SOCIAL GROUPINGS AND CIRCADIAN ACTIVITY OF THE KILLIFISH,FUNDULUS HETEROCLITUS

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Abstract
The effects of group size and intensity of constant illumination on the circadian locomotor activity of the killifish, F. heteroclitus, were examined. Individual fish displayed a free-running circadian rhythm of locomotor activity under constant darkness (DD) and constant illumination (LL). The period length under DD was significantly greater than that under LL, with period length also being dependent on the intensity of LL. Groups of 5 fish displayed a more variable circadian activity than single fish, with no significant differences in period length between DD and LL. The group of 25 fish formed a shoal and displayed precise circadian periods under DD and LL, with period length dependent on the intensity of illumination. The circadian period lengths and relations to LL of shoaling fish were significantly different from those of individual fish. The circadian activity of a single fish cannot be considered as equivalent to, or representative of, an individual in a group or shoal of fish. Social groupings affect both the circadian activity and period length of killifish.