There are probably few places that conjure up images of desolation more than the depths of the earth’s oceans. In these aquatic wastes there is little or no daylight, food is lacking and mates are in short supply.
Even so, going about your business undetected by predators in these places is harder than you might think. At levels where daylight can penetrate, an animal’s silhouette can be detected under the downwelling light, and in the darker depths some predators have developed photophores that they use as searchlights.
Animals have therefore adopted different camouflage strategies in order to deal with this. Transparency vastly reduces an animal’s visibility under ambient light, whereas red or black pigmentation helps reduce the reflection of the directed light emitted by photophores.
However, in the lower mesopelagic zone, found between 600 and 1000 m below the surface, animals are faced with a dilemma. This is where the optimal camouflage strategies overlap, as some daylight reaches these depths, but searchlight-carrying predators can also be found here. An animal can’t both be transparent and have pigmentation at the same time, so what possible solution is there for these animals? Sarah Zylinski and Sönke Johnsen, publishing in Current Biology, have uncovered a mechanism that solves this dilemma.
They had noticed that the appearance of a small octopus, Japetella heathi, and a medium-sized squid, Onychoteuthis banksii, varies from transparent to pigmented. Could this be in response to different light conditions? To test this, they first studied what appearance these animals had under ambient light conditions to simulate downwelling daylight.
They found that both animals were transparent under these conditions, making them ideally camouflaged for the upper levels of the mesopelagic zone.
Next, they exposed transparent animals to a beam of light with a wavelength similar to that produced by photophores, to simulate exposure to a predator searchlight.
They found that these animals changed their appearance to the pigmented form within a second of exposure to the beam, and they eventually took evasive action, as if to protect themselves from predators. This suggests that these animals change their appearance in response to a threat from searchlight-bearing predators.
But does the pigmentation actually help them to remain undetected? To test this, the authors measured the reflectance from live Japetella skin, in either the transparent or pigmented form.
They saw that the reflectance of the pigmented skin was twofold lower than that of the transparent skin when illuminated with light of a wavelength similar to that emitted by mesopelagic predators. This reduces the animal’s visibility when illuminated by predator searchlights and suggests that switching camouflage mechanisms gives the animals a real advantage.
The authors’ findings show that some animals have developed the ability to switch between two different camouflage strategies, allowing them to quickly respond to different predator threats as well as changing optical conditions. Although the mechanism that regulates this switch is currently unknown, it will be interesting to see how widespread these adaptations are and what additional strategies could be deployed to avoid predator detection in dynamic environments.
