By subjecting triggerfish to vision tests, Naomi Green is helping us understand why coral reefs are such colourful places.
Ethograms is a monthly column published in collaboration with the Australasian Society for the Study of Animal Behaviour (ASSAB), showcasing the work of early-career researchers. Andrew Katsis is a PhD candidate at Deakin University, and an outreach officer for ASSAB.
Beneath the gentle waves of the Great Barrier Reef, nature is putting on a show — a psychedelic pinwheel of darting, shifting colour. Reef fish move together in shimmering schools, flitting left and right amongst the coral. To the human eye, it is one of the great visual sights in nature; but these reefs weren’t created for our own aesthetic pleasure.
"Coral reefs are one of the most colourful environments on the planet, and coral reef fish are some of the most colourful animals,” says Naomi Green, a PhD candidate at the University of Queensland. “But we don't actually know if they can see their own colours, or what is the purpose of all these colours.”
In the animal kingdom, colour is very much a language, and one with multiple uses. The poison dart frogs’ vivid hues are a warning to predators of their toxicity, a signal known as aposematism. Chameleons alter their skin colour to communicate social information, such as fighting ability, to other chameleons. In superb fairy-wrens, males that moult into their bright blue plumage early in the breeding season are more likely to score extra-pair matings.
To understand why an animal is so colourful, it helps to know who else can see their colour — and, indeed, whether they can see it themselves. For instance, we now know that the small UV-reflective patch on the dorsal fin of a two-bar damselfish provides an alarm signal to other damselfish, while being invisible to predators who cannot see in the UV spectrum.
Because humans and fish see the world in very different ways, the functions of fish colouration can often be surprising or counterintuitive. "We can have these really brightly coloured fish, that look bright blue and purple and yellow to us, but actually blend into grey to provide camouflage,” says Green. “So sometimes what we think is the purpose of a colour actually has the opposite effect."
To help untangle the mystery of coral reef colour, Green is studying colour vision in one particular reef resident, the blackbar triggerfish (Rhinecanthus aculeatus). “They're about 10-15 cm long,” she says. “They're brightly coloured — they have blue and black stripes across their forehead — and their dorsal fin goes up and down like a trigger, which is where they get their name.”
But how do you quantify how well a fish sees colour? Most previous studies have used a paired choice experimental design, in which subjects are trained to associate food with a certain colour, and then must reliably choose this colour from a line-up.
In the first of her published experiments, Green tested 58 wild-caught triggerfish using this traditional design. To receive a food reward, each fish had to choose between two visual stimuli: one coloured white (rewarded with a tasty food item), and the other patterned yellow in a way that might suggest toxicity (punished with an unpalatable food item). Across test subjects, the yellow patterns varied in several features to test which were most salient to the triggerfish’s perception.
Despite living in such a vibrant underwater environment, triggerfish have “average colour vision, similar to a lot of other fish,” says Green. Her experiments showed that triggerfish were quicker to learn to avoid the yellow pattern when it had a larger internal edge — that is, composed of smaller repeated shapes rather than a single large shape. This implies that, if you are a nudibranch or squid hoping to avoid becoming triggerfish food, these are the warning patterns you would prefer to display.
These paired choice tests are effective but can be very time-consuming, as fish must be trained separately for each colour. For her latest work, Green has streamlined the training process by drawing on a new technique, inspired by the Ishihara test used to diagnose colour blindness in humans. In the Ishihara test, a pattern of coloured dots contains a shape or number that can only be read by people with normal colour vision, and not by those with colour vision deficiencies. Green’s triggerfish, when placed in the testing tank, now face a similar pattern of coloured dots.
“There's one spot in the middle that's different coloured,” says Green. “We train the fish to find the odd-coloured spot, and they have to peck it to get food. Some of the really smart ones will learn it in a week, usually it's about two weeks, and some of the really stupid ones don't ever get it.
Once a fish can no longer distinguish the odd-coloured spot, even with a food reward at stake, Green knows that she has reached the threshold of its colour perception. Mapping out the fringes of each subject’s visual acuity requires considerable time and patience, but at least the triggerfish are willing collaborators.
“They're like little labradors,” says Green. “They're very easy to train, and very fun to work with. Although they do bite my fingers a lot."