The cautionary tale of the wolf in sheep’s clothing warns that a familiar exterior can hide malicious intent. Like humans, other animals also deceive one another in this way, and our new study in Current Biology reveals that the dusky dottyback fish, Pseudochromis fuscus, is a true master of disguise.
The dottyback is a small coral reef fish that lives on reefs from Madagascar to Australia. On the reefs off Lizard Island on the Great Barrier Reef, adults are either yellow or brown. Yellow dottybacks are often found on live coral among yellow damselfish, while brown dottybacks are often found on coral rubble amongst brown damselfish. Both brown and yellow dottybacks primarily prey upon damselfish juveniles.
Previous work had found that yellow and brown dottybacks are different morphs of the same species, but why the different morphs had evolved wasn’t known. It might be for camouflage from predators against the differently coloured habitats. Or, like a wolf in sheep’s clothing, perhaps the colour of the different morphs is to match the similarly coloured adult damselfishes, allowing them to sneak up on their young. There were reports of yellow dottybacks turning brown when placed on coral rubble, suggesting that something interesting was going on.
Hunter or hunted?
To disentangle the possibilities of why the dottyback morphs were coloured differently, we created artificial reefs made of either coral rubble (brown) or live coral (yellow), stocked them with either yellow or brown damselfish and introduced either a yellow or brown dottyback.
After two weeks we found that the yellow dottybacks on reefs with brown damselfish turned brown, and vice-versa. This colour change happened regardless of the dottybacks’ habitat, so it was the colour of the damselfish driving the change.
In most fish, colour change derives from an increase or decrease in abundance of one pigment type. By analysing skin samples in dottybacks we found they changed colour by altering the relative abundance of two different pigment types – something not previously reported in other fish species.
So it seemed as though dottybacks were changing colour to resemble adult damselfish in order to more easily prey upon their young. To test this, we carried out some aquarium experiments.
To catch a wolf
First, we stocked tanks with yellow or brown adult damselfishes, juvenile damselfishes, and a yellow or brown dottyback and left them for 24 hours. Checking how many juveniles had been eaten by the dottyback, we found that adult dottybacks the same colour as adult damselfish were much more successful at catching young damselfish snacks compared to those dottybacks with mismatched colouration.
Next, we placed a dottyback in a tank with one brown and one yellow juvenile damselfish. Again, dottybacks would more often catch juveniles that were the same colour as themselves. This suggests that juvenile damselfish lower their defences when dottybacks resemble their adults of their own species.
At this point we could see the dottybacks colour change was to mimic the colour of damselfish adults and lure in their young to be eaten. Could there be other benefits, however, such as protection from predators?
To test this idea, we put pictures of yellow or brown dottybacks in front of live coral or coral rubble backgrounds. We calibrated these pictures for the visual system of the coral trout, Plectropomus leopardus, which preys upon adult dottybacks and damselfishes. Using these and control images, we tested whether the dottyback’s colouration helped them avoid ending up as another fish’s lunch, as well as helping them find their own. Sure enough, dottyback colour morphs in their usual habitat (for example, yellow dottyback on yellow coral) were less vulnerable to the trout’s attacks than if their colour was mismatched to the colour of their habitat.
This sort of mimicry has been suggested in other species, such as the mimic octopus (Thaumoctopus mimicus) but this is the first time it’s been quantitatively proven. At least for the time being, this study shows that the dottyback is the reigning master of disguise in the animal kingdom.