Quantifying polarocrypsis of silvery fish in the open ocean through massively multi angular measurements (#415)
Reflective camouflage is a common form of camouflage in the open ocean that has significant effects on the reflected polarization signatures of the animal. However, few studies exist quantifying its effectiveness due to the difficulty of measuring a camouflage strategy that is strongly dependent on to the position of the animal relative to the viewer and the environmental light fields. In this study we quantify the intensity and polarization reflective camouflage effectiveness over an expansive range of conditions in the open ocean with the silvery fish bigeye scad (Selar crumenophthalmus ) and lookdown (Selene vomer ).Â In the deep waters of Curacao and the Florida keys restrained carangid fish onto specular mirrors (stainless steel or aluminum) and a stainless steel diffuse metallic reflector (DMR) on a remotely controlled underwater rig.Â Using a polarimeter and an inertial measurement unit we measured the fish, mirror, and DMR against the background water column for over 1500 system angle combinations.Â We found, using the Stokes polarization contrast in the blue polarimeter channel, a significant decrease of contrast of 12% over all measurements of the bigeye scad.Â For chase angles only (relative yaw > 45Â° between the predator and prey) the percent decrease was 18% and also significant.Â This set of angles coincide with fish evasive behaviours from predators and have been predicted to fair better in polarization contrast from the model presented in Brady et al 2013. We also found a correlation with simulations of the Stokes parameter U contrast channel and the measurement of the same channel in the bigeye scad. Histological measurements of the bigeye scad show a specialized arrangement of birefringent guanine platelets that likely produce angle-dependent polarization reflection properties and reduce highly polarizing Fresnel reflections to facilitate the specialized angle-specific reflection properties required for polarization crypsis in the open ocean.