Locomotor activity pattern of Brazilian cave catfishes under constant darkness (Siluriformes, Pimelodidae)

Abstract

Troglobites ‐ cave restricted species ‐ are good models for chronobiological studies focusing on the functions of circadian rhythmicity since they provide a test for hypotheses on external versus internal selection. Cases of reduction in the circadian components of activity have been reported for cave fishes, beetles, and crustaceans, but the methods of data analysis are not fully comparable. We present the results of spectral analysis of locomotor activity in two species of troglobitic catfishes: the less specialized Pimelodella kronei (compared to its eyed close relative, P. transitoria) and a highly specialized Imparfinis, undescribed species. The activity of seven specimens off. transitoria, nine of P. kronei, and six of Imparfinis sp. was automatically recorded in an infra‐red photocell device, for four to ten consecutive days, under constant darkness. All the eyed catfishes showed significant free‐running circadian components, as did seven P. kronei and three Imparfinis specimens. Variable ultradian and infradian rhythms were detected. These data suggest a regression of circadian rhythmicity in part of the blind populations. Some hypotheses may account for the persistence of circadian rhythms in these cave fishes: 1) Evolutionary time in isolation has not been sufficient to genetically fix this regression throughout the population (specially for P. kronei); 2) “Zeitgebers” such as temperature oscillations (for P. kronei) and light‐dark cycles (for Imparfinis sp.) are acting over part of these populations; 3) It is the expression of the selective value of a mechanism insuring periodic movements to and from the cave (for P. kronei); 4) Circadian rhythmicity has been preserved due to its role in the maintenance of internal temporal order.

Abstracting keywords:

• Circadian rhythm
• locomotor activity
• free‐running
• caves
• catfishes
• Pimelodidae

Notes

Departamento Zoologia, Instituto de Biociências da USP, Caixa Postal 11294, 05422–970, São Paulo, Brasil

GMDRB, Departamento de Fisiologia e Biofísica, Instituto de Ciências Biomédicas da USP, São Paulo, Brasil