Field of Science

When Organisms Really Shine... Bioluminescence Pt.1

As near exclusive surface dwellers, we only see the sun-kissed top layers of the vast oceans of our planet. As we descend into the depths, the light from the sun dies away. And as we reach the bottom, we should be plunged into absolute blackness.
But we aren't. There are lights at the bottom of the ocean, and they don't come from nuclear reactions in stars far  in the sky. 
This illumination comes from living creatures. The great deeps of our planet are populated with creatures who have only ever known light coming from other living things. Species as diverse as Sharks, Squids, Shrimp can exhibit this trait known as Bioluminescence. This is what happens when organisms really shine.
This is the first of a number of posts I will produce on bioluminescence.
In this first post I will explore the many ways organisms use bioluminescence to survive.


Camouflage

The hatchet fish is not the most pretty of creatures. Rarely do humans find it in it's natural habitat, the deep ocean. There it hunts what ever prey is unfortunate to cross its path.  But fierce though they are, there are always bigger fish waiting to hunt them.
When swimming through the deep blue, it is actually difficult to camouflage oneself. Many predators spot prey based, not on their colouration, but on the shadow they cast. That's why sharks often swim below their prey before attacking, and why they confuse surfers for seals.
So to truly camouflage oneself in the sea, one needs to get rid of that shadow. The hatchet fish has worked out a clever way of doing this, through the use of a technique called counter illumination.
On it's underside there are organs on which bioluminescent bacteria grow. These organs emit light which cancels out their shadow, so that when predators look at the fish from below, they can't make out a silhouette.  And the Hatchet fish is camouflaged, by light.

Blinding Flash

To hunt in the dark depths of the ocean, predators often need very sensitive vision. Using this, they can spot  prey in the low light conditions. However, some prey emit a bright flash of luminescence when approached by a predator. The effect of this on the predators eyesight can cause it to hesitate, and in that time, the prey can make good it's escape.


Smoke Screen

So imagine a predator, perhaps like the deep sea hatchet fish coming across a smooth nylon shrimp. This juicy looking customer would be tasty pickings. Just as the predator approaches, the shrimp sprays out a glowing slurry of bioluminescent bacteria directly in the face of the predator, encompassing it in a glowing cloud. The predator, startled and distracted by the cloud, loses the prey.


The enemy of my enemy is my friend

There is another reason why the smoke screen above is damaging to the predator is  that in some cases when prey spray these bioluminescent slurries, they stick to the predator. So the predators camouflage is now compromised. It is now clearly visible to other predators, which will hunt it down.  In this way, the prey forms an alliance with a predator further up the food chain to combat a common enemy.


Disposable Limbs

Octopoteuthis is a small bioluminescent squid. It is presumed to be a creature of a nervous disposition, as it literally goes to pieces when its attacked. The bioluminescent tips of its tentacles drop off, and brightly emit light. The predator focuses its attack on these organs, and the squid can make it's getaway with it's remaining limbs.

Lure

This attraction some predators have to light in itself can make them vulnerable to attack. You may be familiar with a predator that uses this weakness.  The deep sea angler fishes have a long filament on the end of which there is a bioluminescent lure. The angler fish uses this to attract its prey into its gargantuan maw.
However, you don't necessarily even need to emit light to take advantage of it. Sperm whales and megamouth sharks are known to have whit colouring around their mouths, which has been speculated as having reflective properties, that make them slightly luminescent. Prey that investigate these sources of light will fall prey to these predators.

Headlights

Some predators have bioluminescent organs situated near their eyes that act as headlamps. Dragonfishes have red luminescence emitters. The great thing about red emitting light is that it causes the shrimps to fluoresce. In the same way that UV lamps make the lint (and other stains) on your clothes glow, the red light emitted by the dragon fish highlights the shrimp, allowing for a precise attack.


Courtship
Bioluminescence can be used by some creatures in elaborate displays to attract mates using elaborate displays.The most famous example of this is the firefly, which emits flashes of light, so that the males and females of the species can find eachother over long distances. The patterns of flashes allow the fireflies to distinguish between different species.

In conclusion

So in summary, bioluminescence has many functions in the natural world. Many of the creatures described above can use their bioluminescence for more than one of the functions described above. the female deep sea angler can use her bioluminescenct lure to attract males. Predators who consume the tips of the octopoteuthis tentacles will unintentionally emit a bioluminescent signal, attracting bigger fishes.
And we don't even know the full diversity of interactions in which bioluminescence plays a role. But it is known that based on the diversity of chemical mechanisms for bioluminescence, it evolved on at least 40 separate occasions.
It is because of these complex interactions that even in the depths of the oceans, far from the sun, the light of life shines on.

Next week (work permitting) I hope to delve a little deeper into the evolutionary origins, and the basis for bioluminescence.

Haddock, S., Moline, M., & Case, J. (2010). Bioluminescence in the Sea Annual Review of Marine Science, 2 (1), 443-493 DOI: 10.1146/annurev-marine-120308-081028


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