Bioluminescence in cave glow-worms: do cave tours have an effect?
Email: 1 akclarke@utas.edu.au 2 d.merritt@uq.edu.au
Abstract
Glow-worm larvae emit light to attract prey into their sticky silken web threads. Glow-worms are found in suitable wet caves as well as in rainforest settings. In wild caves of Tasmania and New Zealand, glow-worms (Arachnocampa tasmaniensis and A. luminosa, respectively) maintain synchronised 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 show that cave glow-worms synchronise their bioluminescence by detecting and matching each others’ glows. Placing artificial lights in caves causes glow-worms to synchronise to the imposed light cycle because they interpret the light as coming from other glow-worms.
From studies in wild caves at Ida Bay, we also show that where A. tasmaniensis are located in the cave mouth, experiencing cycles of daylight and darkness (light:dark), they still possess an underlying rhythmic tendency to glow most brightly during the day, just like their counterparts deeper in the cave. When cave mouth larvae are placed in constant darkness in the laboratory, they show a free-running endogenous rhythm with a peak occurring during the daylight hours. We conclude that this underlying rhythmicity is modulated by exposure to natural light, inhibiting the glowing during daylight hours. The accumulation of drive during the inhibitory period causes the glow-worms to release their light most intensely just after dark and progressively through the night.
In this regard, the rhythm of bioluminescence in A. tasmaniensis is completely different to that of the Australian mainland rainforest species, A. flava. The bioluminescence of A. flava has an underlying rhythmicity that promotes maximum bioluminescence during the night. Natural light inhibits bioluminescence — just as it does in A. tasmaniensis — but the inhibitory period coincides with the trough phase of the underlying rhythm of A. flava. We suggest that A. tasmaniensis shows this unusual rhythmic pattern because it is adapted as a cave-dweller, while A. flava is not.
We demonstrate that in Marakoopa Cave at Mole Creek, the timing of show cave tours happens to coincide with the brightest component of the natural bioluminescence cycle. Further, the artificial lighting in the main glow-worm viewing chamber above Zambezi Falls, does not substantially affect the display. The lack of effect of cave lighting on the bioluminescence intensity is readily explained in terms of the experimental results on the bioluminescence rhythm.