… 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 that we can’t see are lumped together as “ultra-violet”.

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 a recognition signal. 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.