Bioluminescence in cave glow-worms: signs of altered circadian rhythmicity
Abstract
Glow-worms emit light from cells in the malpighian tubules to attract prey into their webs. They are found in suitable wet caves as well as in rainforest settings. Forest glow-worms cease glowing on exposure to light so they glow only at night. They possess a circadian rhythm of light output, demonstrated in the laboratory through their ability to maintain cyclical glowing for many weeks in constant darkness. Because glow-worms reach high population levels in caves where they do not receive strong daily resetting stimuli, we investigated whether cave glow-worms are rhythmically bioluminescent. We developed a remote time-lapse digital imaging setup to record light output levels at 10 minute intervals for up to 5 days. Analysis of light output of the Tasmanian glow-worm, Arachnocampa tasmaniensis, and the New Zealand glow-worm, A. luminosa, in wild caves established that both species maintain strong rhythmic light output. The time of peak light output is different to forest glow-worms: cave populations glow most brightly when it is daylight outside the cave and most weakly during the night: they are completely out of phase with adjacent rainforest populations. We discuss the possible basis of the phase-shift and synchronization within caves.