Hide and seek in the open sea: pelagic camouflage and visual countermeasures.

Camouflage is exceptionally challenging in pelagic environments because of their featureless nature. Thus, it is perhaps no surprise that pelagic species have evolved highly sophisticated cryptic strategies, three of which-transparency, mirrors, and counter illumination-are rare or absent in other habitats. Pelagic visual systems are equally complex, and several visual capabilities, including UV and polarization sensitivity and intraocular filters, are thought to facilitate detection of camouflaged animals. This article reviews the optical nature of the pelagic realm and both the camouflage and camouflage-breaking strategies of its inhabitants, focusing primarily on underlying principles and what remains to be discovered. A theme throughout is that far more is known about the structures of the optical and visual systems involved than about their function, an imbalance that is due primarily to the rarity of observations of undisturbed behavior.