I've been reading a lot about circadian rhythms, when suddenly this question popped into my head: do circadian rhythms exist for deep sea organisms (3000 ft / 1000 meters+)?

For land mammals, the entrainment of circadian rhythms is done through exposure to light (retina or other photosensitive elements), but in the deep ocean there is no light. How are circadian rhythms maintained there - what acts as an entrainment signal? Is it the moon, currents, etc.?

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    $\begingroup$ This, IMHO, is an excellent question. $\endgroup$ Commented May 2, 2013 at 3:03
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    $\begingroup$ very interesting question. just a pointer: en.wikipedia.org/wiki/… plankton migrates between 100 and 1000 meters of depth, so there are regular signals communicated downwards $\endgroup$ Commented May 2, 2013 at 9:34
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    $\begingroup$ This study (sciencedirect.com/science/article/pii/S0967063707002002) shows cyclic activity of a deep-sea fish, with tidal currents as entrainment signals. $\endgroup$
    – biozic
    Commented May 2, 2013 at 17:53

1 Answer 1


A research paper that recently came out suggests that deep-sea life does have a circadian rhythm, but it is regulated much differently than it is by us surface dwellers. We see light, we eat and digest, and the suprachiasmatic nucleus in the brain (our "biological" clock) keeps track of it and eventually establishes a rhythm of the circadian variety.

Most living things at the surface have some sort of circadian rhythm - the mechanisms vary, but they are present and can be readily observed.

In the deep sea, the mechanisms are quite different since they can't see any light, but the rhythm is still established. When these animals (squid in the research) eat a certain type of bacteria containing a light organ, the presence of certain proteins within that organ actually trigger a change in the gene transcription responsible for encoding the protein controlling the circadian rhythm. Thus, when the squid eats, it ultimately determines what the rhythm will be. The entrainment itself actually happens by means of the light organs consumed in the bacteria.

As far as where the bacteria ultimately get there bioluminescence, there is still research to be done, although I would presume there is a mixture of downward bacterial migration and chemical generation contributing to this.

Link to the article: http://mbio.asm.org/content/4/2/e00167-13

  • $\begingroup$ Do you remember the name of the article or a link? $\endgroup$
    – Alex Stone
    Commented Jul 24, 2013 at 1:14
  • $\begingroup$ not off the top of my head, but I will look for it tomorrow $\endgroup$
    – mikedugan
    Commented Jul 24, 2013 at 4:01
  • $\begingroup$ Heath-Heckman, E.A., Peyer, S.M., Whistler, C.A., Apicella, M.A., Goldman, W.E. and McFall-Ngai, M.J., 2013. Bacterial bioluminescence regulates expression of a host cryptochrome gene in the squid-Vibrio symbiosis. MBio, 4(2). $\endgroup$ Commented Nov 29, 2020 at 21:55

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