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キーワード:
allocation pattern; egg size; food level; life table; ovary size; reproductive effort; reproductive threshold; resource abundance; rotifer; Synchaeta; trade-off
要旨:
Individuals that live in resource-limited environments are faced with an allocation problem since they generally do not have enough assimilated energy to fuel each of the processes of reproduction, somatic growth, and storage at the physiological maximum rate. Thus, they have to decide on the proportions at which they allocate energy to the various processes. In laboratory experiments and field observations, I found that the common planktonic rotifer Synchaeta pectinata increases the proportion of energy allocated to reproduction (reproductive effort, RE) as food becomes more limited. This change in the allocation scheme along a gradient of food concentrations was inferred from volumetric measurements of egg size and the size of the elliptically shaped ovary. Synchaeta pectinata used the contents of its ovary either for reproduction (increasing the size of the individually and consecutively produced eggs) or for storage (enhancing survival during starvation by resorption of this material). As egg size was relatively constant across food levels, the higher RE at low food concentrations was not due to the fact that Synchaeta channeled a larger absolute quantity of energy into reproduction, but rather that they lowered the threshold size for reproduction of the ovary and hence reproduced earlier. This pattern was also suggested by the results of the field study: when the resource concentrations declined during the study period, the ovary size of field-caught Synchaeta decreased considerably whereas the size of their eggs, collected at the same dates, stayed relatively constant.
The lower reproductive threshold at low food concentrations resulted in a small ovary size after each egg deposition which implied a twofold cost. First, rotifers with small ovaries were less resistant to starvation because they could resorb only a little material from their ovaries. Second, it took longer for them to produce the next egg since a small ovary took longer to recover in size than a large ovary (for a given food concentration).
The plasticity in the allocation scheme of Synchaeta pectinata can be interpreted as an adaptive strategy to variable food conditions. By lowering their reproductive threshold when food becomes limited, Synchaeta pectinata increase their chance to produce at least one offspring during their lifetime. In contrast, the higher reproductive threshold under good food conditions results in better maternal condition and, therefore, facilitates future reproduction
