International study on Artemia : 15. Effect of light intensity on hatching rate of Artemia cysts from different geographical origin

Arternia cysts from 4 geographical strains were incubated under standard hatching conditions at 5 different light intensities. Hatching rates increased with increasing light intensities. The light-intensity threshold at which the maximal hatching rate was attained, varled from straln to strain. Differences in light sensitivity between the Arternla strains studied can, at least partly, be attributed to variation in chorion characteristics. The hypothesis is discussed that light might act as a diapause inhibitor in marine and freshwater branchlopods. In additlon to temperature, salinity, pH, and oxygen content of the incubation medium, light conditions influence the hatching efficiency of Arternia cysts (Sorgeloos, 1980). Increased hatching efficiencies in light, as compared to controls incubated in darkness, was first described for Arternia by Sorgeloos (1973). In the meantime this phenomenon has been confirmed for the same as well as for several other strains by Meade (1976), Royan (1976), Person-Le Ruyet and Salaun (1977) and Spektorova and Syomik (1979). From a recent review on cyst hatching in Arternia (Sorgeloos, 1980), it appears that hatching rate is much more susceptible than hatching efficiency to fluctuations of temperature and salinity. As a consequence one may expect that varying light intensities also affect the hatching rate in Arternia cysts. Furthermore, since important differences have been reported for various International interdisciplinary study on Artemia strains coordinated by the Artemia Reference Center, State University of Ghent, Belgium ' ' 'Bevoegdverklaard Navorser' at the Belgian National Science Foundation (N.F.W.O.) O Inter-Research/Printed in F. R. Germany cyst-characteristics (Vanhaecke and Sorgeloos, 1980) the light effect may vary from one Arternia strain to another. We have studied the effect of varying light intensities on the hatching rate of Artemia cysts from 4 geographical strains: Buenos Aires (Argentina), Chaplin Lake (Canada), Great Salt Lake (Utah), and San Pablo Bay (California). From the latter strain both untreated and decapsulated cysts (treated according to methods described in Bruggeman et al . , 1979, 1980) have been used for experimentation. Cylindroconical hatching tubes made of glass were placed at different distances from a fluorescent light tube (Philips, 65 W) in order to expose the tubes to light intensities of 20, 100, 500, 1000 and 2000 lux, respectively. For each strain 5 replicate tubes were set up each with 250 mg cysts incubated in 100 m1 natural seawater (35 ppt) at 25 "C. The cysts were kept in suspension by gentle air-bubbling. The hatching rate was followed by subsampling at intervals of 1 h : i. e . 5 subsamples of 250 p1 each were taken with a n automatic micropipet and the number of nauplii counted. This sampling procedure was continued until the maximum hatching efficiency (determined prior to experiments, following the method of Sorgeloos et al., 1978, at 1000 lux) was attained, or until no increases in hatching efficiency were noted in 3 consecutive series of samples. The mean values for the 5 subsamples, taken every hour, were expressed a s percentage of the maximal hatching efficiency. For each light experiment hatching percentages were used to calculate nonlinear regression lines, employing the method of orthogonal polynominals. Subjecting these regression lines to a n analysis of variance revealed the best fit representing the hatching curve. 112 Mar. Ecol. Prog. Ser. 5: 111-114, 1981 The results for the 4 strains studied are graphically represented in Figure 1. As a general trend, hatching rates increase with increasing light intensities until, from a certain intensity onwards, they remain constant. Great variation is noted among the strains studied: (1) Great Salt Lake cysts, revealed minimal differences in hatching rate in different light intensities. (2) Buenos Aires and San Pablo Bay cysts exhibit, increased sensitivities for light; their light-intensity thresholds are situated between 500 and 1000, respectively, 100 and 500 lux. (3) Chaplin Lake cysts appear to be very sensitive to light; not only their hatching rate but also their hatching efficiency varies considerably as a function of experimental light intensities. (4) In San Pablo Bay cysts, chorion removal results in a drop of the light intensity threshold from 100-500 lux in untreated cysts (Fig. 1 C) to 20-100 lux in decapsulated cysts (Fig. 2). According to Sorgeloos and Persoone (1975) Artemia cysts are susceptible to light triggering as soon as they have reached full hydration under aerobic conditions. Sorgeloos and Persoone also noted that a minimal dose of light energy is needed to trigger onset of metabolism in the encysted embryo. We extrapolate from our present findings that, as illumination intensity is lowered below the critical light intensity threshold for maximal hatching rate, progressively more accumulation time is