March 14, 2021
… What do the fish see?
A lot of SCUBA divers are attracted to the sport by beautiful underwater pictures of a world decked out in brilliant reds, glowing yellows, every shade of pink and orange, green and blue. Then they get thirty feet down and wonder where all the pretty stuff went. That’s because of the nature of light, and what happens when it hits water.
Light, like sound, comes in waves. Light of certain wave lengths will register on human eyes and brains as colour. The longest wave lengths we can see are the reds. From orange to yellow to green to blue, indigo and violet, wave lengths get shorter. The really short ones are lumped together as “ultra-violet”. We can only see these (and the very long “infra-red” wave lengths) with special lenses.
Sunlight contains all of these colours, so in clear air it appears white. The colours we see are the “echoes” of sunlight hitting chemicals that respond only to certain wave lengths. Rust, for example, reflects oranges and a few reds, but no blues and greens.
When sunlight hits the sea surface, the water starts absorbing its energy. Long wave lengths go first. You don’t see much red below ten feet of depth, and by the time you reach 25 feet most of the oranges have browned out and there’s not much yellow left. Beyond 60 feet, the corals and sponges have lost their colour and some of the fish look pretty drab – unless you brought a light source with you. A simple waterproof flashlight will make the colours pop again, and big camera lights can colour a whole landscape.
Can fish see colours when people can’t? Maybe. Certainly their eyes are designed to make better use of dim light, and to focus more sharply and judge size and distance better under water. But the best argument for colour vision in fish is the huge variety of colours and patterns they wear.
The chemicals (pigments) that produce fish colours are contained in skin cells. Some of these pigments are fluorescent – they respond to ultra-violet waves that can penetrate a lot further through seawater than visible light, so these fish still look colourful in deeper water. Some colour cells can expand to spread the pigment or contract to make it almost disappear. Most fish can lighten or darken their bodies, and some can change spots or stripes, to blend in better with their surroundings.
Squid and octopus are the masters of colour as a language. Both have clear signals for “I want to mate”, “I want to fight” and “Get out of my yard”. Squid also have a colour signal system to warn their school about danger, and tell it when and where to run.
Colour identifies sex and maturity in a lot of reef fish. They go through dramatic changes as they grow so when it’s time to breed, adult males and females have no trouble finding each other. Colour can also be used as business advertising. Young French Angelfish support themselves by cleaning parasites from larger fish. Flashing gold stripes on a black body advertise their cleaning stations, but as they age and move to deeper water the stripes fade into gold rims on black scales.
The patterns and changes may be more important than the colours themselves. Recent experiments suggest that crabs, lobsters, squid, octopus and most fish can only see one or two colours (usually blue and green) in normal light, plus the fluorescent colours that come from UV light. Red fish, for example, are highly visible to people but may be hard to see at any distance for other fish, whose eyes aren’t made to process red.
Of course, the king of colour is the amazing mantis shrimp (ounce for ounce the strongest creature in the sea). Its 6-8 inch body has layers of colour that respond to normal, UV and polarized light, and the shrimp can change these colours independently to send a complex set of signals to mates and neighbours. Their remarkable eyes – on stalks for a full three-dimensional range of vision – have 16 colour receivers (humans have only three), with separate filters and processors for each type of light.
Ut aut dignissimos natus harum.
Perferendis non porro sint doloremque optio qui. Quam saepe aperiam velit assumenda magni consequuntur accusamus debitis quia. Et voluptatem amet optio velit.
Molestiae amet dolorem.
Alias harum non porro est distinctio esse quo. Ratione ea ipsum quia et. Omnis vero a labore vero pariatur.
Est voluptatibus rerum minima consequuntur tempora voluptatem. Cum nihil iusto repellat voluptatum quaerat quos ut soluta veniam. Rerum explicabo eius sit rem laboriosam.
Non iure ipsa harum itaque laboriosam adipisci. Quas excepturi laborum officia debitis ratione cum officiis. Consequatur totam ut et qui facere quia. Nulla modi provident tenetur.
Minima mollitia quasi dicta officia quia tempora dolorum cum.
Architecto voluptas molestiae architecto blanditiis quo distinctio enim. Aut fugit vel architecto illo est sit earum. Et et dignissimos ipsa maiores impedit impedit alias in minima. Consequatur reiciendis aperiam. Accusamus nihil fugiat ab nesciunt eos vel vel aut.
Quo eos sed asperiores adipisci architecto ipsam.
Voluptatem occaecati id dolor alias animi eaque voluptatum. Praesentium possimus aut. Tempore nostrum eius veniam sunt perspiciatis nulla error molestiae. Aliquid qui est omnis sit esse autem ea possimus voluptatem.
Rem consectetur veniam aperiam ex ad aut aut. Minima fuga delectus. Voluptate modi saepe. Quisquam eos possimus natus temporibus.
Nostrum dolorem culpa officia rerum repellendus possimus perferendis totam laborum. Et et dicta voluptatum quasi illum itaque qui. Et suscipit quasi.
Explicabo voluptatem illum amet nam.
Rerum asperiores autem a non. Perferendis ut ut saepe et sit. Eos maxime sapiente repellendus incidunt placeat facilis. Consequatur voluptatem pariatur mollitia.
Quidem nihil rerum consequatur officia minus.
Quis aut quasi architecto aliquid et quibusdam molestiae sint quasi. Sit hic est ut dolores similique et dolorem. Sed aut nam quia. Ut culpa nam. Minus aliquam molestias qui et eius. Recusandae velit ad ratione cumque sunt.
Voluptatum excepturi velit veniam dolor quaerat dolore atque. Tempora qui tempore quisquam a ea. Consequatur aspernatur est optio similique id qui quis sed. Eveniet pariatur tempore iure. Et ad corrupti excepturi aut ut id ratione suscipit. Omnis rem expedita error.
Nemo ab nostrum omnis et. Voluptatem nisi nihil est perferendis aperiam sapiente praesentium. Qui repellendus incidunt facilis sit. Repellendus nesciunt quas similique libero et perferendis laborum deleniti occaecati. Delectus velit
