Category Archives: Clock Zoo

Rainforest Glow-worms glow at night because their clock says so

ResearchBlogging.orgGlow worms glimmer on cue:

University of Queensland researcher and lecturer Dr David Merritt has discovered that Tasmanian cave glow-worms are energy conservationists: they switch their lights off at night-time.
The discovery was made during a partially funded UQ Firstlink study, which revealed that the glow-worm’s prey-luring light output is governed by circadian rhythms, regardless of ambient light levels.
The study aimed to investigate the physiology and behaviours of cave dwelling glow-worms, which are actually the immature or larval stage of a mosquito-like fly found in Queensland, New South Wales, Victoria, Tasmania and New Zealand.
The study’s leader, Dr Merritt, says that unlike their rainforest dwelling counterparts, the cave-dwelling Tasmanian glow-worm can detect the time of day, even from the deepest stretches of their caves.

Circadian Regulation of Bioluminescence in the Prey-Luring Glowworm, Arachnocampa flava, by David J. Merritt and Sakiko Aotani, Journal of Biological Rhythms, Vol. 23, No. 4, 319-329 (2008), DOI: 10.1177/0748730408320263

The glowworms of New Zealand and Australia are bioluminescent fly larvae that generate light to attract prey into their webs. Some species inhabit the constant darkness of caves as well as the dim, natural photophase of rain-forests. Given the diversity of light regimens experienced by glowworms in their natural environment, true circadian rhythmicity of light output could be present. Consequently the light emission characteristics of the Australian subtropical species Arachnocampa flava, both in their natural rainforest habitat and in artificial conditions in the laboratory, were established. Larvae were taken from rainforest and kept alive in individual containers. When placed in constant darkness (DD) in the laboratory they maintained free-running, cyclical light output for at least 28 days, indicating that light output is regulated by an endogenous rhythm. The characteristics of the light emission changed in DD: individuals showed an increase in the time spent glowing per day and a reduction in the maximum light output. Most individuals show a free-running period greater than 24 h. Manipulation of the photophase and exposure to skeleton photoperiods showed that light acts as both a masking and an entraining agent and suggests that the underlying circadian rhythm is sinusoidal in the absence of light-based masking. Manipulation of thermoperiod in DD showed that temperature cycles are an alternative entraining agent. Exposure to a period of daily feeding in DD failed to entrain the rhythm in the laboratory. The endogenous regulation of luminescence poses questions about periodicity and synchronization of bioluminescence in cave glowworms.

Gotta love a paper in which Drosophila is used only as food for the organism under study (for the food-entrainment experiment)! Reminds me of the old departmental games of “my organism eats yours” back in grad school.
Anyway, all of the experiments in this paper were done on rainforest glow-worms, not the cave-dwelling ones. And as far as I know this is the first attempt to do any chronobiological studies on this organism, so the authors did the logical thing and performed a standard battery of tests in the lab: monitoring the glowing intensity rhythms in constant darkness (showing that the rhythm is driven endogenously, by an internal clock) and in light-dark cycles (showing that the rhythm is entrainable by light and with what phase, i.e., that the insect larvae are nocturnal, although the cave animals glow while it is light outside):
glow-worm.JPG
In addition, since they are interested in cave-dwelling organisms, they tested the ability of temperature cycles fo entrain the rhythm (it worked) as well as scheduled feeding times (this did not work).
But the impetus for the work, unlike what the media article suggests (tourism!), is evolutionary:

We conclude that glowworms exhibit true circadian
regulation of their light output. Light acts as both an
entraining agent and a masking agent. The dominant
role of light in establishing the characteristics of the
light output rhythm raises questions about the rhythmicity
and period of bioluminescence within caves
where glowworms have never been exposed to daylight.
A number of species such as A. luminosa from
New Zealand and A. tasmaniensis from Tasmania,
Australia, have large populations in caves as well as in
rainforest. Based on laboratory analyses of A. flava,
glowworms in caves would either be arrhythmic
because they have never been exposed to photic
entrainment cues, or would be rhythmic but individuals
in a colony would be asynchronous because they
have different free-running periods. It will be of interest
to establish the rhythmicity and phase of luminescence
in cave-dwelling glowworm populations. The
fact that members of the genus Arachnocampa inhabit
both photoperiodic and aphotoperiodic habitats
makes them ideal for examination of the retention of
circadian rhythmicity in cave environments where
very few circadian cues are present.

So, I expect that the authors will next attempt a comparative study – pitting the rainforest and cave-dwelling populations of the same species directly against each other in a similar battery of experiments. I am looking forward to seeing the results.
Merritt, D.J., Aotani, S. (2008). Circadian Regulation of Bioluminescence in the Prey-Luring Glowworm, Arachnocampa flava. Journal of Biological Rhythms, 23(4), 319-329. DOI: 10.1177/0748730408320263

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

Clocks in Bacteria V: How about E.coli?

Clocks in Bacteria V: How about E.coli?Fifth in the five-part series on clocks in bacteria, covering more politics than biology (from May 17, 2006):

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Clocks in Bacteria IV: Clocks in other bacteria

Clocks in Bacteria IV: Clocks in other bacteriaFourth in the five-part series on clocks in bacteria (from April 30, 2006):

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Clocks in Bacteria III: Evolution of Clocks in Cyanobacteria

Clocks in Bacteria III: Evolution of Clocks in CyanobacteriaThe third installment in the five-part series on clocks in bacteria (from April 19, 2006):

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Clocks in Bacteria II: Adaptive Function of Clocks in Cyanobacteria

Clocks in Bacteria II: Adaptive Function of Clocks in CyanobacteriaSecond post in a series of five (from April 05, 2006):

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Clocks in Bacteria I: Synechococcus elongatus

Clocks in Bacteria I: Synechococcus elongatus
First in a series of five posts on clocks in bacteria (from March 08, 2006)…

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