The studied population of O. okupa sp. nov. showed a seasonal variability in prevalence (Fig. 5; Table 9). The highest percentages occurred during late autumn-mid winter. The highest peak was in December 2011 (i.e. 19.2%), in coincidence with the highest number of symbionts (Table 9). Intermediate prevalences occurred in November 2011 (i.e. 7.2%), January 2012 (i.e. 8.2%) and February 2012 (i.e. 6.1%). The remaining prevalences were always < 5% and the lowest one occurred in April 2011 (i.e. 1.6%). The intensity of the infestation was always a single worm per host. In the case of the specimens associated with M. pellucida, the prevalence was 66.7% (Table 9), reaching 89.3% in hosts > 20 mm long (Fig. 6).
There was a non-significant correlation between the percentage of ripe females and prevalence in S. plana along the studied period (Pearson coefficient = -0.407, p = 0.168). However, the percentage of ripe females was, on average, significantly higher (one-way ANOVA, F = 9.253, p = 0.011) in the warm than in the cold period, reaching 31.0±4.8% and 7.5±6.1%, respectively, while the prevalence showed exactly the contrary pattern, 3.2±1.3% vs. 8.9±1.7% (one-way ANOVA, F = 7.066, p = 0.022) (Fig. 5).