High variability in the expression of circadian rhythms in a cave beetle population

Abstract

Cave-dwelling organisms belong to widely different evolutionary lineages but they show parallel or convergent evolution in several morphological and physiological traits such as elongated appendages, lower metabolism, specialized sensory systems, loss of eye and pigmentation. Circadian rhythms in locomotor activity have already been documented in hypogean insects but ultradian rhythmicity in cave-dwelling organisms has been reported only by recent authors. We selected Laemostenus latialis as a good candidate to investigate the biological clock functioning in a species thought to be at its beginning of the process of cave evolution in the darkness. We based our analysis on inferential methods used here to obtain the quantitative resolution of the rhythm features. An automatic electronic device was developed to this purpose so to continuously monitoring the general locomotor activity of individual cave-dwelling beetles. We found significant locomotor rhythms to be present and that Laemostenus maintained a 24-h circadian rhythm but with a different amplitude. We observed two different and opposite behavioural patterns: the “Rhythmic group” showed a typical circadian rhythmicity of surface-dwelling animals with a surprisingly and extremely stable circadian period and with a restricted variance and a high amplitude. Differently, within the “arrhythmic group” we have observed an evanescent circadian period with high variability in tau, a larger variance and a lower amplitude. DFT spectral analysis showed the presence of ultradian rhythms with several significant peaks in the range 1–10 h and a main peak at 12 h, similar to those found in mysid crustaceans, cockroaches and mice. Ultradian rhythm looked independent from the circadian one and we did not observe any regularity. Further experimental studies are still needed to track the variation of this trait along the process of adaptation to cave life, particularly in closely related populations and species of troglophilic animals.