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BIOL 140
Chin Sun

Su Thazin Thin A3 Lina Castro Yasaman Osfogh Site Selection Of Artemia Salina Upon Stimulus Of High Light Intensity Abstract Artemia Salina is known to demonstrate negative phototaxis behaviour when exposed to high light intensity above the threshold level. We attempted to determine the threshold range by exposing the replicates to varying intensities of light. A lamp and photometer were used and the replicates were placed in a petri dish covered with a light palette partitioned into three sections of light,medium, and dark.The preference of site was determined by the area they were in after a minute. Chi-square method was used to calculate and compare the significance of the results. All trials had chi-square values less than 5.99 of criticial value.Results failed to support the alternative hypothesis but we did manage to find the threshold range of 1000-1660 lux where Artemia Salina started to display negative phototaxis above 1660 lux. The discrepancy of results can be explained by various mechanisms and also differences in experimental design. Discussion Our alternative hypothesis was rejected by all four trials . The chi-square values found are 1.67, 0.08,4.88, and 5.132 respective of the order of trials . As they are all less than the critical value of 5.99, we concluded to fail to reject the null hypothesis. However, we saw a pattern in the organisms' responses in accordance with our prediction which was that the zooplankton will start to move away as soon as the photoreceptors in the compound eyes detect an undesirable light intensity. The pattern was determined by Su Thazin Thin A3 Lina Castro Yasaman Osfogh comparing the graphs of all four trials. In each trial more replicates chose the section of lux value between 1000-1660 until we used 1970 lux. As stated by other studies, Artemia Salina displays negative phototaxis at high light intensities and positive phototaxis at low light intensities (Dumont et al.,1995) .Light intensity is detected by their photoreceptor cells in the compound eye. Artemia salina is equipped with two widely separated compound eyes fixed on flexible stalks. Within the compound eyes are 300 ommatadia( a cluster of photorecepter cells) composed of crystalline cones and retinula cells (Abatzopoulos et al., 2002). The crystalline cones , in particular are integral for producing a response to light. Refractive lens-like structure called glycogen body is suspended in each of the cone cells of the main crystalline tract inside the crystalline cones. Each ommatidium has six retinula cells which make up a fused rhabdum (Abatzopoulos et al., 2002).In response to dark environment, the crystalline tract shortens ,the rhabdom lengthens, and the glycogen body curvature shifts simultaneously (Nilsson et al., 1982).The distal part of the rhabdom widens and also become funnel shaped (Nilsson et al., 1982). A new research has shown that multivesicular bodies in the light adapted retinula cell reduces in number when the organism adapts to dark and reappears when it re-adapts to light (Nilsson et al., 1982). The photocerebrum of the nervous system receives nerves from the compound eyes and sets off a behavioral response.Behavioral response is first explained by phototaxis behavior of Artemia sp. A detection in optimal light intensity will induce the organism to swim towards the source using appendages called phyllopods projecting from the thorax, resulting in what is called a positive phototaxis response (Emslie 2003). Su Thazin Thin A3 Lina Castro Yasaman Osfogh A detection in undesirably high light the intensity will induce the organism to swim away from the source of light resulting in negative phototaxis response. Light affects Artemia salina in a number of ways ranging from inducing behavioral changes to affecting feeding, growth, and survival.Planktivores which can withstand high salinity waters such as Mullet, Milkfish, and Tilapia prey heavily on Artemia Salina (Forward et al., 1992). Predation can compromise their growth and survival leading the organism to adopt a mechanism called Diel Vertical Migration. Photoreceptors from the compound eyes are mainly using during migration and Artemia sp has been found to display maximum sensitivity to the highest light intensity during the process of diel vertical migration (Forward et al., 1992).During the day when light intensity is high) , due to the presence of predators the organisms tend to sink (Forward et. al., 1992). The results present behavioral evidence to demonstrate that sinking behaviour at sunrise during diel vetical migration functions for predator avoidance and rising behavior functions for feeding (Forward et. al., 1992). Even though the normal behavior of Artemia Salina is to ascend during the day to feed where there is high nutrition (Knott, 1998) and sink at night, the presence of predators induce them to act reversely and light is the main cue for this mechanism. To begin with , performance of the predators will be determined by the level of light , if the level of light is undesirable for the predators , Artemia sp is likely to be saved and if not they will be readily fed upon by the predators ( Garcia-Mateos et. al., 2011). The most important factor considered to be involved in diel vertical migration is a change in light intensity but others also Su Thazin Thin A3 Lina Castro Yasaman Osfogh speculate that it could be triggered by light dependent mortality instead of the common belief of predation (Dumont et al., 1995). High intensity of light has can also cause damage the photoreceptors of the compound eyes. Increase in depth ie. decrease in light intensity may contribute to reducing photodamage and visual predation strain (Dumont et al., 1995). Very high light intensities can result in anatomical, physiological and behavorial changes ( Meyer- Rochow 1994) .Once imposed the effect could persist even after the source of light has been removed (Meyer- Rochow 1994). One of the possible effects is total blindness which would dramatically affect migration and feeding (Meyer-Rochow 1994). However it is noted that behavorial or anatomical change cannot be explained by light-induced damange alone (Meyer-Rochow 1994). The discrepancy of results stems from various biological variations. Sex contributes to the difference in results. A study made on fairy shrimp, another Anostraca , found that males were less negatively phototactic than females (Dumont et al., 1995). Variabili
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