Determining how Freshwater Copepods Follow Planar Dextran Trails

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Young, Madison R.
Goodisman, Michael
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In this study two species of freshwater copepods, Hesperodiaptomus shoshone (H. shoshone), and Hesperodiaptomus arcticus (H. arcticus) will be used to determine how some species of freshwater copepods interact with planar dextran trails. When a copepod swims through water, hydrodynamic disturbances with a variety of structures are created; some are vortical, planar, or laminar. Initial studies show that these copepods avidly follow laminar trails in an upstream fashion [Pender Healy]. However, when copepods execute turns or fast swimming, vortices are created. When copepods execute slow turns, planar wakes are formed. The direction of flow in the wakes and the location of the wakes provide information on the location of the copepod that is generating that wake. The intent of this research is to determine if the signals in the wakes can lead the following copepod to the source of the disturbance. Hence, all analyses focused on events where a copepod responded to the signal. Responses include reorientation or angle of entrance, time spent in trail, preference for a particular trail width and edge following behavior [Strouhal number]. The goal is to understand more about their how they sense and respond to changes in their environment and it is hypothesized that both species will interact more with the wide trail and that H. arcticus will spend more time in the trail and enter at a greater angle. It is also hypothesized that both species adhere to ideal Strouhal values. To test this, two dextran (a polysaccharide) trails will be simultaneously dropped into a tank containing the copepods. One of the trails will be 2mm and the other will be 4mm. It is expected that the copepod will wobble or traverse the trail to contact the edges; edge following enables the copepod to stay on track. Alternatively, the copepod may follow the center of the trail where the flow is the fastest and therefore is relying on flow speed rather than the shear found in the edge of the trail. Analyses of the location of the follower relative to the edge versus the center of the trail can assess whether the copepod is sensing flow shear versus flow speed. A MatLab script will be used to find more detailed information (Figures 11-22). Both species prefer to follow the wider trail (Figure 5 and 6), and H. arcticus spend more time in the trail than H. shoshone. Hesperodiaptomus shoshone followed the 4mm trail eighteen times for an average of 2.14 seconds and the 2mm trail five times for an average of 1.36 seconds. The number of encounters was also determined, which confirms that the proportion of encounters resulting in follows is much higher for the 4mm trial than the 2mm trial in both species (Table 3). This data illustrates statistically significant results (p<0.05) that H. shoshone prefer to follow the wider trail and follow for longer periods of time compared to the smaller one. These results were compared to the results for H. arcticus, which followed the 4mm trail twelve times for an average of 5.08 seconds (Figure 6). This comparison between both species on the 4mm trail confirmed that H. arctius spend more time in the trail than H. shoshone (p<0.05). The average angles of entrance were 29.16° for H. shoshone on the 4mm trail and 21.08° on the 2mm trail, and 39.69° for H. arcticus on the 4mm trail (Table 2). When compared, the results demonstrated that H. arcticus enters the trail at a greater angle than H. shoshone on both the 2mm and 4mm trails (p<0.05). There was not enough of the wobbling behavior shown to find Strouhal Values at this time.
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