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):


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

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Thanks for finding our work interesting enough to comment on! Your summary is perfect: better than I could do. At the time the Journal of Biological Rhythms paper was submitted, we already had some evidence that cave glow-worms cycle their light but we didn't have enough observations to be sure enough to publish. So in July, I revisited Tasmania with my caving buddies, Arthur Clarke and James Young, and set up time-lapse camera equipment in several different caves. We confirmed that cave and forest glow-worms' rhythms are out of sync. (We also got bogged several times, got really cold, wet and muddy, and just missed hitting a kangaroo one night on the road.) The big question that lies ahead of us is: What is the entraining stimulus? Cave temperatures are notoriously constant, so we will have to look at other possible cues. I can't wait to visit Tasmania again in January to do some more observations. Hopefully we will manipulate the cave glow-worms' rhythms by imposing an artificial light:dark cycle for a few days. I'll keep you posted.