Polarization sensitivity as a contrast enhancer in pelagic predators: lessons fromin situpolarization imaging of transparent zooplankton

Abstract

Because light in the pelagic environment is partially polarized, it has been suggested that the polarization sensitivity found in certain pelagic species may serve to enhance the contrast of their transparent zooplankton prey. We examined its potential during cruises in the Gulf of Mexico and Atlantic Ocean and at a field station on the Great Barrier Reef. First, we collected various species of transparent zooplankton and micronekton and photographed them between crossed polarizers. Many groups, particularly the cephalopods, pelagic snails, salps and ctenophores, were found to have ciliary, muscular or connective tissues with striking birefringence. In situ polarization imagery of the same species showed that, while the degree of underwater polarization was fairly high (approx. 30% in horizontal lines of sight), tissue birefringence played little to no role in increasing visibility. This is most likely due to the low radiance of the horizontal background light when compared with the downwelling irradiance. In fact, the dominant radiance and polarization contrasts are due to unpolarized downwelling light that has been scattered from the animal viewed against the darker and polarized horizontal background light. We show that relatively simple algorithms can use this negative polarization contrast to increase visibility substantially.

Null point of discrimination in crustacean polarisation vision

“…This is a process somewhat analogous to colour vision and adds to the well-known uses of the polarisation of light for navigation, orientation and habitat localisation (Wehner, 1976;Schwind, 1983;Labhart and Meyer, 1999). Animals may use this channel to improve the detection of objects through veiling light or glare (Schechner and Karpel, 2005;Alkaladi et al, 2013), to enhance the contrast of transparent prey (Shashar et al, 1998;Tuthill and Johnsen, 2006; but see Johnsen et al, 2011), to break camouflage (Shashar et al, 2000;Jordan et al, 2012;Temple et al, 2012) or to allow the detection of polarised body patterns for communication Marshall et al, 1999).…”

Maturation of polarization and luminance contrast sensitivities in cuttlefish (Sepia officinalis)

“…Cuttlefish are visually driven predators that prey upon various small moving crustaceans (Hanlon and Messenger, 1996). Of them, mysid shrimp use transparency for camouflage (Wells, 1962) but their tissues generate localized polarization via scattering, reflection and birefringence, which could allow polarization-sensitive predators detect them (Johnsen et al, 2011). Likewise, adult cuttlefish use PS to detect silvery fish that also generate a polarization pattern (Shashar et al, 2000).…”

Out of the blue: the evolution of horizontally polarized signals inHaptosquilla(Crustacea, Stomatopoda, Protosquillidae)

“…For instance, in shallow, clear marine waters, the intensity and spectral composition of the downwelling light can vary dramatically, both as a function of the time of day, and because of environmental factors such as turbidity (Cronin et al, 2014). In such changing conditions, the polarization of light remains more constant than other visual dimensions over short ranges (Waterman, 1954;Cronin, 2001), which renders it a reliable provider of information (Shashar et al, 2011;Johnsen et al, 2011). Previous research in this field has focused on either the underlying retinal mechanisms of polarization sensitivity (for reviews, see Horváth and Varjú, 2004;Roberts et al, 2011) or the optical mechanisms by which polarization and multi-component polarization and/or colour signals are produced (Chiou et al, 2005;Mäthger and Hanlon, 2006;Chiou et al, 2007;Mäthger et al, 2009;Cronin et al, 2009).…”