Speaker – Sönde Johnsen, Duke University (hosted by T. Wolcott), 1108 Jordan Hall and Zoom

Seminar Title – Bioluminescence and ultra-black camouflage in deep-sea fish

Abstract – At oceanic depths greater than 200m there is little ambient sunlight, but bioluminescent organisms provide another light source that can reveal animals to visual predators. Transparency and mirrored surfaces– common camouflage strategies under the diffuse sunlight of shallower waters– are conspicuous when illuminated by directed bioluminescent sources due to reflection from the surface of the body. Pigmentation, however, allows an animal to absorb light coming from bioluminescent sources, rendering itself visually undetectable against the dark background of the deep-sea. We present evidence that suggests pressure to reduce reflected bioluminescence has led to the evolution of ultra-black skin (reflectance <0.5%) in at least 16 species of deep-sea fishes across seven distantly related orders. Histological data suggest this profoundly low reflectance is mediated by a continuous layer of densely packed melanosomes in the exterior-most layer of the dermis, and that this layer lacks the unpigmented gaps between pigment cells typically found in other darkly-colored fishes. Using finite-difference time-domain modeling, and comparisons with melanosomes found in other ectothermic vertebrates, we find that the melanosomes making up the layer in these ultra-black species are optimized in both size and shape to minimize reflectance. The low reflectance results from melanosomes scattering light within the melanosome layer, thus increasing the optical path length and therefore light absorption by the melanin. By reducing skin reflectance, ultra-black fish can reduce the sighting distances of visual predators more than six-fold compared to black fish with 2% reflectance. This biological example of highly efficient light absorption via a simple architecture of strongly-absorbing and highly-scattering particles may inspire new synthetic ultra-black materials.