Circadian variation in the behaviour and physiology of Bulinus tropicus (Gastropoda: Pulmonata)

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Abstract
Locomotor activity, feeding, defaecation, excretion, oviposition, and hatching have been studied over the circadian span for specimens of the tropical freshwater snail Bulinus tropicus entrained to a 8.5 h light: 15.5 h dark photoperiod. With the exception of defaecation, which is apparently arrhythmic, the activities studied followed a diurnal pattern with an approximately parallel phase relationship over the circadian period. The rhythms persist in constant conditions and periodogram and autocorrelation analysis showed the feeding and locomotor activity cycles to have an almost identical free-running period of approximately 23.5 h. In contrast to the rhythm of ammonia excretion, which is maximal early during the light or subjective light period, oviposition in B. tropicus follows a bimodal pattern and the hatching times tend to be similarly distributed throughout the day. The hatching behaviour of B. globosus is similarly bimodal, although this species shows only a single peak of egg laying at about midday. A multiple oscillator mechanism is suggested to account for the results.