What are the defining characteristics of fresh-water fish? Go ahead, think about it, I’ll wait right here…Done? Have a good head-scratcher about this? So, what did you come up with? Probably things like “relatively small,” “muted colors,” “easier to bread and fry.” Stuff like that.
This here’s an arapaima, also called pirarucu, and it is one of the largest freshwater fish in the world, capable of growing to about 15 feet long. Shaped like a torpedo, the arapaima has an infamous habit of leaping out of the water when in danger–as if being larger than the local crocodilians wasn’t already a safe guard–which can present a real hazard to people who accidentally hook these things while fishing. Their scales are highly mineralized, with a corrugated surface and underlying layers of collagen fibers arranged at right angles to each other. Translation: these things are both f**king flexible and f**king tough.
But even having partially metallic scales isn’t enough to get you on one of my evolution posts (scorpions transmit minerals they absorb from the insects they eat into their stingers to give them more punch), you gotta have something few others creatures in your corner have…like the fact that arapaimas breathe air.
We’ve all heard of lungfish (if you haven’t, you can always sign up at the college) which are fish capable of obtaining oxygen from the air, as well as the water. It isn’t even that unusual, electric eels (which are actually more closely related to catfish) and mudskippers do it all the time, the rub being that they must respire through the water regularly. The arapaima’s swim bladder is modified to act as a lung, and is so effective that arapaima must return to the surface every 5 to 15 minutes, these fish can, and do, drown.
Let’s say that again, the arapaima is a fish that can drown, which, as you may well be aware, is something fish are not supposed to do!
Where is the logic or intelligence in that design? It serves a blatant evolutionary purpose, the Amazon River (where the arapaima lives) can become poorly oxygenated regularly as a result of the wet season when it overflows with silt, becoming highly stagnant. As a result, creatures like the arapaima needed to adapt and find new ways of surviving, but the key difference here is that other air-breathing fish can hypothetically survive without returning to the surface, whereas the arapaima would drown.