How do fish see people?

So, you’re a Napoleon wrasse, swimming around a reef in a leisurely manner… when suddenly, you spot a scuba diver paddling towards you with a net. The panicked ocean-dweller tries to escape, without losing sight of its pursuer. But what do fish actually see? A fish’s eyes are located on either side of its head, each boasting a 180-degree field of perception. They have just one blind spot, right behind the tail fin. A fish’s vision varies significantly from species to species. Ornamental fish – such as goldfish – have even better vision than humans. The reason? Their eyes have additional photoreceptors for colour vision, enabling them to see ultraviolet light, which gives them a huge advantage when underwater. Unlike us humans, goldfish can make out the tiniest of details in clear water that’s rich in UV light. Without goggles, we can see barely anything. For a Napoleon wrasse (pictured here), colour vision is particularly important. “These fish communicate using colour,” explains zoologist Guido Westhoff. “For example, their blue-green colouration intensifies during the mating season to make them more noticeable.” The young wrasse in this photograph clearly does not view the diver as a potential mate. Not only is the diver much bigger than the wrasse, he also ‘looks weird’ – and that automatically marks him out as an enemy. According to Westhoff, however, the Napoleon wrasse will lose its fear of divers as soon as it becomes fully grown. “By then it’ll be about two metres long and will no longer see the diver as a threat,” says Westhoff. “On the contrary, Napoleon wrasses are very curious, and are known to approach humans.”

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