The relationship between locomotor activity rhythm and burying behavior in the wrasse,Suezichthys gracilis

The wrasse,Suezichthys gracilis, is a diurnal fish which buries itself in sand during the night-time. The present paper deals with the locomotor activity rhythms ofS. gracilis, examined by using an actograph with infra-red photo-electric switches in a dark room. The fish were kept in eight experimental tanks (each 30l in capacity), with three different bottom conditions: sand (grain size about 1 mm in diameter and 5 cm deep); 1 or 2 stones (about 10cm in diameter) without sand; and transparent acrylic pellets (2 × 2 × 3 mm in size, 5 cm deep). The light intensities were 550–700 lux just above the water surface, decreasing to 21.3% under the acrylic pellets at a water depth of 20cm. The water temperatures were kept at 22.0–25.0°C during the experiments for 7 to 14 days. In the aquarium with bottom sand, diel activity rhythms ofS. gracilis were mostly synchronized to LD (LD12:12; 06:00–18:00 light, 18:00–06:00 dark), free-running activity rhythms continued distinctly under LL (constant illumination), and locomotor activity was greatly suppressed, with disappearance of the activity rhythm, under DD (constant darkness). In the aquarium without sand, locomotor activity ofS. gracilis could be summarized as follows. The fish moved throughout almost the entire period under LD, though more frequent movements were observed in light conditions than in dark ones. Under LL they showed continuous locomotor activity during the experiment, with no obvious periodicity. Under DD the activity of the species was somewhat suppressed, but irregular movement or indistinct periodicity was observed. In the aquarium with transparent acrylic pellets, locomotor activity under LD and DD, respectively, bore a close resemblance to activity patterns under the same light conditions with sand, whilst activity under LL was identical to that under LL without sand. Accordingly, it seems that maintenance of normal activity rhythms in the wrasse was due not only to the darkness, but also to the presence of bottom sand. It therefore seems that the biological clock inS. gracilis is not related to locomotor activity, but to burying behavior.