Scientists shed light on how the blackest fish in the sea 'disappear'

Rodiano Bonacci
Luglio 17, 2020

The Invisible Man isn't the only expert in making himself imperceptible with optics science.

Reference: "Ultra-black Camouflage in Deep-Sea Fishes" byAlexander L. Davis, Kate N. Thomas, Freya E. Goetz, Bruce H. Robison, Sönke Johnsen and Karen J. Osborn, 16 July 2020, Current Biology.

Very little sunlight penetrates more than 200 metres below the ocean's surface. So in these areas, evolutionary adaptations such as transparency and mirrored surfaces aren't as useful at keeping away assailants looking for dinner.

This deep-sea dragonfish has ultra-black skin capable of absorbing the bioluminescent light that might blow its cover.

At such depths, bioluminescence - light emission by living organisms - is the only light source.

"In the deep, open ocean, there is nowhere to hide and a lot of hungry predators", said zoologist Karen Osborn of the Smithsonian Institution, at Washington's National Museum of Natural History, a co-author of the research published in an acclaimed journal Current Biology, while adding that, "An animal's only option is to blend in with the background". Tucker trawl nets allow scientists to catch fish from mid-water levels, where ultra-black fish can be found.

"You'd either have to blast them with tons of light to see any detail or you'd just get a silhouette", he said. "It didn't matter how you set up the camera or lighting - they just sucked up all the light".

Knowing the ineffectiveness of common camouflage tactics, the researchers hypothesised that pressure to limit reflected light might have led to the evolution of pigmented body surfaces with near-zero reflectance in ultra-black fish. Reflectance is the measure of the proportion of light striking a surface and reflecting off of that surface.

"Ultra-black arose a few times across the fish family tree", Davis said.

"Effectively what they've done is make a super-efficient, super-thin light trap", Osborn said.

One of the fish that was recently discovered, the Oneirodes species, reflected between.044% to.051%, according to the study.

These fish - like the fangtooth, the Pacific blackdragon, the anglerfish and the black swallower - have modified the shape, size and packing of the pigment in their skin to the point that it reflects less than 0.5 per cent of light that hits it, researchers said today.

Ultra-black skin, as the new research shows, is made possible by specialised melanosomes - tiny packages of pigment, called melanin, within the fishes' skin cells. Both have tiny structures within their cells that contain melanin - the same pigment that lends human skin its color.

Other cold-blooded animals with normal black skin have tiny pearl-shaped melanosomes, while ultra-black ones are larger, more tic-tac-shaped.

"When things are so dark like they are in the deep sea, if you reflect even a handful of photons, something can detect you", Davis said.

That miniscule reflection surpasses the darkness of ultra-black butterflies, which reflect.06% to.5% of light that hits them, and about equals the blackest birds of paradise, which reflect.05% to.31%, the study reported. Osborn and her colleagues discovered that this pigment is not just abundant in the skin of ultra-black fish, it is distributed in a unique way.

The ultra-black Pacific blackdragon (Idiacanthus antrostomus), the second-blackest fish studied by the research team.

The researchers suggested that ultra-black fish biology could advance the development of less expensive, flexible and more durable ultra-black materials for optical technology including cameras and telescopes, and for camouflage.

Adopting this efficient design strategy could improve the manufacture of ultra-black materials, which now use an architecture more like what is found in ultra-black birds and butterflies, Osborn said.

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