Invited Speakers

Sönke Johnsen, Professor of Biology, Duke University, Durham, NC

Originally trained in mathematics and art, Dr. Johnsen has studied camouflage, signaling, and non-human visual modalities for over 20 years. He is particularly interested in vision and camouflage in the open ocean, but has also worked on coastal and terrestrial species, magnetoreception, nocturnal illumination, sexual signaling, eye evolution, and human cataracts.  His research combines mathematical analyses with behavioral and morphological studies and in situ measurements and observations. His field work primarily involves open-ocean research cruises that use SCUBA and deep-sea submersibles.  In addition to exploring the evolution and diversity of the optical and visual tricks that animals perform, Dr. Johnsen is interested in improving communication between theoretical and experimental scientists and between scientists and artists. Outreach is a strong focus, and his research has been presented in numerous magazines, newspapers and television shows. In his spare time, he is an avid nature photographer.

ABSTRACT:

Through the Looking Glass: Polarization vision versus transparency and mirror-based camouflage in the open sea

Sönke Johnsen, Professor of Biology, Duke University, Durham, NC

Animals in the open ocean have evolved a number of camouflage strategies that are absent or rare in other environments. Two of the most interesting are whole-body transparency and mirrored sides. Both are highly successful forms of radiance camouflage and many animals have evolved sophisticated variations of them. However, both strategies may also be vulnerable to animals with polarization vision: transparent tissues may be birefringent or depolarizing and mirrored surfaces may change the polarization of reflected light.  Thus it has long been suggested that polarization vision in pelagic animals has evolved to enhance the contrast of transparent and mirrored prey.

We examined the potential for this in transparent species 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 (~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 compared to the downwelling irradiance. In fact, the dominant radiance and polarization contrasts of the object were due to unpolarized downwelling light that had been scattered from the animal viewed against the darker and polarized horizontal background light. We found that relatively simple algorithms can use this negative polarization contrast to substantially increase visibility.

Using the in situ polarization imaging system, we photographed various species of fish with mirrored sides on the Great Barrier Reef. In addition, we used transfer matrix theory to model how the polarization of light reflected from semi-random stacks of guanine platelets  (such as those found in fish) depended on the number of plates and the distribution of platelet thicknesses and angles. The in situ imaging showed that many mirrored fish species were less conspicuous to animals with polarization vision than would be predicted. The transfer matrix modeling showed that, while stacks of 10 or 20 platelets strongly affected the polarization of light for most angles of incidence, stacks of 50 or greater platelets with a moderate amount of randomness in both platelet thicknesses and angles reflected nearly 100% of both polarization components. While further analysis is required, these structures may thus act as polarization-preserving reflectors that would provide camouflage to both animals with normal vision and polarization vision.

In conclusion, while polarization vision may serve to break camouflage strategies in the pelagic realm, the situation is more complex than was originally appreciated, and certain species may have evolved sophisticated counter-measures.