shedding light on bioluminescense

To me, one of the most horrific environments to imagine is the bottom of the ocean. It’s completely foreign and black, it’s miles from the surface, it’s under water, it’s highly pressurized, and there are scary ass animals. Among them, the deep-sea fish: most of which have one redeeming quality: light-up body parts. When it comes to glow-in-the-dark deep-sea fish, bioluminescent body parts are generally used as lures and so are feeding purposed.

Humpback Angler Fish. Yeah, good luck ever sleeping again.

Last week, an evolutionary baggage reader (Matt Martin, holla!) asked a thoughtful question. “How did deep sea bioluminescence evolve?” How can a consumer survive in the bathypelagic zone of the ocean, where virtually no light reaches, and life is scarce? What is a vertebrate doing down there to begin with?  How was bioluminescense even given a chance to evolve in such an extreme place?

Not very well. Luckily, the road for biolumeniscent evolution had already been paved by the time these deep-sea fishes sank from the middle depths to the deep. A gradual shift towards selection for those fish that could exist a little deeper and occupy a specialized niche began to occur. And so, they descended.

Within that shift, selection for increased concentration of luciferinases occurred, gradually turning into specialized organs with high concentration of coelenterazine and the luciferinases to light them up. Whoa, s*!# just got real. Let’s back up.

Luciferins are the key chemicals in the reactions that produce bio-light. A specific luciferin called coelenterazine has strong antioxidative properties (just like the antioxidants in blueberries and prunes- they bind free radicals and keep them from wrecking your cells. They’re good for you. Eat lots.) Way back in the day, the predecessors to glow-in-the-dark deep-sea fish had coelenterazine all throughout their bodies serving as antioxidants.

In the deeper parts of the ocean, oxidative stress is way less than it is closer to the surface. This is because 1. There is less UV exposure 2. There is less oxygen. When the ancient fish began to sink into the deep-sea consumer niche, the lessening physiological demand for coelenterazine as an antioxidant gave it the chance to fulfill the new demand for having a glowy dangly thing. So it began to locally accumulate, forming light-emitting organs. Of course, this was all very gradual. The glowy dangly things probably started off as a very dim photogenic patches of skin, but evolved into a specialized structure- technically known as an “esca.” Or a glowy dangly lure thing. Whichever.

Lots of other critters can glow, too. Plants, fungi, marine invertebrates, bugs, worms, you name it. But that’s a whole ‘nother can of… well, worms.

Here's the paper if you're feeling ambitious:

The origins of marine bioluminescense: turning oxygen defence mechanisms into deep-sea communications tools. Rees, De Wergifosse, Noiset, Dubuisson, Janssens, Thompson. The Journal of Experimental Biology 201, 1211-1221 (1998).