A theoretical evaluation of the respiration rate partition in the Gasterosteus aculeatus-Schistocephalus solidus host-parasite system

The oxygen consumption rate in a wild Mediterranean non-migratory population of a teleostean fish, the three-spined stickleback (Gasterosteus aculeatus), was measured to investigate if the infection by the cestode worm Schistocephalus solidus plerocercoids increases the oxygen consumption (Vo(2)) of the host fish. Previously, an increment in Vo(2) has been reported but never found to be statistically significant. A new approach was carried out in the present study, using a small, but statistically representative, sample of uninfected and infected fish. We used the data on the parasite oxygen consumption available in the current literature to estimate the possible contribution of plerocercoids to the oxygen consumption of our experimental animals. The maximal plerocercoid putative contribution to the measured Vo(2) of the infected host (i.e., host plus parasite) was assessed to be around 2.8%. Interestingly, a key role in the respiration rate of infected sticklebacks turned out to be played by the infection burden. In other words, the host specific Vo(2) (sVo(2)) increment was significantly correlated with the number of plerocercoids harboured by the host. The observation implies that a reliable comparison of the physiological parameters between infected and uninfected stickleback populations can be only carried out if the number of parasites among the single individuals is homogeneous. Unfortunately, the random occurrence of the parasites makes this effort unpractical.