Effects of temperature, photoperiod, and Myxobolus cerebralis infection on growth, reproduction, and survival of Tubifex tubifex lineages

Tubifex tubifex is the definitive host for Myxobolus cerebralis, the causative agent of whirling disease in salmonid fish. Several mitochondrial lineages of T. tubifex exhibit resistance to M. cerebralis infection. Release of the triactinomyxon form of the parasite from T. tubifex varies with water temperature; however, little is known about the interactive effects of temperature and photoperiod on the susceptibility of T. tubifex lineages to M. cerebralis infection. In addition, the environmental effects on the growth, reproduction, and survival of T. tubifex lineages are unknown. Monocultures of lineages III and VI were subjected to infection (0 and 500 myxospores per worm), a range of temperatures (5, 17, and 27 degrees C), and various diurnal photoperiods (12:12, 14:10, and 16:8 dark : light) over a 70-d period by using a split-split plot experimental design. Lineage VI resisted infection by M. cerebralis at all temperatures, whereas lineage III exhibited infection levels of 4.3% at 5 degrees C, 3.3% at 17 degrees C, and 0% at 27 degrees C. Lineage VI exhibited significantly higher adult survival, weighed more initially, gained more weight, and had higher natality (production of immature tubificids) than did lineage III regardless of temperature, photoperiod, or infection treatment. There was no detectable effect of lineage, infection, or photoperiod on cysting. Both lineages III and VI cysted at 5 degrees C but not at 17 degrees C or 27 degrees C. Competition between lineage VI and other lineages for resources may serve to decrease the overall infection levels among T. tubifex populations, thereby reducing both triactinomyxon production and the occurrence of whirling disease among susceptible salmonids.