Tory Olson Graduate Student, Biology
Invasive species are a problem in many ecosystems. Bythotrephes longimanus, an invasive zooplankton originating from Eurasia, invaded the Laurentian Great Lakes in the 1980s (Lehman 1987; Berg and Garton 1988). Bythotrephes has since been observed in inland Minnesota lakes (Branstrator et al. 2006). Bythotrephes is a predacous Cladoceran that preys upon and depletes native zooplankton populations, such as Daphnia. Zooplankton diversity is also depleted with the introduction of the spiny water flea.
Two major morphological features of Bythotrephes are its large, dark compound eye and long, stiff caudal spine which serves as a postcontact morphological defense against fishes (Branstrator 2005). Due to the high visibility of Bythotrephes from their large body size and eyespot diameter, fishes larger than 100 mm in body length seem to have a strong preference for Bythotrephes, even when they are the least abundant prey (Mookerji et al.1998; Coulas et al. 1998). In fact, Bythotrephes has often become the dominant food source for fish planktivores once they have established themselves in a lake (Bur and Klarer 1991; Berg and Grimaldi 1966). Research has been done regarding this matter of visibility; however, either linear weight or eyespot diameter regression models were used to determine visibility. No methods have been developed to analyze total body visibility. Using software in the VDIL gave me the opportunity to analyze whole specimens in an attempt to quantitatively determine if Bythotrephes were more visible than Daphnia, based on total body grayness.
Photos were taken at several magnifications (1-5x) of multiple Daphnia and Bythotrephes under a dissecting microscope in Donn Branstrator’s laboratory (UMD). Photos of first, second, and third instar Bythotrephes were taken. The color photos were converted to black and white in Adobe Photoshop (Version 9.0). The background noise was reduced in Photoshop by using the magic wand function to select the organism. The selected organism was then pasted into a new blank page. Sometimes similarly colored background would also be selected because the magic wand could not differentiate between the organism and the similarly colored background. If this occurred, the eraser tool was used to delete as much background gray as possible. Finally, the pictures were saved as *.tif files to be compatible with SCION (Figure 1).
The free version of SCION was downloaded from the SCION website (http://www.scioncorp.com/). The grayscale of Daphnia and Bythotrephes were compared using 3 Bythotrephes individuals for each instar and 3 Daphnia total. First, I had to select “Options” > “Threshold” to allow for a histogram to be created. By selecting “Analyze” > “Show histogram,” the pixel histogram was created as a means to quantify the total “grayness” of an individual. I summed the pixel count at each gray level from 106 to 250 (i.e., sum of level*pixel count) of the total gray scale ranging from 0 to 255. Magnification was corrected for by dividing the sum by the square of the magnification (e.g., if 2x, divide by 4; if 3x, divide by 9). The mean and variance of total grayness for each group of organisms was then calculated and graphed in Microsoft Excel 2003.
As seen in Figure 2, Daphnia had the lowest total grayness [sum of (gray level) x (pixel count at that level)], with a mean of 21.08 and a variance of 1.55 (all results in millions). First instar Bythotrephes had a total grayness mean= 24.22 (variance= 58.44), while second instar Bythotrephes total grayness mean equaled 65.32 (variance= 6.29). Finally, third instar Bythotrephes had the greatest total grayness, with a mean of 83.48 and a variance of 5.48. Thus, third instar Bythotrephes are approximately 4 times more visible than Daphnia based on total grayness. Second instar Bythotrephes are approximately 3 times more visible than Daphnia, while the total grayness of Daphnia and first instar Bythotrephes were not significantly different. Overall, Bythotrephes increase in visibility with age (i.e., instar), as expected. Furthermore, the increase between first and second instar Bythotrephes (41.1 million) is 2.25 times greater as compared to the increase between second and third instar (18.16 million).
These results support the theory that fish preferentially select for Bythotrephes due to their increased visibility as compared to native Daphnia. If not gape-limited, fish should select for the largest, most visible Bythotrephes available. These results may be used in future studies to predict the ratio of food present in the planktivore’s stomach (i.e., dietary proportions). Assuming all else to be equal (e.g., zooplankton densities equal, no outside selection pressures, fish large enough to easily consume all sizes of zooplankton, etc.), the planktivore’s expected stomach content ratio of 3 instar Bythotrephes: 2 instar Bythotrephes: 1 instar Bythotrephes: Daphnia would be 4:3:1:1. However, more work must be done with larger sample sizes and more native species to better explain total grayness variation in Bythotrephes and native zooplankton.
1) Berg, D.J. and Garton, D.W. (1988) Seasonal abundance of the exotic predatory cladocerans, Bythotrephes cederstroemi, in western Lake Erie. Journal of Great Lakes Restoration, 14, 479-488.
2) Berg, A. and Grimaldi, E. (1966) Ecological relationships between planktophagic fish species in the Lago Maggiore. Verh. Internat. Verein. Limnol., 16, 1065-1073.
3) Branstrator, D.K. (2005) Contrasting life histories of the predatory cladocerans Leptodora kindtii and Bythotrephes longimanus. Journal of Plankton Research, 27, 569-585.
4) Branstrator, D.K., Brown, M.E., Shannon, L.J., Thabes, M. and Heimgartner, K. (2006) Range expansion of Bythotrephes longimanus in North America: Evaluating habitat characteristics in the spread of an exotic zooplankter. Biological Invasions, 8, 1367-1379.
5) Bur, M.T. and Klarer, D.M. (1991) Prey selection for the exotic cladoceran Bythotrephes cederstroemi by selected Lake Erie fishes. Journal of Great Lakes Restoration, 17, 85-93.
6) Coulas, R.A., Macisaac, H.J., and Dunlop, W. (1998) Selective predation on an introduced zooplankter (Bythotrephes cederstroemi) by lake herring (Coregonus artedii) in Harp Lake, Ontario. Freshwater Biology, 40, 343-355.