Peacock Flounder: Evolutionary Marvel of Camouflage, by Ben Szykman

There are numerous species of tropical flounder, but a small number have adapted for life specific to coral reefs. One such species, Bothus mancus, or the Peacock Flounder, has evolved the unusual ability to drastically change its body color. Although the ability to change color with chromatophores is relatively common in terrestrial reptiles, it is much less prevalent in aquatic organisms. Some species, particularly cephalopods, are known to communicate through changing iridescence and color patterns on their bodies; but the Peacock Flounder uses its chromophores for camouflage. Although some flatfish can vary their color to a limited extent, the combination habitat and trait exaggeration make the Peacock Flounders a novel fish. While little research has been done on why and how the Peacock flounder uses its camouflage, this paper will provide evidence to show that this unique trait has evolved in parallel with habitat substrate for the purpose of reducing interspecific competition, as well increasing chances of reproduction and survival.

            The answer to why this evolutionary adaptation is useful in the first place lies in the Peacock flounders unique environment. Most species of flatfish remain in relatively shallow coastal water for their whole life, where the substrate is mostly homogenous. The pattern and coloration of a particular species will resemble that of the substrate in its environment (Cott 1966).  This allows flatfish to function as ambush predators, so they won’t be immediately visible to their prey. The environment of the Peacock Flounder, however, has variably sized and colored substrate. While there is the usual sand, rubble, and rocks, there is the addition of dead and crushed coral as well.  The composition of the substrate can be radically different just a few meters away from the reef, providing additional problems to benthic residing organisms. With less physical living space and a higher predator abundance, it seems unlikely that a flatfish can compete adequately in such a highly specialized habitat. Yet, the Peacock Flounders chromophores allow it to go where no normal flounder has gone before.  Where a normal flatfish would be restricted to inhabit an area where its body pattern resembles the bottom, the Peacock Flounder’s ability to change color allows it to travel to places with different substrate composition. This helps solve the competition problem of living in such a diverse habitat like a coral reef. Even though there are many organisms competing for resources at every niche in a reef, the flounder can drift between these niches in search of less competitors (Alatalo et al 1986). Moving niches will change the dynamic of competition the fish is experiencing; if there is more competition and conditions are undesirable, the flounder can simply keep moving until an appropriate niche is found.

Peacock Flounder in four different locations. Photographs taken a few minutes apart.  By Brocken Inaglory [CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons

The ability to migrate between more niches than the average flounder not only decreases competition, but can also provide a broader array of diet options. A more diverse diet means less reliance on one particular prey species, which makes the predator less susceptible to harm if one species becomes scarce. A particular study found that the Peacock Flounder’s preferred habitats with either sand or broken coral benthic composition, and substrate either gray/white or red/brown in color (Tyrie et al 2014). This range shows the flounder can comfortably hunt in the soft sands extending past the reef, or the more colorful rocky substrates immediate to the coral itself (Tyrie et al 2014). Not only can the flounder access fish that physically feed and live in the coral, such as angelfish, they can also prey on species that school in and around the reef. In one instance, a Peacock Flounder was observed preying on a pufferfish in a sandy area (Gochfield et al 2008). This case reflects how the flounder can adapt to a broad range of diet, since a pufferfish’s defense mechanisms make it a less than desirable prey species.  The ability to change color not only allows this fish to access more resources in its habitat, but also enables it to remain undetected to a larger number of species which can be preyed upon.

            This access to a larger amount of niches provides access to many other resources besides prey and hunting areas; the major of which is living space (Sale 1978). In an environment with a low inhabitable area and high diversity like the coral reef, shelter is a hot commodity. While most fish are confined to searching for a home in the reef itself, the Peacock flounder can reside in the much less populous sand and rubble. The sand itself provides little shelter for mating and raising fry, but the different colored and patterned broken coral substrate is more than ideal to lay eggs. This goes back to the idea that the flounder can use its chromatophores to change niches in order to find a more suitable environment. The white sands may be prime hunting grounds, but the dead coral provides resources necessary to the life history of the flounder. The ability to switch between these two sub-habitats allows the flounder to minimize its efforts on everyday activities, as it may be easier to hunt in one and reproduce in the other. One study found that the Peacock flounder prefer the rubble/broken coral substrate for reproduction, as it provides a higher vantage point to spot a mate than on the flat sand (Tyrie et al 2014). The larger substrate provides additional shelter for mating and the laying eggs, as there are more crevasses to hide in. When the eggs hatch into fry, this substrate becomes important in continuing to shelter them until they are large enough to hunt in more open environments. The chromatophores provide additional protection when the fish is in the adult stage, since it provides the obvious advantage of hiding from predators. In a reef ecosystem, many predators like Blacktip Reef Sharks and Barracudas are visual oriented predators. This means that the flounders color changing camouflage is not only advantageous to finding prey in the reef, it also helps to prevent becoming prey itself.

Peacock Flounder substrate selection (below) compared with substrate availability (Tyrie et al. 2014).

The evolution of the remarkable trait has provided multiple adaptive benefits to the Peacock flounder and its environment. Not only does it allow the fish to migrate between niches to reduce competition, but it also broadens the availability of species that can be accessed to eat. When looking at individual life history traits, the chromatophores are useful in creating a safe mating area, finding a mate, and reducing mortality from predators. It does not seem it was one factor that led to the evolution of this trait, but rather an accumulation of overlapping benefits that allowed this flatfish to adapt for life in the coral reefs.

References

Alatalo, R. V., L. Gustafsson, and A. Lundberg. 1986. Interspecific Competition and Niche Changes in Tits (Parus spp.): Evaluation of Nonexperimental Data. The American Naturalist 127(6):819–834.

Cott, H. B. 1966. Adaptive coloration in animals. Methuen & Co Ltd, London.

Gochfeld, D. J., and J. B. Olson. 2008. A novel feeding behavior by an ambush predator on toxic prey. Coral Reefs 28(1):155–155.

Sale, P. F. 1978. Coexistence of coral reef fishes? A lottery for living space. Environmental Biology of Fishes 3(1):85–102.

Tyrie, E. K., R. T. Hanlon, L. A. Siemann, and M. C. Uyarra. 2014. Coral reef flounders, Bothus lunatus, choose substrates on which they can achieve camouflage with their limited body pattern repertoire. Biological Journal of the Linnean Society 114(3):629–638.