Change in filter‐screen morphology and depth selection: Uncoupled responses of Daphnia to the presence of filamentous cyanobacteria

Abstract

In many eutrophic lakes, ability to cope with filamentous cyanobacteria is crucial to Daphnia fitness, as the filaments can reduce food intake by interference with food-particle retention. Two basic mechanisms were proposed to explain food collection by Daphnia: mechanical sieving, in which filtering appendages act as sieves, and direct interception, in which appendages act as paddles and water does not pass through. As was recently suggested, both mechanisms may be active, and their relative importance is determined by the Reynolds number on filtering appendages. Mechanical sieving seems particularly sensitive to the interference from filamentous cyanobacteria, which can clog the meshes while passing through the filtering chamber. We therefore hypothesized that in the presence of filaments, Daphnia minimizes the interference with filtration by decreasing the Reynolds number on the filters and by thus reducing the relative importance of sieving. To test this hypothesis, we examined the responses of nine clones of the Daphnia longispina group to the presence of nontoxic, filamentous cyanobacteria. The presence of cyanobacteria triggered in Daphnia responses of both behavioral (descending to deep, cold waters) and morphological nature (decreased intersetal and intersetular distances in the filtering apparatus). Both responses led to a decrease in the Reynolds number on the filters. Moreover, the two responses were inversely correlated: individuals with larger meshes descended to colder strata than did those with smaller meshes. These modifications of phenotype are regarded as adaptive, since they allow for minimized filamentinduced interference with the filtration process.

Aphanizomenon gracile increases in width in the presence of Daphnia. A defence mechanism against grazing?

“…We speculate that the thickening and shortening of Aphanizomenon filaments help individuals to avoid mechanical destruction. Bednarska and Dawidowicz (2007) observed in Daphnia a reduction in intersetal and intersetular distances in the filtering apparatus to reduce the interference from filamentous cyanobacteria. The shortening and thickening of a filament may make it easier to pass through the filtering chamber without being destroyed or eaten.…”

Setae thickening in Daphnia magna alleviates the food stress caused by the filamentous cyanobacteria

“…The reduction in mesh sizes allows Daphnia to intercept small bacteria (Brendelberger 1991), which may compensate for unfavorable nutritional conditions. The presence of filamentous cyanobacteria caused decreased intersetal and intersetular distances in the filtering apparatus, and elongation of filtering setae in Daphnia (Machá-ček 1998;Bednarska and Dawidowicz 2007). The latter authors suggested that such morphological changes reduce the interferential effect of cyanobacterial filaments on the filtration process.…”

“…Although modifications of the filtration apparatus of Daphnia have been broadly studied on numerous Daphnia species (Macháček 1998;Repka et al 1999a, b;Ghadouani and Pinel-Alloul 2002;Bednarska and Dawidowicz 2007), little has been known about the changes in the thickness of setae induced by filamentous cyanobacteria. Because the distance between setae decreases in response to the presence of cyanobacteria (Bednarska and Dawidowicz 2007) and the number of setae on filtering appendages is constant in adult animals (Pop 1991;Lampert and Brendelberger 1996), it is reasonable to expect the setae to become thicker in such conditions.…”

“…Because the distance between setae decreases in response to the presence of cyanobacteria (Bednarska and Dawidowicz 2007) and the number of setae on filtering appendages is constant in adult animals (Pop 1991;Lampert and Brendelberger 1996), it is reasonable to expect the setae to become thicker in such conditions. In our opinion, thickening of setae, induced by the presence of filamentous cyanobacteria, makes filtering comb stronger, which enables it to effectively fulfill its function, especially if it lengthens under low-food conditions.…”

Threat or treat: the role of fish exudates in the growth and life history of Daphnia

“…Prior to measurement, the animals were preserved in 4% sugar formaldehyde and the endopodites were dissected from the third pair of thoracic legs of each individual (two endopodites per Daphnia) and transferred in a drop of glycerol onto a microscope slide (Bednarska and Dawidowicz 2007). The filter screens were then photographed and inter-setular distances were measured at 10 locations in the middle of the setae using MultiScan image analysis software.…”