Phosphorus acquisition and competitive abilities of two herbivorous zooplankton, Daphnia pulex and Ceriodaphnia quadrangula
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Contrary to an expectation from the size-efficiency hypothesis, small herbivore zooplankton such as Ceriodaphnia often competitively predominate against large species such as Daphnia. However, little is known about critical feeding conditions favoring Ceriodaphnia over Daphnia. To elucidate these conditions, a series of growth experiments was performed with various types of foods in terms of phosphorus (P) contents and composition (algae and bacteria). An experiment with P-rich algae showed that the threshold food level, at which an individual’s growth rate equals zero, was not significantly different between the two species. However, the food P:C ratio, at which the growth rate becomes zero, was lower for Daphnia than for Ceriodaphnia, suggesting that the latter species is rather disfavored by P-poor algae. Ceriodaphnia showed a higher growth rate than Daphnia only when a substantial amount of bacteria was supplied together with a low amount of P-poor algae as food. These results suggest that an abundance of bacteria relative to algae plays a crucial role in favoring Ceriodaphnia over Daphnia because these are an important food resource for the former species but not for the latter.
KeywordsCompetition Growth rate hypothesis Zooplankton P:C stoichiometry Size-efficiency hypothesis
We thank the two anonymous reviewers for their valuable comments and suggestions on the early version of our manuscript. We wish to thank Wataru Makino and the members of the Community and Ecosystem Laboratory, Tohoku University for technical help and suggestions. This research was financially supported by a grant-in-aid for scientific research A (No. 19207003) and by the Global COE program J03 of MEXT Japan.
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