Why would bioluminescence evolve in a Europa-like world?

Question

In my sci-fi universe, I have a planet called Tateos Prime. It is a Pluto or Europa-like world, which orbits too far from its twin red suns to support liquid water at its surface. However, geothermal heat emanating from the core, caused by the decay of uranium 238, creates a subsurface ocean environment in which life can exist.

I would like some of these life forms to have bioluminescence; the ability to produce their own light. One idea i had which I particularly liked was “scumlight”; light emitted by clouds of chemosynthetic bacteria that floated above the sea floor.

However, in regards to how such a trait would evolve, I’ve run into a problem:

On Earth, bioluminescence arose because it was advantageous to organisms that had vision. Either they used it to light their surroundings, to attract prey or ward off predators. However, in the seas of Tateos Prime, there is no sunlight, and therefore vision (at least in the visible-light spectrum) would never have evolved to begin with.

Why then would bioluminescence evolve in this environment?

Answer 1

"On Earth, bioluminescence arose because it was advantageous to organisms that had vision" - maybe. We don't really know why bioluminiscent bacteria evolved on Earth.

According to https://en.wikipedia.org/wiki/Bioluminescent_bacteria "the biological role and evolutionary history for specifically bioluminescent bacteria still remains quite mysterious and unclear".

There are 4 hypotheses listed:

• light emission is a side product of the metabolism
• it helps to survive if there is too much oxygen in environment
• part of a bioluminescent-mediated DNA repair system
• "bait hypothesis" - attracts predators to help dispersal of the bacteria

3 out of 4 hypotheses have nothing to do with vision, luminescence is a byproduct of normal life cycle of the bacteria. You don't have to give your monsters vision just to explain it.

Writing an interesting reason for it seems rather difficult anyway. It is a bacteria, one of the simplest organisms possible. Avatar is full of luminescent creatures, no one cares why.

Answer 2

Because even in the deep ocean, there are natural sources of light, and you don't need a complex eye to make use of them.

First, you've got geologic sources. Geothermal vents emit red and near-IR light, which doesn't travel extremely far, but neither is water completely opaque to it. Non-biological chemical reactions between minerals, seawater, and dissolved chemicals emitted from vents can also produce characteristic photon emissions. Chemoautotrophs may become sensitive to those weak light signals to find chemicals they need to live. On Earth, there are even bacteria which perform photosynthesis using the infrared light from deep-sea thermal vents. Other organisms may then develop basic directional sensitivity to light in order to find autotrophs to eat.

Then you've got accidental biological sources. It may seem wasteful and anticompetitive to dump energy into light for no reason, but sometimes the basic unit of energy you get from some catabolic reaction is more than you actually need at the moment, to power whatever other biological process it's hooked up to, and the excess will be wasted as either heat or light. Those accidental emissions can then be cooperatively repurposed for communication, or adversarially repurposed by predators.

And as soon as any organisms can detect light for any of those reasons, there is an incentive for other organisms to produce light on purpose to manipulate them.

Note that it doesn't matter what frequency range the initial evolution of basic vision and bioluminesce exploits. Most deep-sea fish can't see red, because red light doesn't travel very far underwater, but some specifically produce red bioluminesce precisely because it is invisible to their prey. So once any sort of light-based signalling system is in place, it can radiate into forms that use a wide range of frequencies for different purposes, regardless of whether that light exists naturally in the environment--and in fact, there are good reasons to specifically evolve towards using light that is visible to humans, in the green and blue ranges, because it has the greatest range in water.

Answer 3

The problem in other words: why would a light-sensing organ develop if there is no light source, and why would bioluminescence evolve if there were no light-sensing organs, and how could they evolve at the same time so that they would drive the evolution of the other.

Fortunately, there is a neat starting point that makes sense: sensing infrared. Some snakes on Earth have organs with specific receptors that sense infrared.

Your organisms could have evolved to sense infrared to detect heat sources and other nearby organisms whose metabolism produces heat. Then some of them could have evolved a mechanism to control their infrared output which has encouraged evolution of more and more complex sense organs. At some point it may turn into a some form of communication, and eventually the bandwidth may have shifted towards what we call visible light, because it can give an advantage over species that only can detect infrared.

Answer 4

However, in the seas of Tateos Prime, there is no sunlight, and therefore vision (at least in the visible-light spectrum) would never have evolved to begin with.

You don't need sunlight for sight to be useful, and the existence of sight isn't a necessary precondition for bioluminescence. By mass something like 98% of all bioluminescent life on Earth is sightless after all. The exceptions - things like the angler fish (shudder) - are more likely to use their bioluminescence as a lure rather than to allow them to see.

Figuring out an evolutionary path shouldn't be too hard.

First thing you need in an ecosystem is autotrophs - life forms that produce their own food using energy and basic chemicals from the environment. In our world that means plants, algae, some bacteria and so on. Mostly they use photosynthesis and a little chemosynthesis in vent ecologies. Your hot ocean lacks a source of bright light for photosynthesis, but has plenty of hot spots and thermal gradients so thermosynthesis is at least a possibility.

Let's assume that thermosynthesis is the base of your food chain. It might start around hot spots, thermal vents and so on, but eventually micro-organisms analogous to our algae and autotrophic bacteria will fill every region with a thermal gradient. If some of these produce light as a byproduct of their synthesis process then that explains the bioluminescent clouds.

And once light exists, sight will follow.

Whether you're swimming around filter feeding or dragging through the muck as a bottom feeder, being able to detect variations in the light coming from the bioluminescent regions will increase your survivial chances. For a filter feeder being able to see where the highest concentration of healthy autotrophs are will reduce the cost of finding food. For both of them, being able to detect predators by the occlusion of the glow means greater chance of evasion or defense.

Likewise the predators will have better survival rates if they can detect prey in the same way. Some prey will no doubt become home to photosynthetic bacteria in much the same way that flamingos get their coloration from environmental factors (carotenes in their food mostly, but still).

All it takes is a little light that shifts the survival odds in favor of sighted creatures and evolution will do the rest for you.