Why are reef fish so bright and beautiful? Why do they have such bold and seemingly conspicuous colour patterns?

Life at Lizard is a blog series containing stories about life at the Australian Museum’s Lizard Island Research Station (LIRS).

The coral reef is the most spectrally diverse environment on the planet and coral reef fish display a stunning array of colours and patterns. The function of these colours and patterns however, is unclear.

To the human observer, many coral reef fish appear to be highly conspicuous and easy to spot from a distance. However, avoiding being eaten is the key to survival on the reef. Many reef fish, therefore are likely to have body patterns that provide visual concealment from predators: either by matching an aspect of the background (background matching); or disguising their body outline (disruptive colouration).

To decipher the function and effectiveness of concealment in animal body patterns, you need to take into account two important aspects of the scene: 1) the background against which the animal is viewed; and 2) the visual system of the observer.

Objectively quantifying animal body patterns from the observer is important as animals are able to see colours that humans cannot (ultraviolet and infrared). Also, many animals, including fish are not able to see as much detail as the human eye.

The banded humbug (Dascyllus aruanus) has a highly contrasting black and white striped body pattern. These fish look very conspicuous to a human observer, but, like the terrestrial zebra, the body pattern is likely to help them to avoid being eaten.

Humbugs live in small coral heads that are made up of nearly-vertical branches, suggesting their pattern may help them blend into this background. In my PhD, I have been using behavioural experiments to test this theory.

I have trained two common coral reef predators, the coral trout (Plectropomus leopardus) and the slingjaw wrasse (Epibulus insidiator) to find a humbug against specific backgrounds. I manipulated the backgrounds so that they were either closely matching the humbug’s body pattern, or very different from it. In each trial, the predator had to choose between two humbugs on different backgrounds. The first fish that the predator found was determined as the easiest for the predator to see.

The results of these behavioural experiments have shown that humbugs are hardest to find when they are against backgrounds that are slightly different to their body pattern. When they are against backgrounds that have very similar pattern statistics, they actually are easier for the predator to spot.

Why would this be the case? It may be that there is something about the prey body pattern that makes them harder to spot when they are mis-matching their background. To help me answer this next question, I will be looking at where humbugs live in the wild. Do humbugs live in coral heads that have branches that look similar or slightly different to their stripes?

So…if you want to hide on the reef, make sure you look a lot like the background, but not perfectly matching it…I’ll be keeping my yellow scuba fins then!

Genevieve Phillips
PhD student intern
University of Queensland

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