Diel activity rhythms and orientation of sea catfish (Arius felis) under constant conditions of light and darkness

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Abstract
Three groups of sea catfish (A. felis) were entrained to an artificial, exogenously controlled light-dark (LD) 12:12 photoperiod. The fish were then individually monitored in a 16-choice, compartmentalized rosette tank for 72 h under either constant darkness (DD), constant, non-directional light (LL) or the entraining photoperiod (LD) depending on the experimental group. The locomotor activity and angular orientation of the fish in each group were then statistically compared. Fish monitored under DD and LL conditions generally failed to maintain the nocturnal activity pattern of the species, although both groups continued to to exhibit the typical relative maxima of activity between 0600-0900 h and 1300-1500 h CST daily. Fish in the DD group were significantly more active overall (as measured by the number of events per hour) than those in the LL and LD groups, except for the first 5 h of the record, although LD group fish were significantly more active during scotophase. The LD group fish were significantly more active, overall, than those monitored in continuous light, although there was no significant difference in activity between these 2 groups during the 1st 12 h of each 24-h period. Neither experimental group (LL and DD) exhibited the dramatic increase in activity accompanying the transition from photophase to scotophase seen in the LD and DD groups was not significantly different. That of the LL group fish differed significantly from the other groups, exhibiting a significant, gradual shift toward angles of smaller size. This shift in turning angles occurred 6-10 h after the transition from photophase to scotophase should have occurred. Possible explanations of this behavior are discussed in terms of a visual control mechanism of orientation, shelter-seeking by the LL group fish or an hypothesized imbalance of the levels of serotonin in the brains of these animals due to the inhibition of melatonin synthesis under conditions of continuous illumination. At least 2 distinct control mechanisms governing the locomotion and orientation of sea catfish that are light-regulated (directly or indirectly) are indicated; the length of the photoperiod (angular orientation) or the transition from light to darkness (diel activity).