Lifting the cloak of invisibility: the effects of changing optical conditions on pelagic crypsis.

While transparency, cryptic coloration, and counterillumination are all highly successful cryptic strategies for pelagic species, they become less effective when confronted with varying optical conditions. Transparent species are susceptible to detection by reflections from their body surface, particularly at shallow depths. Colored and mirrored species are vulnerable to detection when viewed from certain angles, or at certain times of day. Counterilluminating species must cope with the changes in the angular distribution and spectra of downwelling light at different depths. In all cases the vulnerabilities are more pronounced at shallow depths and essentially negligible at depths greater than 200 m. The results suggest interesting adaptations both for crypsis (e.g., anti-reflection coatings, variable coloration, variable filters for photophores) and for visual detection (e.g., circling, crepuscular predation), all of which are potentially fruitful topics for future research.