Evolution stole this dude’s circadian rhythm

I got a laugh out of this one; not for the actual content, but because of how the authors worded their findings.  They sure love their fishies.

We have two very closely related fish, both Mexican tetra, Astyanax mexicanus, one with eyes who lives on the surface, and another who’s blind and lives in dark caves (“Pachon”).  It’s thought that they were the same species one day; divergent evolution.


note: eyeless

note: eyeless

The blind ones are circadian arrhythmic (Moran et al., 2014).  Surface-dwellers are more active during the day than night (blue line, left figure below), and their free-running circadian clock maintains this in the absence of photic input (blue line, right figure).  The blind ones, on the other hand, exhibit no circadian rhythm in the light or dark (orange lines):


Circadian rhythm metabolism


Cave-dwellers are circadian arrhythmic.  This is both in their natural photoperiod (ie, darkness) and in light-dark conditions (which is technically an environmental mismatch, but since they’re eyeless, it doesn’t really matter).

Admittedly, I might be missing something big about these fish, but I had to chuckle when the authors kept referring to circadian arrhythmia like it was a good thing

Eliminating circadian rhythm


Their circadian rhythm in metabolism was eliminated and they stopped evolving.

“Impressive?”  Hmmmmmm… I don’t really have any keen insight into these fish, but to be able to spin their findings so well, these authors must be poets.

The cave-dwellers lack eyes, and as such, don’t respond to photic inputs.  They probably went blind because one day, a few surface-dwellers got sucked into a deep dark cave, and they just kept on keepin’ on.  Eyes are expensive, and if you don’t need ‘em, no selective pressure to keep ‘em… right?  Maybe it’s just better to avoid getting sucked into a deep dark cave.


Study #2: Circadian arrhythmic Blind Cavefish are fasting-intolerant (Salin et al., 2010).

Epigean = surface-dweller

Hypogean = cave-dweller, “Pachon”

Starving fishies: physical activity declines more rapidly and robustly in cave-dwellers.  Worded another way,
circadian arrhythmia: remove constant food supply = life fail.


Physical activity during starvation

From the above experiment, we learned that cave-dwellers expend less energy than their circadian-normal counterparts.  That’s also confirmed here; but this study added that it’s not due to reduced physical activity…
circadian arrhythmia: exercise more, burn less.
Sound familiar?


Oxygen consumption during starvation


Anyway, during starvation, they seem to wail through fuel stores, spill ketones out the wazoo, and if it weren’t for severe hypometabolism, they’d probably die quite soon… despite this, they don’t lose any more weight than controls.  What they did lose, was muscle (or technically “total body protein,” because I’m pretty sure they didn’t use some elaborate technique to assess body composition: fishies probably just thrown in a blender and total protein content of the fish slushy measured).

Anyway, surface-dwellers maintained body protein during starvation, whereas cave-dwellers wasted away.  This is not due to a hypermetabolic state as they clearly efficiently suppressed their metabolism.

Researchers really love these fish!  They try to make everything about them sound like a good thing…

Glycogen starvation

The circadian arrhythmic cave-dwellers lost more muscle than surface-dwellers.  This does not translate to an enhanced ability to “withstand long-term starvation.”

Alas, they finally concede:

Poor survival during starvation


Circadian mismatch, for whatever reason, has profound consequences for metabolic health.  See Nedeltcheva’s study:
sleep restricted dieters lost more muscle & less fat than controls.
See mice with an adipose-specific deletion of the circadian transcription factor Bmal1:
exercise more, burn less,
just like people who’ve just lost a lot of weight.  And now these arrhythmic fishies: same as the above, and also
muscle wasting.


calories proper

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  • Tuck

    OK, color me surprised!
    Life must be pretty good in those caves, if their ability to survive starvation has atrophied…

    • http://www.caloriesproper.com/ William Lagakos

      Ha! yeah, some have speculated as much

      but life is pretty good for us humans nowadays, and we can survive starvation, probably better than ever before!

      • Cheesy

        mmm, the fact that we are still around as a species shows that we have been already pretty good BEFORE :) What makes youthink that we are better than ever before ? What with all the chronically sick people around ? people in need of daily medication ? etc … I don’t know, I can’t see how that pans out …

        • http://www.caloriesproper.com/ Bill Lagakos

          Cahill has speculated (and I agree) that adiposity determines, in part, how long an animal can survive during starvation… and in the context of Tucker’s comment, we have no shortage of adiposity.

          People can still take their meds, water, vits & mins, etc.; this was specifically referring to caloric starvation.

          • Cheesy

            I see, like this fat dude that did not eat for more than a year to lose weight … in that respect, you are 100% right :) I would not starve for very long before I collapse …

          • This Old Housewife

            After witnessing the home hospice death of my M-I-L, I can say it takes about two weeks for someone to die with absolutely NO inputs (food, water). The body starts feeding on itself, and vital organs get consumed until the kidneys, liver, or heart finally shut down. I’m sure there’s a lot more biochemical stuff going on besides (ammonia levels, etc.), but I’m not familiar enough with them to describe accurately.

  • fredt

    “and they stopped evolving.” – and how can anyone make this claim?

    • http://www.caloriesproper.com/ Bill Lagakos

      They certainly haven’t “stopped evolving” entirely! There was a bit of sarcasm :-)

      …the statement was specifically in reference to this quote: “…this species has not needed to undergo the more radical physiological rearrangements typical of animals that have diversified into food-limited environments.”

      it was really just to show that the authors may have been trying to put a way-too-positive spin on the phenotype of these fish

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  • http://www.caloriesproper.com/ Bill Lagakos

    Centipede Strigamia maritima genome contains no circadian rhythm genes reut.rs/1y1M77k

  • Andrew Beale

    I think you’ve slightly misinterpreted the Moran et al study – the cavefish have only lost a circadian rhythm in metabolism, not a circadian rhythm per se (see Beale et al 2013 to see circadian rhythms in gene expression in these fishes http://www.nature.com/ncomms/2013/131114/ncomms3769/full/ncomms3769.html). The loss of the link between the core clock and the output rhythm of metabolism is where evolution has acted in Moran’s study – but they’re not sure how. Their speculation is that losing the link allows energy savings which is actually really interesting evolutionarily.

    • http://www.caloriesproper.com/ Bill Lagakos

      “energy savings” = interesting evolutionarily, but in modern context, this roughly translates to “exercise more, burn less” … maybe good for starving fish, but not for the obesity epidemic.

      Thanks for the link! Interesting –

      Circadian rhythms in Mexican blind cavefishAstyanax mexicanus in the lab and in the field

      Biological clocks have evolved as an adaptation to life on a rhythmic planet, synchronising physiological processes to the environmental light–dark cycle. Here we examine circadian clock function in Mexican blind cavefish Astyanax mexicanus and its surface counterpart. In the lab, adult surface fish show robust circadian rhythms in per1, which are retained in cave populations, but with substantial alterations. These changes may be due to increased levels of light-inducible genes in cavefish, including clock repressor per2. From a molecular standpoint, cavefish appear as if they experience ‘constant light’ rather than perpetual darkness. Micos River samples show similar per1 oscillations to those in the lab. However, data from Chica Cave shows complete repression of clock function, while expression of several light-responsive genes is raised, including DNA repair genes. We propose that altered expression of light-inducible genes provides a selective advantage to cavefish at the expense of a damped circadian oscillator.”