The curious case of the see-through fish

Foto University of Aberdeen Oceanlab

Above is a picture of a deep sea fish called the Mariana snailfish. This fish lives at 8 kilometers deep, or roughly 5 miles, and is transparent. It got me thinking about science fiction.

The Mariana snailfish

The Mariana snailfish lives at incredible depths in the ocean. An unshielded human body can survive no deeper than roughly 60 meters (190 feet). The deepest dive with scuba equipment is now at a little over 300 meters (1000 feet). However, the Mariana snailfish goes a over hundred times deeper than an unshielded human can go.

At 8 kilometers depth, the pressure is literally crushing. At that depth, it reaches about 80 MPa (mega-Pascal). That’s 80.000.000 Newton of force per square meter, or in other words, the same as a weight of 8 million kilograms pressing down on every square meter. How the heck does this fish survive that?

Well, some geneticists found that the Mariana snailfish has some very interesting DNA. For one thing, their DNA stops the calcification of bones very early in their development. Human calcification progresses during our life, which is why children are made of rubber and the elderly can break bones when they sneeze. Snailfish have a much more gelatinous skeleton.

Those of us with skeletons that live on the Earth’s surface have closed skulls. That protects our brains from getting mushed if we bump our heads. However, in the deep sea, your skull would crack under the pressure. Mariana snailfish, on the other hand, have an open skull.

Finally, you might think the black dots in the picture are the fish’s eyes. Guess again, those are possible leftovers of eyes, nothing more. There is no light at 8km deep, and no need for eyes. Similarly, there is no need for pigment, and so our little friend is see-through.

The evolved Mariana snailfish

So far, so good. Of course, as a science fiction writer, I like to look further than the science. Let’s do a little thought experiment. Imagine that humanity manages to kill all life on the surface – if you need inspiration for this, read an IPCC climate report. Then imagine a few million years passing and new forms of evolving… deep in the Mariana trench. Imagine our little see-through fish becoming sentient.

Now, there are some hurdles to overcome for this to happen. You see, developing an open skull to survive at 8 km deep is one thing, developing a large enough brain capacity to rival humans is quite another. On top of the pressure problems, there is the matter of fuel. A massive brain is like a massive video card in your computer, it eats fuel and produces lots of heat. The heat is pretty handy in the deep sea. but the fuel could be a problem.

So, let’s imagine that problem away. Our Mariana snailfish friends somehow find a source of fuel and their brains grow. Appendages like hands and fingers are pretty important. Stephen Hawking could control his gear without hands, but that’s the pinnacle of evolution, not the start. Our ancestors needed hands to start using tools and develop a society. Unfortunately, developing an arm in the deep ocean is probably a bit hard.

The next step is a society. That’s where we hit snag number two. We humans have harnessed the great power of tools and fire. This might sound a bit simple, and it is an oversimplification, but our evolution to where we are today does hinge on those things. Fire enabled us to cook food, helping simplify the digestion. Tools helped us become smarter and more efficient. We created clothes, shelter, and finally nuclear reactors. All those things are pretty hard to do at 8km depth.

The spacefaring Mariana snailfish

So, okay, let’s pretend our snailfish has progressed to modern times despite these hurdles. Their society could be anything. They might have tentacles and weird tools, but whatever. Now they go and conquer the universe. What would their space ships look like?

One gigantic problem is that they would require a spaceship that has water on it. Not just water, water under high pressure. Mariana snailfish can survive in shallower water than 8 km, but not anywhere near the surface. They kind of come apart near their.

So, a spaceship filled with water. A space shuttle is pretty tough to push into space. Imagine making it several times heavier by loading it with high-pressure water. You have to make a bigger rocket. Oh, and one that you launch from the deep sea. Yeah. That sucks pretty much. Of course, if pollution has destroyed our ozone layer and/or magnetosphere, you might have no atmosphere left, which could make things a little easier (no air friction). Still, overcoming gravity is a b*tch with this much weight.


Well, interesting thought experiment that was. I don’t see the rise of the deep sea snailfish in our future. It could make an interesting novel, maybe, if you can think of solutions to all these problems. In any case, I thought it was an interesting thought experiment.

Martin Stellinga Written by:

I'm a science fiction and fantasy author/blogger from the Netherlands