Why do earthworms come up to the surface after the rain?

ResearchBlogging.orgBelieve it or not, this appears to have something to do with their circadian rhythms!
Back in the 1960s and early 1970s, there was quite a lot of research published on the circadian rhythms in earthworms, mostly by Miriam Bennett. As far as I can tell, nobody’s followed up on that work since. I know, from a trusted source, that earthworms will not run in running-wheels, believe it or not! The wheels were modified to contain a groove down the middle (so that the worm can go only in one direction and not off the wheel), the groove was covered with filter paper (to prevent the worm from escaping the groove) and the paper was kept moist with some kind of automated sprinkler system. Still, the earthworms pretty much stood still and the experiments were abandoned.
Dr.Bennett measured locomotion rhythms in other ways, as well as rhythms of oxygen consumption, light-avoidance behavior, etc. With one of my students, some years ago, I tried to use earthworms as well – we placed groups of worms in different lighting conditions (they were inside some soil, but not deep enough for them to completely avoid light) – the data were messy and inconclusive, except that worms kept in constant light all laid egg-cases and all died (evolutionary trade-off between longevity and fecundity, or just a last-ditch effort at reproduction before imminent death?). Worms in (short-day and long-day) LD cycles and in constant dark did not lay eggs and more-or-less survived a few days.
I intended to write a long post reviewing the earthworm clock literature, but that was before I got a job….perhaps one day. But the news today is that there is a new paper that suggests that clocks may have something to do with a behavior all of us have seen before: earthworms coming out to the surface during or after a rain.
In the paper, Role of diurnal rhythm of oxygen consumption in emergence from soil at night after heavy rain by earthworms, Shu-Chun Chuang and Jiun Hong Chen from the Institute of Zoology at National Taiwan University, compared responses of two different species of earthworms, one of which sufraces during rain and the other does not. They say:

Two species of earthworms were used to unravel why some earthworm species crawl out of the soil at night after heavy rain. Specimens of Amynthas gracilis, which show this behavior, were found to have poor tolerance to water immersion and a diurnal rhythm of oxygen consumption, using more oxygen at night than during the day. The other species, Pontoscolex corethrurus, survived longer under water and was never observed to crawl out of the soil after heavy rain; its oxygen consumption was not only lower than that of A. gracilis but also lacked a diurnal rhythm. Accordingly, we suggest that earthworms have at least two types of physical strategies to deal with water immersion and attendant oxygen depletion of the soil. The first is represented by A. gracilis; they crawl out of the waterlogged soil, especially at night when their oxygen consumption increases. The other strategy, shown by P. corethrurus, allows the earthworms to survive at a lower concentration of oxygen due to lower consumption; these worms can therefore remain longer in oxygen-poor conditions, and never crawl out of the soil after heavy rain.

So, one species has low oxygen consumption AND no rhythm of it. It survives fine, for a long time, when the soil is saturated with water. The other species has greater oxygen consumption and is thus more sensitive to depletion of oxygen when the ground is saturated with water. Furthermore, they also exhibit a daily rhythm of oxygen consumption – they consume more oxygen during the night than during the day. Thus, if it rains during the day, they may or may not surface, but if it rains as night they have to resurface pretty quickly.
Aydin Orstan describes the work in more detail on his blog Snail’s Tales, and he gets the hat-tip for alerting me to this paper.
Chuang, S., Chen, J.H. (2008). Role of diurnal rhythm of oxygen consumption in emergence from soil at night after heavy rain by earthworms. Invertebrate Biology, 127(1), 80-86. DOI: 10.1111/j.1744-7410.2007.00117.x


13 responses to “Why do earthworms come up to the surface after the rain?

  1. Jim Thomerson

    I read somewhere that earthworms found during the day, dead in shallow surface pools, had died of sunburn.

  2. i thought that it depends on the soil type. the more clay in the soil the less oxygen there is, so when it rains it necessitates rising to the surface for air.

  3. Related to earthworms surfacing, but not circadian rhythms, purposeful ground vibrations can also serve as a bait-gathering strategy. In fact, worm grunting as it is called is celebrated at the Sopchoppy (FL) Worm Gruntin’ Festival. (Sorry, shameless self-promotion warning – this old post of mine discusses the practice and the link to experiments by Darwin, plus a video of the technique.)

  4. Marion Delgado

    When I was a kid i was told earthworms abhor acidic pH soil and something about how the water trickled down and became slightly acidic when it reached them.
    I never gave it a 2nd thought after that. Interesting.

  5. Marion Delgado

    Actually, I think i was told later that they need air around them in the soil instead of water to breathe properly, come to think on it. What this seems to be is a differentiation of earthworms into species that are bothered by rain at nite and those that can hold their breath until they wake up?

  6. earthworms come to the surface nightly. it’s just after a heavy rain they have house cleaning to do. gotta get the drains working again…the observation of worm activity that i like the best: is when it’s been cold and for whatever reason the leaf litter is flattened to the ground, then the temperature moderates enough so that the next morning the flat litter has a new bumpy texture.

  7. jfrancishill

    a follow up on purposeful vibration of ground to gather
    worms. i always start my weedeater on the ground and one
    day after starting it i left for a few seconds and on
    return there were 15-20 worms in a three foot or so circle
    of the running weedeater. cheap(big)bait.

  8. I thought that this post was very interesting. I knew that certain animals had circadium rythms but not earth worms. And on this website I saw that earthworm’s burrows let nutrients and water into the soil. but wouldn’t that drown the earthworms?

  9. I thought that this post was very interesting. I knew that certain animals had circadium rythms but not earth worms. And on this website I saw that earthworm’s burrows let nutrients and water into the soil. but wouldn’t that drown the earthworms.

  10. This is a very interesting aricle. I read from the same website above that the worms are skin breathers and if they dry out, they will die. So, it is easier for them to absorb water when they go out of their burrows in the ground after a heavy rain and they can move around easier, too.

  11. This is a cool article. I read some information on this website: http://snailstales.blogspot.com/2008/06/why-do-earthworms-come-out-of-soil.html, That this certain question might now have just ONE answer but it may have many. Darwin states that they earthworms come up to the surface maybe because of ” the time of day”, ” the temperature”. ” the season” and etc. So, I think that there might not be one answer but many theories. I read that earthworms are “measured the survival times of worms submerged in water as well as their oxygen consumption rates. They also tested the survival of worms in soil moistened with water at different pH values or water with different concentrations of cadmium.” ( found in website)
    so I thought this was really cool! I also learned a lot of new facts about earthworms.

  12. I read on this website
    http://www.backyardnature.net/earthwrm.htm, that earth-worms play a huge part in history!! They create little holes in the ground which allow water to reach the plants. Without these little holes the ground would get to hard and the roots of the plant wouldn’t be able to grow through it. This would cause the extinction of plants, then the extinction of humans. I never knew how important earth-worms were to the World!!

  13. we did a biology experiment on earthworms today and some stupid imbeciles killed a lot earthworms , innocent earthworms , they smashed them and stepped on them it was so cruel if they only knew how important they were im getting angry just thinking about it