Locomotor activity rhythms in cave fishes from Chapada Diamantina, northeastern Brazil (Teleostei: Siluriformes)

Abstract

Previous studies on the locomotor activity of troglobitic (exclusively subterranean) species have shown that circadian rhythmicity may be reduced in populations evolving in the absence of zeitgebers such as daily cycles of light and temperature; therefore, circadian activity rhythms, although not infradian nor ultradian rhythms, seem to have been selected by external, ecological factors. We studied the locomotor activity of a highly specialized Heptapteridae catfish (undescribed genus and species) from Chapada Diamantina, NE Brazil, compared to another specialized Brazilian troglobitic heptapterid, Taunayia sp. Locomotor activity was continuously measured in the laboratory with an infra-red photocell system. Seven specimens of the new genus were tested, each one during 14 consecutive days according to the following schedule: three days in DD → seven days in LD (12:12 h) → four days in DD. Data were submitted both to fast Fourier transform periodogram followed by Siegel's test of significance and Lombs – Scargle periodogram techniques in order to identify spectral composition of the time series. In general, results were similar to those obtained for Taunayia sp.: (a) for most specimens, absence of significant circadian components in locomotor activity under DD; (b) for all specimens, significant circadian components under LD, with higher levels of activity during the dark phase, as expected for species belonging to nocturnal epigean taxa; (c) for most specimens, no residual oscillations recorded when free-running conditions were reinstalled. Circadian locomotor activity detected under LD may thus be interpreted as a direct, masking effect of the LD cycle. This suggests a pattern for highly specialized troglobitic species, isolated for a long time in the subterranean habitat, with a progressive reduction of circadian time-keeping mechanisms. Our studies also demonstrate the potential of subterranean organisms for investigation of the origin, evolution, functioning and genetics of circadian rhthmicity.

Keywords:

• Free-running
• activity
• fish
• Brazil
• entrainment
• spectral analysis

Acknowledgements

This study was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (grants # 93/4515-0 and 96/8535-3) and by the Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPQ (ET fellowship # 306066/88-2: LMB fellowship # 304553/2002-6).