Polarization contrast of zooplankton: a model for polarization-based sighting distance.

Transparency is commonly used by zooplankton for camouflage in open waters. Polarization vision allows planktivorous animals to increase their prey's detectability. Polarization properties of zooplankton were analyzed by measuring changes in the transmitted light. The transmitted light was subjected to depolarization and phase retardance, resulting in a species-specific polarization contrast between animal and background; from 5% in Corycaeus sp. to 92% in Undinula vulgaris (Copepoda). This contrast diminishes exponentially with distance, reaching 50% of the inherent value at 1 and 2m, for moderately turbid and clear waters, respectively. However, at reactive distances of planktivorous fishes this contrast is reduced by less than 20%.