Relationships between life–cycle and infestation characteristics
The infestation in O. okupa sp. nov., seemed to be connected with the reproduction. The months with lower prevalence (i.e., mid spring and summer) showed a markedly higher percentage of ripe females (>30%) (Fig. 4, 5; Table 9), reaching a 50% in August 2011, while the coldest month (i.e. October 2011 to February 2012) showed a high prevalence coupled with low percentages of ripe females (Fig. 4, 5; Table 9). The marked seasonal pattern found in the population of O. pugettensis (Johnson, 1901) infesting the starfish Patiria miniata (Brandt, 1835) at Dana Point (California) seemed also to be connected with the commensal reproductive dynamics (Lande and Reish, 1968). This species reached the highest (≥80%) and lowest (≤30%) prevalence in November-December and prior to the decrease of water temperature in summer, respectively. Its abundance was maximum in winter (2-3 worms per host) and minimum at mid-summer (<0.5 worms per host).
Despite sampling on different years is certainly required to infer regularities in the relationships between prevalence and life cycle in O. okupa sp. nov., these observations suggests that ripe females may have a reduced mobility (i.e., they tend to remain inside the host during their whole life), whereas males could be more mobile and thus leave their host, likely to increase fertilization success. Taking into account that O. okupa sp. nov. has a 1:1 regular distribution, the higher mobility attributed to males may contribute to an increase fertilization success by increasing the possibility of male/female partnership. Two possible mechanisms may explain this: 1) males may directly enter a host occupied by a ripe female, or 2) males may approach a host when females are releasing their sexual products. In both cases, we suggest that chemical cues may be involved. A similar behaviour was previously reported for B. seepensis, and, as mentioned above, also contributed to explain the lack of host-bivalve size relationships for adult males (Britayev et al., 2007). A similar situation was also reported for Haplosyllides floridana Augener, 1922, whose male stolons were never found inside the host sponge Neofibularia nolitangere (Duchassaing & Michelotti, 1864) and, conversely, were found free-swimming in the water column (Martin et al., 2009). Accordingly, the highest prevalence in the studied population of O. okupa sp. nov. occurred when both males and females occupied their respective hosts.
Despite this general pattern of reproduction vs. prevalence, O. okupa sp. nov. seems to reproduce actively during the whole year, but with an increasing effort during spring-summer and a higher intensity in summer. The absence of ripe females in April 2012 contrasted with the almost 40% found in 2011. However, ripe females occurred both before and after April 2012, allowing us to suggest that its absence in April 2012 could have been biased by the low number of symbionts found in this month (Fig. 4; Table 9). Persistent high temperatures can stimulate oocyte growth, subsequently causing the advancement of reproductive period in polychaetes, as reported for the Mediterranean populations of Eupolymnia nebulosa (Montagu, 1819) (Cha et al., 1997). However, our data does not allow assessing whether environmental constraints such as differences in temperature could affect the reproductive cycle of O. okupa sp. nov. during the studied period.