Aerobic scope does matter in temperature-size rule, but only under optimal conditions
We united the theoretical predictions on the factors responsible for the occurrence and evolutionary significance of the temperature-size rule. We tested the causal connection among them assuming that (i) the temperature-size rule is the response to temperature-dependent oxygenic conditions, (ii) body size decrease is a consequence of cell shrinkage in response to exposure to hypoxia, (iii) this response enables to keep the wide scope for aerobic performance, and (iv) it prevents the decrease in fitness. We conducted our tests on three clones of the rotifer Lecane inermis with different thermal preferences. These clones were exposed to three experimental regimes: mild hypoxia, severe hypoxia driven by a too high temperature, and severe hypoxia driven by an inadequate oxygen concentration. The results showed that our causative reasoning was generally correct, but only under mildly hypoxic conditions. In more stressful environments, rotifers had clone- and condition-specific responses, which in fact were equally successful in terms of the levels of fitness. Our results join for the first time all factors connecting the cause and effect in the temperature-size rule. They indicate the importance of the conditions under which it should be tested. The most important messages from this study were that (i) a decrease in the body size was one of but not the only option for preventing fitness reduction under hypoxia, and (ii) such a response to higher temperature enabled the maintenance of wide aerobic scope in clone-specific, thermally optimal conditions.