Report witch between Transparency and Pigmentation to Optimize Camouflage in the Deep

Animals in the lower mesopelagic zone (600–1,000 m depth) of the oceans have converged on two major strategies for camouflage: transparency and red or black pigmentation [1]. Transparency conveys excellent camouflage under ambient light conditions, greatly reducing the conspicuous-ness of the animal's silhouette [1, 2]. Transparent tissues are seldom perfectly so, resulting in unavoidable internal light scattering [2]. Under directed light, such as that emitted from photophores thought to function as searchlights [3–8], the scattered light returning to a viewer will be brighter than the background, rendering the animal conspicuous [2, 4]. At depths where bioluminescence becomes the dominant source of light, most animals are pigmented red or black, thereby reflecting little light at wavelengths generally associated with photophore emissions and visual sensitivities [3, 9–14]. However, pigmented animals are susceptible to being detected via their silhouettes [5, 9–11]. Here we show evidence for rapid switching between transparency and pigmentation under changing optical conditions in two mesopelagic cephalopods, Japetella heathi and Onychoteuthis banksii. Reflectance measurements of Japetella show that transparent tissue reflects twice as much light as pigmented tissue under direct light. This is consistent with a dynamic strategy to optimize camouflage under ambient and searchlight conditions. The animals of the mesopelagic realm hold a certain fascination for many, and perhaps none more so than for the visual ecologist. In this vast, three-dimensional wilderness, animals have evolved an impressive and diverse range of solutions to the problems associated with life where sunlight is low or nonexistent, food is scarce, and mates are hard to find. As well as great diversity, we also see shared solutions in the face of shared problems. Of these, convergences in camouflage strategies are maybe the most striking [1]. In the upper mesopelagic, virtually every phylum present has a transparent representative, whereas in deeper waters, this trend shifts to a majority of the taxa represented being pigmented deep red or black [2]. Mesopelagic animals that are confined to a camouflage strategy of either transparency or pigmentation risk being sighted by predators under either biological searchlights or downwelling light, respectively (summarized in Figure 1). Furthermore, the boundary between environments where one or the other strategy would be most useful is neither sharp nor fixed, changing with factors such as time of day, cloud cover, and turbidity [1, 9, 14]. Being able to switch between strategies in response to specific threats or changing optical conditions would be highly advantageous …