Reference: Biol. Bull.. 158: 69-76. (February, 1980) SOCIAL GROUPINGS AND ORCADIAN ACTIVITY OF THE KILLIFISH, FUNDULUS HETEKOCLITUS MARTIN KAVALIERS Department of Zooloyy and Entonwloijy, Colorado State University, Fort Collins, Colorado 80523 Until relatively recently little was known about the characteristics and controls of circadian rhythms of fishes (Schwassman, 1971a; Thorpe, 1978). Investigations with a number of Arctic species have revealed that the circadian system of fishes is highly labile, adapting to the prevailing environmental conditions (Eriksson, 1978; Miiller, 1978a). Studies from more temperate regions have shown that seasonal differences exist in the length of the free running circadian period of the locomotor activity of cyprinid fish (Kavaliers, 1978). The majority of those studies have dealt with the circadian activity of single fish recorded primarily under constant darkness. However, in the wild many fishes are normally found in groups ranging from aggregates to shoals and schools. What effect group size has on the circadian activity and period of fish and other verte-brates is not well known (Aschoff, 1979). In a brief report Siegmund and Wolff (1973) did suggest that both intensity of illumination and group formation can affect the circadian activity of the European minnow, Leucaspius delineatus. The Atlantic killifish, Fnndnhts hctcroclitus, a species of fish that is widely used in physiological and behavioral research, shows evidence of endogenous circadian rhythms. Killifish display a circadian rhythm of color change under constant illumination (Kavaliers and Abbott, 1977). Davis and Bardach (1965) demonstrated a "time coordinated pre-feeding activity rhythm" in killifish that they suggested was determined by an endogenous circadian rhythm. Killifish also occur in shoals and aggregates and thus are an ideal species in which to examine the effects of social grouping on circadian rhythmicity. A shoal is considered to be a facultative social group of fish within which a variety of behavior takes place (Pitcher, 1979). The present study investigates the effects of group size as exem-plified by single killifish, aggregates of five fish, and shoals of 25 fish on circadian activity and period length. MATERIALS AND METHODS Killifish were obtained from the Marine Biological Laboratory, Woods Hole, Mass., and placed in 120-liter opaque plastic tanks under a 12-hour light 12-hour dark cycle (LD 12: 12) at 20 C (2 C) and 20 ( / lt . salinity. At a random time of day fish were fed a commercial fish food (Purina Trout Chow, St. Louis, Mo.). Activity determinations were carried out in shielded, opaque, polyethylene tanks (125 X 165 X 145 cm) with a gravel substrate and a glass stand-pipe at a water depth of 90 cm. A constant water flow (15 ml/min) at 20 C (2 C) and 20%c was provided by a regulated water supply. Illumination was provided by overhead fluorescent tubes (120 W Sylvania Gro-lite) and incandescent lights (45 W Sylvania), whose intensity could be controlled by neutral density filters (Kodak W ratten) and dimming controls. 69
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