A circadian rhythm of swimming behavior in a predatory leech of the family Erpobdellidae

A wide variety of organisms exhibit circadian rhythms of motor behavior. These daily rhythms are advantageous because they allow animals to synchronize their activities with predictable changes in the environment, including daily fluctuations of light, temperature and prey availability. With the exception of earthworms, however, research on circadian rhythmicity in annelids is surprisingly limited (Brady, 1981). In this study, we used time-lapse video recordings to monitor the swimming activity patterns of Nephelopsis obscura, a freshwater predatory leech of the family Erpobdellidae. Leeches exposed to a LD 12:12 cycle swam significantly more at night than during the day (P < 0.001). Moreover, a circadian pattern of swimming behavior persisted for several days after leeches were placed in constant darkness (DD). We therefore conclude that leeches have a circadian clock influencing the expression of swimming behavior. To our knowledge, this is the first demonstration of an endogenous circadian rhythm of motor behavior in any leech species.