Versatile fish finetune vision as environment varies Free

Put on goggles, immerse your head under water and the world of colour changes. As you descend, red and violet shades vanish, until only the deepest blues and mauves remain. Mix in floating plankton, stirred up sand, or dissolved tannins and colours become even murkier. This is the ever-shifting visual world inhabited by fish. Remarkably, as the surrounding light shifts, some fish can adjust the colours they perceive by altering the production of light-sensitive proteins – known as opsins – in the retina. ‘We can't adjust to a completely red-only environment by switching photoreceptor types’, says Julián Torres-Dowdall from University of Notre Dame, USA; but fish can. So, how well do fish make these transitions and are there any underlying patterns as they adjust their vision to changes in their environment? Torres-Dowdall and César Bertinetti, also from University of Notre Dame, joined forces to re-examine the wealth of research that has already investigated the colour sensitivity of fish eyes to try to find how fish react to changes in light conditions throughout their lives.

‘César brought [this study] to life’, says Torres-Dowdall, recalling how Bertinetti identified 36 research papers covering almost 40 species investigating how fish's colour sensitivity altered under different circumstances. These included the colour of light that the fish were reared under and hormones that affect their growth and reproduction, when some species migrate to a new environment with completely different light conditions. Then, he began the Herculean task of identifying the factors that had the biggest impact on the light-sensitive proteins in the fish retinas as they fine-tuned the colours they saw most vividly.

Summarising their key discovery, Torres-Dowdall says, ‘Most fish can adjust their vision during their lifetime to cope with changes in water colour’. So, fish vision is incredibly versatile and alters depending on the prevailing conditions. Two light-sensitive proteins – those detecting UV and red wavelengths – were particularly adaptable. Bertinetti explains that as fish grow their diet often changes from UV-reflecting zooplankton to larger creatures that reflect less UV light, so fish lose their ability to see UV colours as they grow older. In contrast, the vision of fish that were given sex hormones, which drive reproduction, shifted to be more sensitive to shades of red, which makes sense, as many fish develop red markings during the mating season; being well attuned to that colour could help them to identify that all-important mate. ‘Fish colour vision is overall very flexible, yet fish prioritize certain colours when trying to deal with specific environmental changes’, says Torres-Dowdall.

The duo also noticed that a fish's vision was more sensitive to its hormones than changes in the environment. But they were surprised that fish that had lived under a particular colour of light all of their lives did not adapt more swiftly than fish that had rarely experienced it before when their surroundings returned to that colour. Also, fish lost their ability to see certain shades when that light colour vanished from the surroundings, so changes were not permanent. ‘Fish are able to switch back and forth throughout their life’, says Torres-Dowdall.

In short, fish can adjust their vision depending on their environment and respond as the predominant wavelengths of light in the water change. And now Torres-Dowdall is keen to find out what factors play a role in the fish's ability to respond to light changes in their environment. ‘How quickly can they adapt? Do they use the same mechanisms? And what advantages do fish have from these adjustments?’ he wonders.