When we think of camouflage, there are a number of images that spring to mind. Perhaps we think of a young mammal or bird, its markings cryptic and beautifully matching the background of its hiding place. Maybe we picture the breathtaking colour-changing feats of cephalopods, as cuttlefish and octopuses rapidly adjust their appearance to blend near perfectly into their marine home. We could even think about masquerade. This is the realm of insects that look like sticks, a gecko that resembles leaves, right down to the veins and nibble-marks, or, uniquely, spiders that spend their days pretending to be a bird-dropping. Perhaps what we don’t consider is that these are all visual forms of camouflage, relying on becoming harder to see by confounding the visual searches of a potential predator. However, not all animals rely on vision as their primary sense in the way that humans do. What if your own smell could help you disappear completely from a predator’s perception?
Rohan Brooker, of the Georgia Institute of Technology, wanted to investigate this after being inspired by the chemical camouflage used by some invertebrates. The Bison robustum caterpillar disguises itself as the plant it feeds on, not in visual appearance, but by inheriting its smell after eating. Chemicals from the consumed plant are incorporated into its exoskeleton. Predatory ants will clamber right over the curious caterpillar, unable to detect its natural scent.
“Most of the literature on camouflage focuses on visual methods, but many animals use smell more. For these animals, chemical camouflage may be far more important to stay hidden,” said Brooker.
To investigate the theory of a scent-based disguise, Brooker and his fellow researchers headed out to the Great Barrier Reef, establishing themselves at the Lizard Island Research Station. There, they set to work to discover if a vertebrate could also make use of chemical camouflage.
The orange-spotted filefish (Oxymonacanthus longirostris) is a beautiful, coral-dwelling fish, with orange markings adorning vivid blue skin. It is native to reefs of the Indo-Pacific, and feeds almost exclusively on a single type of Acropora coral. . “I was working on a separate project with the filefish as part of my PhD, looking at dietary specialization. I saw how narrow their diet was and also how well they visually blend into the corals they were eating. The filefish seemed like the perfect choice for testing for diet-induced chemical camouflage,” said Brooker.
The team collected a number of filefish from the surrounding reef, and housed them in large aquariums. They were divided into two groups, one that was fed exclusively on its standard diet of Acropora spathulata coral, and one that was able to eat only Pocillopora damicornis, another native coral species that forms only an occasional part of the filefish diet.
In a clever component of the methodology, the researchers recruited two species of tiny crab from the same reef habitat, and used them to determine if the filefish actually smelt like their coral homes. The two species, Tetralia glaberrima and Trapezia cymodoce, normally inhabit the two types of corals, Acropora and Pocillopora respectively, and would be naturally inclined to head towards the familiar smell of their home corals.
The sniffer-crabs were put to work, released into both experimental groups to see where their took them. As expected, T. glaberrima, the Acropora-dwelling crab, homed in on the filefish that shared the coral’s smell, and were not as interested in the fish that instead had been eating Pocillopora. The reverse was also seen, with T. cymodoce crabs preferring to head towards fish that smelled like their favourite coral, Pocillopora. The scented signal coming from the Acropora group filefish was so strong that it was enough to leave the confused sniffer-crabs attempting to get as close to the fish as possible, perhaps believing them to be coral and treating them like shelter.
Brooker and colleagues were pleasantly surprised: “The whole mechanism seemed pretty logical and we thought there was a good chance that something was going on, but I think we were all a bit surprised at how strongly the fish’s odour matched the coral.”
Stage one successful, the researchers had established that something was indeed happening as a result of the filefish diet. The fish appeared to smell like the coral it fed on, certainly enough to befuddle the researchers’ sniffer-crabs. But would it be effective at hiding the filefish from its predators?
For stage two, the sniffer-crabs, mission accomplished, were returned to the reef. Instead, to investigate if the adopted aroma of the filefish also kept them hidden from predators, a new species was introduced to the experiment: a native predatory cod (Cephalopholis spp.).
The cod was placed into a tank along with corals and a filefish that was hidden from the predator’s view, safe from becoming a snack. The researchers wanted to examine if and how the cod would respond to each of the groups of filefish that had been feeding on differing diets. They found that in situations where the filefish diet did not match the corals present the cod were restless, indicating that they could smell food nearby and were trying to determine its location. In stark contrast, when the coral the filefish were eating and hiding in matched, the cod remained passive, hidden away in a cave inside the tank. This behaviour suggested that the cod could not smell the filefish, and could not discriminate it from the aroma of the Acropora corals present.
Taken together, the behaviour of the crabs and cod around the filefish and corals provide evidence for what is a fantastic finding – a different form of camouflage in the natural world. Whether the fish is actively altering its scent via a behavioural choice as it selects food, or if it is the beneficiary of a fortunate effect of its diet remains unclear and is still to be tested. Acropora corals make excellent hiding places due to their large, branching structure, but are generally a poor food source. Yet they make up the bulk of the filefish diet, perhaps suggesting that the filefish is trading off dietary quality for that which provides them greater camouflage. Either way, the filefish can reap the benefits of the chemical changes that enable a greater chance to remain undetected by its predators.
This is perhaps just the beginning, and the first of many instances in the world where a species relies on masking its natural scent from twitching noses and chemical detectors. The next step will be to understand how the filefish achieves its scent-matching to the corals without the benefit of an exoskeleton, possessed by the inspirational B. robustum caterpillar. Brooker believes an answer might lie with amino acids in the mucus of fish, though much work is still to be done. “One of the exciting things about this study is that it shows you don’t need an exoskeleton to use diet-induced camouflage, like in the studies to date. This opens up the possibility that a wide range of animals could be doing similar things,” he said.
Even in the fascinating world of camouflage, this finding is something special. It is easy to consider strategies that enable an animal to avoid predation only from a visual perspective, yet there are many species out there that rely on senses other than vision. Some animals can have an olfactory range that detects a myriad of scents we may find it difficult to even imagine. The brightly coloured, orange-spotted filefish has given an incredible insight into an evolutionary adaptation that allows an animal to remain undetected by predators that hunt by scent. It has become one of the first vertebrates to be discovered to employ chemical camouflage as its defence: to avoid becoming a meal for others, the filefish smells just like the corals it eats.
Rohan M. Brooker, Philip L. Munday, Douglas P. Chivers, Geoffrey P. Jones. (2014). You are what you eat: diet-induced chemical crypsis in a coral-feeding reef fish. Proceedings of the Royal Society B, DOI: 10.1098/rspb.2014.1887
Chemical Camouflage was originally published in the January 2015 issue of Biosphere magazine. Subscribe to it here for the latest research from the natural world and stunning photography: