The muscle dwelling myxozoan, Kudoa inornata, enhances swimming performance in the spotted seatrout, Cynoscion nebulosus.

Parasites usurp host resources and, as a consequence, enhance their transmission and increase their fitness while reducing the fitness of their host. Performance capacity is a key predictor of fitness. Thus, the effects of parasites on host fitness may often be mediated by alteration of host performance. We tested the effect of the skeletal muscle dwelling myxozoan, Kudoa inornata, on the swimming performance in spotted seatrout, Cynoscion nebulosus. We predicted greater infection would result in reduced swimming performance. Unexpectedly, increasing density of K. inornata myxospores in seatrout skeletal muscle was related to increased fish swimming performance. The experiment was repeated and confirmed these unexpected results. The mechanisms underlying enhanced performance of an infected host are not understood, but their occurrence emphasizes the role of parasites as selective pressures on host evolution.