Written by: Shivranjani C Moharir
In the oceanic depths below 200 meters, where even the sunlight cannot penetrate, thrives a world of beautiful bioluminescent organisms, which swim across the deep waters, cutting the darkness with the light they emit and playing hide and seek with the predators and preys!
In an attempt to understand the life of organisms in the deep oceans, a collaborative group of researchers from Duke University (Durham), The Natural History Museum (London), Smithsonian National Museum of Natural History (Washington DC), and Monterey Bay Aquarium Research Institute (Moss Landing) have collected and studied fishes from the Gulf of Mexico and Monterey Bay, CA. They observed that the fishes collected from deep ocean were not just black, but were ultra-black! Consistently, most of the ultra-black fishes collected from deep oceans exhibited the reflectance of less than 0.5% at 480 nm, which is the peak wavelength of the ambient sunlight at deep sea and that of most oceanic bioluminescence, and the reflectance of 0.051% to 1.04% across the visible spectrum. This low reflectance of deep-sea fish matches with that of the blackest known animals. One of the deep sea fish species, Oneirodes, is even darker than ultra-black butterflies, and its darkness equals that of the blackest birds of paradise. To give a comparison, a black paper can reflect ~10% of incident light and the blackest synthetic materials manufactured from carbon nanotubes reflect 0.045% of light.
The authors suggest that this low reflectance by the skin of deep ocean ultra-black fish is attributed to closely packed layers of melanosomes. The melanosome layers are located immediately between the epidermal basement membrane and the stratum spongiosum, a loosely organized collagenous layer of the dermis in the skin of the fish. The authors also suggest that the size and aspect ratio of the melanosomes play a crucial role in producing the low reflectance. They have simulated the reflectance from 350-700 nm wavelength of light from closely packed melanosomes with 169 unique combinations of aspect ratio and size using ‘Finite-Difference Time-Domain Modeling of Close-Packed Melanosomes’ and found that the melanosome geometries of ultra-black deep-sea fishes are well optimized to produce the lowest reflectance.
What are the benefits obtained by ultra-black fish by camouflaging in the darkness of the deep sea? To answer this question, the researchers used a computational model to calculate the relative sighting distances by the predators of fishes with skin reflectance ranging from 2% to 0%. Sighting distance models suggested that low reflectance reduces predator sighting distance up to 6-fold. Thus, being dark could give a survival benefit to these ultra-black fishes!
Acknowledgement:
Our sincere thanks to Dr. Karen J Osborn for permitting to use the image.
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