Internal clocks, or circadian rhythms, are nearly ubiquitous across taxa (e.g., animals, plants, fungi, and cyanobacteria), and it is widely believed that a biological clock benefits organisms by enabling them to schedule behavioral and physiological changes in anticipation of predictable changes in environmental conditions. Theory and evidence suggest it is important that the internal clock resonate closely with the 24-h daily cycle. Recently, however, Cyclosa turbinata (Walckenaer, 1841) (Araneidae) was revealed to have a circadian clock with a period of about 19 h, which was presumed to be anomalous. Here, we report on the behavioral rhythms of a nocturnal orbweaver, Metazygia wittfeldae (McCook, 1894), from the same family. Under laboratory conditions of a 12:12 h light:dark cycle, we found that locomotor activity initiates shortly after dark, reaching a peak early in the dark phase, continuing at a lower level throughout the remaining dark phase, and then diminishing shortly after lights-on. Locomotor activity continued to cycle under constant dark conditions with a mean free-running period of 22.7 h. We also found a second component in the free-running activity (mean 11.5 h) which correlated very tightly with the free-running period. Thus, M. wittfeldae has what can be considered a typical circadian clock resonating with the 24-h day. Notably, however, there were two outliers close to the 19-h period observed in C. turbinata, suggesting that there may be sufficient variation in clock period among araneid spiders upon which selection could act leading to the short-period clocks in C. turbinata.
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Vol. 46 • No. 1