Shedding Light on the Threespine Stickleback Circadian Clock

The circadian clock is an internal timekeeping system shared by most organisms, and knowledge about its functional importance and evolution in natural environments is still needed. Here, we investigated the circadian clock of wild-caught threespine sticklebacks (Gasterosteus aculeatus) at the behavioural and molecular levels. While their behaviour, ecology, and evolution are well studied, information on their circadian rhythms are scarce. We quantified the daily locomotor activity rhythm under a light-dark cycle (LD) and under constant darkness (DD). Under LD, all fish exhibited significant daily rhythmicity, while under DD, only 18% of individuals remained rhythmic. This interindividual variation suggests that the circadian clock controls activity only in certain individuals. Moreover, under LD, some fish were almost exclusively nocturnal, while others were active around the clock. Furthermore, the most nocturnal fish were also the least active. These results suggest that light masks activity more strongly in some individuals than others. Finally, we quantified the expression of five clock genes in the brain of sticklebacks under DD using qPCR. We did not detect circadian rhythmicity, which could either indicate that the clock molecular oscillator is highly light-dependent, or that there was an oscillation but that we were unable to detect it. Overall, our study suggests that a strong circadian control on behavioural rhythms may not necessarily be advantageous in a natural population of sticklebacks and that the daily phase of activity varies greatly between individuals because of a differential masking effect of light.

Survival and growth of Pangasianodon hypophthalmus cultured under controlled photoperiod

Changing in photoperiod duration may affects the physiology of nocturnal fish such as Pangasianodon hypophthalmus. A study aims to understand the effects of controlled photoperiod towards survival and growth of P. hypophthalmus has been conducted from June to August 2020. There were 3 treatments applied, namely natural photoperiod, 18 hours dark (18D), and 24 hours dark (24D) with 3 replications in each treatment. The rearing tanks used in this study were 100 L circular plastic tanks. In 24D treatment, the tanks were placed under dark colored tarp tent continuously. For the 18D treatment, the tanks were placed under dark tarp tent, but the tent was opened for 6 hours/ day (the tanks were in dark condition for 18 hours/ day), while the control tanks were positioned under natural photoperiod. P. hypophthalmus fingerlings, 6-8 cm TL and 4-5 g BW were used in this study. Thirty fishes were reared in each rearing tank, they were feed with commercial pellets, 2 times/day, at satiation. Fish survival was monitored every day, while samplings for fish growth were conducted weekly for a 8 weeks period. Results indicate that the survival of fish was 100% in each treatment applied. Fish growth, however, shown differences. The growth of fish reared in 24D and 18D was better than that of the control. By the 9th week, the fish in 24D was around 70.71g BW with 19.27 cm TL (daily growth rate 9.35%), while those of the 18D was 69.41 g BW, 18.77 cm TL and 9.29% daily growth rate. The fish reared under natural photoperiod was around 61.95 g BW with 18.19 cm TL and 7.33% of daily growth rate. Data obtained indicate that the application of longer dark is positively improve the growth of P. hypophthalmus.Keywords:Nocturnal FishLight Dark Catfish

Controlled iris radiance in a diurnal fish looking at prey

Active sensing using light, or active photolocation, is only known from deep sea and nocturnal fish with chemiluminescent ‘search’ lights. Bright irides in diurnal fish species have recently been proposed as a potential analogue. Here, we contribute to this discussion by testing whether iris radiance is actively modulated. The focus is on behaviourally controlled iris reflections, called ‘ocular sparks’. The triplefin Tripterygion delaisi can alternate between red and blue ocular sparks, allowing us to test the prediction that spark frequency and hue depend on background hue and prey presence. In a first experiment, we found that blue ocular sparks were significantly more often ‘on’ against red backgrounds, and red ocular sparks against blue backgrounds, particularly when copepods were present. A second experiment tested whether hungry fish showed more ocular sparks, which was not the case. However, background hue once more resulted in a significant differential use of ocular sparks. We conclude that iris radiance through ocular sparks in T. delaisi is not a side effect of eye movement, but adaptively modulated in response to the context under which prey are detected. We discuss the possible alternative functions of ocular sparks, including an as yet speculative role in active photolocation.

Controlled iris radiance in a diurnal fish looking at prey

1.SummaryActive sensing using light, or active photolocation, is only known from deep sea and nocturnal fish with chemiluminescent “search” lights. Bright irides in diurnal fish species have recently been proposed as a potential analogue. Here, we contribute to this discussion by testing whether iris radiance is actively modulated. The focus is on behaviourally controlled iris reflections, called “ocular sparks”. The triplefin Tripterygion delaisi can alternate between red and blue ocular sparks, allowing us to test the prediction that spark frequency and hue depend on background hue and prey presence. In a first experiment, we found that blue ocular sparks were significantly more often “on” against red backgrounds, and red ocular sparks against blue backgrounds, particularly when copepods were present. A second experiment tested whether hungry fish showed more ocular sparks, which was not the case. Again, background hue resulted in differential use of ocular spark types. We conclude that iris radiance through ocular sparks in T. delaisi is not a side effect of eye movement, but adaptively modulated in response to the context under which prey are detected. We discuss the possible alternative functions of ocular sparks, including an as yet speculative role in active photolocation.