Why is marine life so colourful?

05 December 2017

Question

Why is ocean marine life more colorful than in lakes?

Answer

Chris Smith asked Marine Biologist, Kate Feller, to dive into an answer for this question from Jack on Facebook. Astrophysicist, Matt Middleton, then explained how the planets get their colour.

Kate - This has a lot to do with the light environment underneath the water. In a coral reef system, which is probably most famous for being super colourful, the water is what we call ‘blue, blue blue, dead, dead, dead.’ It’s super clear and really blue and gorgeous and that’s why we like to go on vacation there. When you live in a clear water you can be really colourful and send lots of visual signals to your neighbours or to someone you want to mate with, and all that kind of stuff. But in lakes, in particular, and freshwater systems that are very closely tied to the land, you end up with a lot of dissolved organics. There’s a really great word for that, it’s called ‘gelbstoff’ that just means yellow guk or yellow stuff. So when you're in this darker, murkier, light absorbing water, you can’t send a signal as readily, so the fish tend to be a little bit browner.

Chris - They don’t bother?

Kate - It’s more like they evolve to fit in the environment that they live in.

Chris - If you’re not using bright colours, how do they find each other then?

Kate - A lot of chemical signals or sound vibrations and a lot of other sensory things. But with that, I must make you aware of the freshwater darter, which is one of the most beautiful, colourful types of fish I have ever seen in my life. They’re only found in the southeastern United States and the males of these species they just buck up to these rainbow, beautiful things in the spring with like red and green. They’re rainbow fish in these streams so that they can find the ladies.

Chris - Patrick?

Patrick - The colours are, obviously, great for attracting mates. Doesn’t it make them more likely to get attacked by a predator as well, or is there some sort of tradeoff or dance that they do?

Kate - You can use colourful signals as a sexual signal, which there’s risk with that, but if it didn’t work and they all got killed off then they wouldn’t survive. So obviously there’s some behavioural thing they’re doing to overcome the risk of being really flashy. But you can also become colourful to blend into your backgrounds, so if your background is really colourful then maybe you’ll be really flashy as well.

Chris - Lovely. One of the things that’s very colourful, given we have someone who’s a space scientist here, Matt, when we see pictures of our solar system depicted the planets are given colours. Is that accurate?

Matt - It can be. Some people like to fiddle with the colours of various things. If you’ve ever seen a picture of a Nebula, you do get red and blue from scattered light, but when you look at the planets what you're mostly seeing is either the rock - in the case of Mars which has a very thin atmosphere, you’re seeing the chemicals themselves in the atmosphere. Neptune and Uranus are the case in point - they’re very blue. Neptune is bluer than Uranus because it has a high content of methane. What happens is that that methane absorbs the red light coming from the Sun and it bounces back the blue, and this is why we see it.

Chris - So it’s not inaccurate; it’s not just artistic licence when we give planets colour. There is some genuine information coming back in the light from those planets which means we can say: “ah look, they must have a lot of chemical X in the atmosphere” and we give them that colour with reason?

Matt - Absolutely Chris. Absolutely.

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