Host/symbiont size relationships. Positive host/symbiont size relationships may either indicate 1) active size segregation behaviour, as reported both for the symbiotic coral dwelling crab Trapezia bidentata (Forskål, 1775) (Adams et al., 1985) and the fish Gobiodon histrio (Valenciennes, 1837) (Hobbs and Munday, 2004), which seem to be able to migrate from colony to colony to choose one of an appropriate size, or 2) parallel growth of hosts and symbionts, as reported both for the nemertean Malacobdella grossa (Müller, 1776), hosted by Arctica islandica (Linnaeus, 1767) (Sundet and Jobling, 1985) and the pontoniin shrimp Anchistus custos (Forskål, 1775), hosted by the bivalve Pinna bicolor Gmelin, 1791 (Britayev and Fahrutdinov, 1994). In turn, the absence of host/symbiont size correlation could be caused by symbionts that either colonize the hosts during a single phase of their life-history (e.g. juveniles, adults), that are highly mobile (thus, colonizing the available host independently of their size), or growth faster than the host, even if the infestation starts with juveniles colonizing small-sized hosts (Britayev et al., 2007).
Among symbiotic polychaetes, positive host/symbiont size-relationships are not common (Martin and Britayev, 1998) and, when occurring, they may be modified by differential behaviour linked to, for instance, reproductive activities. This seems to be the case of Branchipolynoe seepensis Pettibone, 1986, an obligate symbiont of deep-sea hydrothermal vent mytilids of the genus Bathymodiolus, whose females have a longer life span than males and remain inside the same host along their whole life, while males leave their original hosts for mating and then colonize the available hosts independently of their size. Consequently, positive host/symbiont size correlations occur only for females and for non-mature males (Britayev et al., 2007). In most known cases, however, the relationship between size structure of symbiotic polychaetes and their hosts is unclear (Martin et al., 1991, 1992; Emson et al., 1993; Rozbaczylo and Cañete, 1993; Britayev and Zamyshliak, 1996) so that the life histories of the former have been considered to be independent of that of the latter, with the hosts tending to live longer and commonly hosting successive symbionts (Martin and Britayev, 1998).
The absence of host/symbiont size-relationships previously reported for the intertidal population of O. okupa sp. nov. (Martin et al., 2012) was here confirmed by the absence of significant correlations found along the whole study period. The only exception was April 2012, which we consider as a spurious relationship due to the low number of collected worms (n<10) (Fig. 4; Table 9). However, other months with a similar number of individuals (e.g. n = 8 - 11 in March, October, and August) (Fig. 4; Table 9) showed non-significant relationships. These results support Martin et al. (2012) in that the worms do not grow together with their hosts in their intertidal environment. In turn, O. okupa sp. nov. was reported to infest specimens of S. plana > 20 mm in shell length and to be more frequent in intermediate-sized shells, 26-36 mm long (Martin et al., 2012), which has also been confirmed along the whole period of study. Limiting host sizes have been reported for the starfish Asterias rathbuni (Verrill, 1909) in Vostok Bay (Sea of Japan), whose specimens with disc radii lower than 35 mm were not inhabited by Arctonoe vittata (Grube, 1855), and those with radii up to 90 mm harboured one single symbiont, whereas the largest starfishes could host up to four polychaetes (Britayev et al., 1989). Another example occurs in Chile, where the scale-worm Harmothoe commensalis Rozbaczylo and Cañete, 1993 did not occur in specimens of the clam Gari solida (Gray, 1828) < 60 mm in shell length (Rozbaczylo and Cañete, 1993). The size-limit for S. plana as a host of O. okupa sp. nov., also implied that there were no small juveniles infesting small-sized hosts, which allows us to discard the fast-growing hypothesis for this new species.
On the other hand, the infested specimens of M. pellucida collected in a subtidal environment during January 2013 included a considerable amount of small-sized hosts (Fig. 6). Moreover, there was a significant positive size correlation between hosts and symbionts. This relationship may be caused either by the symbiont selecting the host to be infested according to the appropriateness of its size, or by host/symbiont parallel growth, which cannot be assessed in light of our present data. Despite the low number of collected bivalves, the existing size relationship and the very high infestation rates (Table 9), suggest together that M. pellucida and the subtidal environment could be the preferred host and habitat for O. okupa sp. nov., with the symbiont selecting the most suitable hosts when conditions are optimal.
Prevalence of the infestation. The prevalence of commensal polychaetes is highly variable and has been considered as a species-specific characteristic. However, a commensal species may also show different prevalences, which may vary according to bathymetric, spatial and temporal (i.e. intra- and inter-annual) patterns (Martin and Britayev, 1998). Studies including temporal trends are, however, scarce. With the exception of the impressive fidelity of the year-to-year counts of a regular seasonal trend reported for the infestation prevalence of Ophiocoma echinata (Lamarck, 1816) by Branchiosyllis exilis (Gravier, 1900) in Panamá (Hendler and Meyer, 1982), the only known data on year-to-year variability for a symbiotic polychaete are probably those on Arctonoe vittata (Grube, 1855). In Vostok Bay, this worm infests the starfish Asterias amurensis Lütken, 1871 with a progressive increase in prevalence from 0% in 1975-76 to 8.4% in 1978 and to 79.1% in 1980 (Britayev, 1991). Based on our study period, O. okupa sp. nov. seems to show a seasonal trend in prevalence, with the highest percentages occurring during the coldest seasons, i.e. from late autumn to late winter (Fig. 5; Table 9). The present dataset only covers a single year and therefore inter-annual regularities cannot be inferred.
Theoretically, we may expect an influence of the host’s population structure on that of the symbiont (Martin & Britayev, 1998). A positive relationship between host density and infestation characteristics was reported for A. vittata and its host starfish A. rathbuni in Vostok Bay (Britayev et al., 1989). The highest prevalence, mean intensity and abundance depended upon host’s density, which could be caused by the accumulation of a chemically-mediated host-cue more effectively attracting the settling symbionts. High host densities may also reduce the influence of external factors. For instance, commensals associated with less abundant host populations may experience a more relevant decrease in fitness than those harboured by dense host populations (Martin and Britayev, 1998).
Among the few relationships with known data, high infestation indexes tend to be positively correlated with the availability of large and numerous hosts to be occupied by the symbionts (Martin and Britayev, 1998). In the present case, the overall low prevalence would lead to the expectation of a low number of S. plana > 20 mm long in the study area. Conversely, S. plana was very abundant independently of size (Subida et al.,2011; Drake et al., 2014) and so numerous adequate hosts were available through the whole study period (Fig. 6; Table 9). Therefore, the prevalence seems not to be connected with the host availability. In turn, this low prevalence could be related to the extreme daily changes in temperature and/or the alternating long desiccation/immersion periods that characterize the intertidal environment, as well as with biota disturbance due to bait digging of this zone at Río San Pedro (Carvalho et al., 2013). The high prevalence in the subtidal population of M. pellucida (i.e. higher than 85% in specimens > 20 mm long), seems to confirm this hypothesis, with the permanent immersion in the subtidal environment favouring the presence of the symbiont.
The fact that no specimens of S. plana < 18 mm in shell length were found during the annual cycle, is in agreement with all previous data obtained in punctual samplings (Martin et al. 2012, 2015). The present approach did not clarify this peculiarity of the studied bivalve population. However, the high density of specimens restricted to the narrow intertidal area at Río San Pedro led to postulate the existence of a negative interaction between the established population and the settling larvae, which could be actively ingested by their conspecific adults during their normal filter/suspension feeding activities (i.e. passive cannibalism) (Cargnin-Ferreira, 2005; Santos et al. 2011).
The association of a symbiont with various hosts in the same locality could affect the prevalence, which strongly depends on its level of affinity for the different hosts. Although the symbiotic population of O. okupa sp. nov. at Río San Pedro only infested S. plana, this factor could not be eventually discarded due to the presence of the nearby subtidal population of M. pellucida at the opening of Río San Pedro.
Infestation intensity. The infestation intensity of symbiont polychaetes has been more widely reported than the prevalence. It may range from one to hundreds of symbionts per host, but is clearly dominated by the association of a single symbiont per host either due to the usually low symbiont densities or to the influence of an intraspecific aggressive behaviour (Martin and Britayev, 1998). Like the prevalence, intensity may also oscillate within the same population due to seasonal changes in relative abundances, linked or not to reproduction and recruitment events. A common situation for symbionts with 1:1 regular distributions like O. okupa sp. nov., is that adults may occasionally share the host with one to several juveniles, as previously reported for polychaete species of the genera Acholoe, Adyte or Branchiosyllis, among others (Martin and Britayev, 1998).
In the case of O. okupa sp. nov., Martin et al. (2012, 2015) reported single findings of one male and one female and one male, one female and one small worm, likely a juvenile sharing the same host, while the constant regular distribution of one single symbiont per host was attributed to intraspecific aggressive behaviour. There are several cases of symbiotic polychaetes with male and female couples living together associated with the same host, such as the Mediterranean fish parasite Ichthyotomus sanguinarius Eisig, 1906 (Eisig, 1906; Culurgioni et al., 2006) or the deep-sea hexactinellid sponge symbiont Robertianella synophthalma McIntosh, 1885, reported as Harmothoe hyalonemae in Martin et al. (1992). However, in both cases, most hosts harboured couples, while in O. okupa sp. nov. couples seemed to be the exception.