] light’, Nagloo explains. Knowing that crocodiles cannot see clearly when submerged but they have a fantastic assortment of adaptations that help them to see and hunt in dim conditions, Nagloo and his colleagues Shaun Collin, Jan Hemmi and Nathan Hart decided to gaze into the eyes of two Australian crocodile species to find out more about their colour vision and visual clarity.
Investigating the structure of the retina at the back of the eyes of juvenile saltwater crocodiles (Crocodylus porosus) and freshwater crocodiles (Crocodylus johnstoni), Nagloo discovered that instead of having a compact fovea (a depression in the retina where there is a high density of photoreceptors that provide a high-resolution view of the world), the foveae of saltwater and freshwater crocodiles are stretched across the back of the eye in line with the horizon. Describing the elongated structure as a foveal streak, Nagloo says, ‘[it] gives greater visual clarity’, adding that the elongated shape of the structure allows both species to keep a close eye on events on the riverbank without moving their heads.
However, when Nagloo investigated the sensitivity of the different photoreceptors in both species’ eyes, he was surprised to find that the crocodiles have relatively sophisticated colour vision, provided by three colour-sensitive cones that are tuned to violet, green and red wavelengths. Also, the sensitivity of the saltwater crocodiles' colour photoreceptors was slightly shifted to shorter (bluer) wavelengths compared with that of the photoreceptors of the freshwater crocodiles, even though neither species can focus underwater, suggesting that they may use their vision underwater more than had been thought.
Considering the subtle differences between the visual systems of the two Australian species, Nagloo says that the crocodiles have finely tuned visual systems that give each species an advantage while in their own environments. And he is now keen to find out how the vision of both species adapts as they grow. ‘We know that eye size has a big effect on visual clarity and we also know that saltwater crocodiles reach much greater maximum body sizes than freshwater crocodiles, so it would be interesting to find out whether the eye of saltwater crocodiles also gets much bigger than the freshwaters’ and what impact that would have on visual clarity,’ he says.
