Spud Guns & Blind Cave Fish

Last night's episode of House is proof that the series hasn't lost an ounce of its punch. I think I counted six distinct story lines, all of them fascinating, and all woven seamlessly together. If all that weren't enough, it involved someone shooting someone with a potato cannon. That's what I call fine TV!

This brings me to tonight's topic: the sleeping habits of blind Mexican cave fish. Blind cave fish provide, perhaps, the most elegant and compelling demonstration of natural selection at work. They are very similar to species of fish that live outside the caves, except for the blindness and lack of pigmentation. They do have "scars" where once their ancestors had eyes.

Apparently, the genes coding for working eyeballs and those for pigmentation are very sensitive to mutation. Outside the caves, eyes are useful for avoiding predators and finding food and mates. Color is useful for attracting mates. Accordingly, mutations that damage eyesight and pigmentation are constantly selected out. Inside the caves eyesight and color are useless and so the mutations quickly accumulated in the species. That is almost as cool as a spud gun.

One difference between the spelunking Astyanax mexicanus and its less agoraphobic siblings is that A mexicanus doesn't get much shuteye. From Wired:

[Researchers] monitored the different species of fish under simulated 12-hour day and night cycles and found that all three populations of cave fish slept way less than the surface fish. Cavefish slept an average of 110 to 125 minutes a day, while the surface fish averaged 800 minutes.

I am not sure that "surface fish" is the right term. Siesta fish sounds more like it. The fish outside the caves are getting better than twelve hours of sleep a day. Only a beagle can compete. But how can you tell when a fish is asleep? I am interested in this because I have the same problem with students in the back row of some of my classes.

It turns out sleeping fish have a stereotypical posture: They stop moving, drop to the bottom of the tank and drop their tail. The scientists found that once a fish had been in this position for 60 seconds, it was definitely in a different state: Like rousing a sleepy teenager, it took three times as many taps on its tank to get a sleepy fish moving after that 60-second mark.

Well, yeah, that's pretty much how you can tell when I'm asleep. I drop my tail and sink to the bottom of the tank. What happens when a fish suffers from sleep deprivation?

To verify that the fish were indeed sleeping, the researchers also explored what happened when they deprived the fish of sleep. They put the fish on top of a Vortex mixer set to its gentlest setting and had it vibrate for 10 seconds out of every minute, all night long. The next day, fish of all four species were far less active. "It's like they were out at night partying," said co-author Eric Duboue. "The next day they had to sleep it off."

I was in fact subject to a similar experiment back in college. It was called a "dorm," which is derived from the Latin word for sleep and was a lie. I note that the researchers failed to consider an alternative explanation. Perhaps the fish were merely attending lectures on public administration.

Why do cave fish sleep only a couple of hours a day? Adaptation provides an answer.

At the surface, fish have to use energy to escape predators during the day, but have plenty of food available. It's the opposite in a cave, where the fish are the top predators but food is scarce. A fish that's awake more has a better chance of snagging the rare morsel as it wanders by.

Ok. The sleep patterns are probably adaptive. But how do we know that the work of adaptation is preserved at the genetic level rather than being a conditioned response to different environments?

To confirm that the differences in sleep habits they observed were genetic, the researchers bred the various cavefish populations with the surface fish. The offspring slept like cavefish, indicating a dominant gene for sleeplessness. A second generation, bred by mating that first hybrid generation, showed sleep behaviors in between the two populations. The researchers concluded that a few specific genes must be responsible for the sleep change.

That nails down the Darwinian angle. The cave fish and the siesta fish have a recent ancestor in common, which is evident from the fact that they can be bred together. Try breeding with a hamster. That was a rhetorical suggestion. The results of cross breeding show that the sleep patterns are heritable. Natural selection alone explains the synergy between inheritance and adaptation.

The Wired article goes out of the way to suggest that this can offer hope to human beings who can't sleep even when they aren't sleeping atop a blender. I think that discovery alone is good enough.

Can Darwinian natural selection explain all the differences between sleepy fish and sleep college students? No. It can only explain those that are genetic in origin. That, however, turns out to count for a lot. Now, can we get the fish into a potato gun?

Comments

[Miranda]

Interesting! How do the eyeless fish find mates? Since eyed and eyeless fish can mate - do eyeless fish with bright colors win a greater number of mates than pale eyed fish?

[KB]

Not being an expert, exactly, I would guess that cave fish find mates and food through hearing and smelling. I don't know what the obstacles are to mating surface fish with cave fish. Probably you have to convince the surface fish that the cave fish has money.

[duggersd]

It is simple. You breed the MALE surface fish with a FEMALE cave fish and only provide the ugly cave fish to the males. Anybody knows they guy is not going to care about the looks if all he has is the plain ones to mate with. Men are pigs! We just want some action.