I remembered watching this Discovery Channel program on cable a few years back and it was seriously one of the most brilliant and incredible documentary I’ve ever seen. It was of a dark, alien world filled with strange looking creatures, with some being absolutely hideous. This was one part of an eight-part Blue Planet TV series produced by BBC – The Deep. It was a documentary on life one to three thousand metres beneath the ocean surface, and as quoted from the video, “more people have traveled into space than ventured this deep”, hence was extremely intriguing.Out of all the animals introduced, many caught on tape for the very first time; the marine hatchetfish piqued my interest the most as I was very interested about the evolution for avoidance of predators and it was, as you will soon discover, truly a Master of Hide and Seek.
Side view of a marine hatchetfish
The picture above shows the marine hatchetfish, which is from the Sternoptychidae family and is the protagonist here. It inhabits the deep ocean at depths of about 200 to 1,000m and can be found in tropical and subtropical waters of the Atlantic, Pacific and Indian Oceans. There are 45 known species but most, like the marine hatchetfish here do not exceed 10cm in length.
[A point to note: There is another type of species named the freshwater hatchetfish but it is from a different family and should not be confused with the marine hatchetfish.]
At 1,000m beneath the ocean surface, the twilight zone is reached, where sunlight barely shines through. It is dark and in here, most animals are highly, if not, completely transparent (like the amphipod as seen below). This is accruing to the fact that they have to avoid being seen by their predators, hence by being transparent, serves as the best camouflage for them and increases their chances of survival.
An amphipod
The hatchetfish however, is not transparent.
This fish is no longer than 10cm and has a very flat body that is so thin that head-on, they are barely visible in the dark waters.
Three barely visible hatchetfishes. Look hard!
But what about from the side? Can its predators see it since it is not transparent?
Even though it is not transparent like the other animals in the twilight zone, it has highly silvered sides that work to the same effect. They reflect remnants of any existing light such that the hatchetfish becomes invisible from the side-view as well.
Still visible
Invisible! When mirrored sides reflect any existing light
Then again, its mirrored sides may still not be adequate to hide from its predators. This is because many of its predators have tubular eyes pointing upwards, which, even in such a dark environment, are able to search for it against any light from above, as they will be able to detect its silhouette.
Visible silhouette when seen bottom-up
However, through millions of years, the hatchetfish has evolved to ‘better itself’ in terms of avoidance from its predators.
It developed a way to deceive the eyes of any predators looking up in search of their presence with the help of photophores (bioluminescent organs). These photophores are located on the bottom of the hatchetfish’s belly and they are light-producing cells that can precisely match the changing colors of the light from the surface far above. The intensity of the light produced is controlled by the hatchetfish itself and the appropriate brightness is selected according to the amount of light that reaches it from above. This is called counter-illumination and with such a counter-shading, the silhouette is ‘broken up’, thus allowing the hatchetfish to be almost invisible from below as well!
Photophores highlighted in purple
Close-up view of photophores
Close-up view of photophores after adjusting to the light from surface
And there it goes! Hatchetfish becomes invisible again! Even from bottom-up
On the other hand, in a continuous evolutionary arms race (please click here for more information on this theory), despite the hatchetfish’s sophisticated counter-illumination to hide from its predators, some of them (predators) can, as in one example shown in this same video, react to this and also evolve accordingly in an attempt to ‘outsmart’ the hatchetfish.
Hatchetfish's yellow-eyed predator
As seen from above, this predator has enormous yellow tubular eyes and they are actually able to distinguish between light produced by photophores and sunlight. Therefore, this predator can counter the counter-illumination employed by the hatchetfish to beat it in this game of hide and seek.
[A point to note: This particular predator was not identified in the video, and even though I tried to search for it and tried to use the Internet resources to match it with the video image, I chose to not name it here so as to avoid providing inaccurate information as I could not be sure of its identity.]
In the evolution for avoidance of predators, the hatchetfish has acquired sheer abilities as evidenced from above to hide from its predators in the twilight zone. That said the predator could also evolve itself to counter this.
Such an evolutionary arms race is not unique in the case of the hatchetfish and its predator, but has been waging on for millions of years amongst other species as well in this eternal game of hide and seek between the hunter and the hunted in the animal kingdom.
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Disclaimer: All pictures on this blog are screenshots captured from the youtube video on the hatchetfishes. They are not obtained from any other websites/webpages, hence there are no photo credits.
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This blog entry is based on this youtube video of "The Deep". The snippet on the hatchetfish will only start from 5'28'' onwards (ends at 8'06'') so please allow the video to load, then skip there directly to watch this evolution for avoidance from predators unfold.