SOCIAL GROUPINGS AND CIRCADIAN ACTIVITY OF THE KILLIFISH, FUNDULUS HETEROCLITUS

¹ Department of Zoology and Entomology, Colorado State University, Fort Collins, Colorado 80523

The effects of group size and intensity of constant illumination on the circadian locomotor activity of the Atlantic killifish, Fundulus 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 (r) under DD was significantly greater than that under LL (26.3 ± 0.20 hr for DD as compared to 24.6 ± 0.40 hr at 100 lux). Period lengths were significantly dependent on the intensity of constant illumination. Groups of 5 fish displayed more variable circadian activity with no significant differences in r values from DD and LL. The group of 25 fish formed a shoal and displayed precise circadian periods under DD and LL. The DD period was significantly greater than that under LL (25.7 ± 0.20 hr for DD as compared to 24.7 ± 0.20 hr for 100 lux). r also decreased with increasing intensity of illumination. 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.

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