Meeting Abstract

51-5  Friday, Jan. 5 11:15 - 11:30  From the Moon in the Sky to the Deep Blue Sea: Using Lunar Optics to Understand the Blackness of Mesopelagic Fish JOHNSEN, S*; OSBORN, KJ; THOMAS, KN; ROBISON, BH; Duke Univ.; Smithsonian Inst.; Duke. Univ.; MBARI sjohnsen@duke.edu http://sites.biology.duke.edu/johnsenlab/

Although color is increasingly studied in both animals and plants, black is often overlooked, even though it can serve important functions. In the case of deep-sea fish, black surfaces strongly absorb (and thus do not reflect) the bioluminescent searchlights of predators, therefore providing a form of camouflage. So, it is perhaps not surprising that many deep-sea fish appear to be exceptionally black. We investigated this in several species of mesopelagic fish in three ways: 1) reflectance spectroscopy 2) scanning and transmission electron microscopy, and 3) optical modeling. The spectroscopy showed that many species reflected very little light, in some cases less than 0.5%, which is one tenth of that seen in most black surfaces found in normal human experience. Interestingly, even though the reflectances were already quite low, they were generally lower in the blue-green portion of the spectrum that comprises bioluminescence, suggesting further optimization. The microscopial studies showed that the surfaces of the black fishes were quite complex. In addition, many had thick sub-epithelial layers of close-random-packed spheroidal melanin granules that were approximately 0.4 to 0.8 microns in diameter. Using methods developed for understanding the reflective properties of lunar soil, which is similarly composed of close-packed strongly absorbing spheroidal particles with high refractive index, we found that the melanin granules found in the black fish were the optimal size for achieving the greatest absorption of incident light with the least amount of material. Together, this study highlights the importance of blackness to deep-sea fish and the strong evolutionary pressures for camouflage in even this dark environment.