Characteristics of the infestation
The infestation by Branchipolynoe seepensis varied substantially among mytilids from different hydrothermal fields and sites, depending on host species and size. For instance, Bathymodiolus azoricus was significantly more infested than B. puteoserpentis. According to our data, however, this seems to be more likely related to local environmental differences than to any biological peculiarity of these species as hosts. The estimated symbiont density (i.e. 1071–1191 individuals m−2), even underestimated, is comparable with the highest reported to date for any free-living scale-worm (i.e. 800–1300 individuals m−2 for Antinoella sarsi in Sarvala 1971), and is also very high even for an inhabitant of the densely populated vent communities.
The infestation by Branchipolynoe seepensis may vary within a given mytilid population among different zones separated only by a few meters. However, it usually tends to be higher in larger than in smaller hosts, as is common in many other symbiotic polychaetes (Martin & Britayev 1998). In spite of this, small mytilids are also regularly infested, which is a very rare trend for a symbiotic association and may be related to the suggested continuous reproduction and settlement of these symbionts (Jollivet et al. 2000).
The infestation intensity may reach up to six symbionts per host (Jollivet et al. 2000; our samples). However, most hosts harbored only one Branchipolynoe seepensis. Regular distributions such as this often result from intra-specific antagonistic behavior related to territorial defense of the host (Palmer 1968; Dimock 1974; Britayev & Smurov 1985). The existence of such antagonistic behavior in B. seepensisis indirectly supported by the presence of trauma to the parapodia and other appendages that are similar to those reported for the intra-specifically aggressive shallow-water symbiotic scale-worms Arctonoe vittata and Gastrolepidia clavigera (Britayev & Smurov 1985; Britayev & Zamyshliak 1996).
Symbiont/host relationships: commensalisms or parasitism?
Stable carbon isotope analyses of Branchipolynoe spp. and their mytilid hosts demonstrate a close nutritional link (Fisher et al. 1994; Van Dover 2002). Our previous observations on the B. seepensis intestinal contents and predominant orientation inside the host (i.e. head-to-siphon and head-to-mouth) suggested that symbionts may consume either the filtered agglutinated suspended particles that are transported toward the host mouth, or the suspended organic particles that are transported to the siphon opening (Britayev et al. 2003). Therefore, the behavior of symbionts may be classified as kleptoparasitic (stealing food from the host rather than feeding on the host itself). Similar kleptoparasitic behavior has been reported for other symbiotic invertebrates including the polychaetes Branchiosyllis exilis, a symbiont of the brittle star Ophiocoma echinata (Hendler & Meyer 1982); Haplosyllis villogorgicola, an associate of the gorgonian Villogorgia bebrycoides (Martin et al. 2002); the nemertean Malacobdella grossa, associated with bivalves (Gibson & Jennings 1969); and the gastropod Trichotropis cancellata, a symbiont of the tubeworm Serpula columbiana (Iyengar 2002).
The only previous report of Branchipolynoe trophic preferences is the finding of pseudofaeces and bits of mytilid demibranches in the gut of B. symmitilida associated with Bathymodiolus thermophilus (Desbruyères et al. 1985). In contrast, neither fragments of host tissues nor pseudofaeces occurred in the intestine of B. seepensis (Britayev et al. 2003). This difference is likely related to intraspecific differences in trophic preferences within the genus and, thus, supports the possible existence of different host/symbiont relationships among similar associations.
The idea that Branchipolynoe seepensis has a negative influence on their mytilid hosts is supported by the finding of damaged gill filaments in Bathymodiolus spp. infested by B. seepensis (Ward et al. 2004) and, indirectly, by the mytilid gill bits found in the gut of B. symmitilida (Desbruyères et al. 1985). Our studies (Britayev et al. 2003; present paper) demonstrate that the incidence of trauma was about four to five times higher in infested than in non-infested mollusks, so it may be assumed that symbionts cause the traumas on host tissues. However, according to our data, host tissues seem not to be a habitual food source for the symbionts. More feasibly, host traumas may be an accidental result of the ‘normal’ feeding activities of these powerfully jawed symbionts. The presence of traumas apparently derived from symbiont activity in non-infested mytilids that could be attributed to polychaetes that either escaped from the host before being collected or were lost during the collection process. By stealing host food, consuming oxygen and accidentally damaging their tissues, the symbionts may have a negative influence on the host metabolism and, thus, on their growth. Among bivalves, decreasing growth rates may lead to changes in shell proportions (Bierbaum & Ferson 1986; Zolotarev 1989). The shells of some of the studied populations of B. azoricus were proportionally wider than longer in infested mytilids relative to non-infested ones, this also corresponding to a proportional increase in width/length ratio (Britayev et al. 2003). Providing that modifications in shell shape may be caused by the inhibition of growth in infested hosts, the symbiont behavior should be more reliably considered as closer to parasitism than to commensalism. Special attention should be addressed to the tunnel-like structures induced by worms in the soft tissues of the host mussels. Symbiotic polychaetes may induce the formation of tubes or tunnel-like structures by stimulating a differential growth of the host's tissues (e.g.Zibrowius et al. 1975; Britayev 1981; Eckelbarger et al. 2005). Although these structures are usually a defensive reaction of the host to the presence of the symbiont, there is an obviously protective function for the worms. The presence of tunnel-like structures in the association of B. seepensis with B. azoricus is, however, the first example of such structures induced in the soft tissues of the host bivalves. Taking into account that the symbionts are located inside the mussel mantle cavity, the protective role of the structures seems doubtful. However, although these structures may not be homologous among the different hosts, their formation may have exactly the same origin: a response of host's tissues induced by a consistent position of the worm.
Peculiarities of the Branchipolynoe seepensis life cycle
Female Branchipolynoe seepensis are substantially longer and relatively wider than males. Thus, in addition to a different number of gonoduct papillae, sexual dimorphism also seems to be expressed as differences in body proportions and size. Size dimorphism was previously attributed to a sexually biased mortality (Jollivet et al. 2000), this leading to a different life span for males and females. In fact, the polymodal size–frequency distribution of symbionts (Jollivet et al. 2000; present paper) probably indicates a periodic recruitment. The first modal groups (Fig. 9) likely represent the same cohort both for males and females. However, females have two or three additional modes and large animals belonging to an older cohort were not rare (Fig. 9). These differences in cohort number between males and females resemble those previously reported (Jollivet et al. 2000), but our data differ in having one and four modes instead of two and three for males and females, respectively. In any case, female size frequency includes one or three more cohorts than males’, evidencing the shorter life span of the later.
Female Branchipolynoe seepensis show a positive size relationship with their hosts, while neither males nor juveniles showed a similar trend (our data from Lucky Strike 2002; Jollivet et al. 2000). Surprisingly, males and juveniles showed positive size relationships with their respective hosts in our samples from Lucky Strike in 1995. The origin of this discrepancy derived from the host size range of the different data sets. The first two only included mytilids larger than 40 mm, while the 1995 samples started at about 4 mm and never exceeded 45 mm in length. While the size of B. seepensis females seems to be well correlated with host size all along their respective range, similar relationships only occurred for males and juveniles living in small hosts (i.e. whose length does not exceed 45–50 and 30–35 mm, respectively) and never occurred for larger hosts.
Positive symbiont/host size relationships are not common among symbiotic polychaetes (Martin & Britayev 1998) and have not yet been discussed for the association between Branchipolynoe seepensis and Bathymodiolus spp. Two hypotheses have been suggested to explain such positive relationships. First, there may be active size segregation behavior, as reported both for the symbiotic coral-dwelling crab Trapezia ferruginea (Adams et al. 1985) and the fish Gobiodon histrio (Hobbs & Munday 2004), which seem to be able to migrate from colony to colony to choose one of an appropriate size. Secondly, there may be a parallel growth of hosts and symbionts, reported both for the nemertean Malacobdella grossa, hosted by Arctica islandica (Sundet & Jobling 1985) and the pontoniin shrimp Anchistus custos, hosted by the bivalve Pinna bicolor (Britayev & Fahrutdinov 1994). However, we suggest a third hypothesis, that fast growth of the symbionts explains this pattern. In that case, the maximum size of the symbiont may be strictly limited by the host size.
Some Branchipolynoe seepensis (mainly juveniles and males) seem to be able to leave their host mollusks to infest new ones, either driven by intra-specific competition (interference) in juveniles, or by an active searching of a reproductive partner in males. However, the migration process seems not to be common, as direct observations and specific searching from the submersible ‘Mir’ in the 2002 cruise of the R/V ‘Mstislav Keldish’ did not reveal the presence of B. seepensis outside their hosts (E. M. Krylova & A.V. Gebruk, personal observations). Our data on locations and host trauma also indicate that symbiont position inside the mantle cavity usually tends to remain unchanged for a long time. In other words, B. seepensis spends a substantial part of its life inside the same host, even in the same location, in contrast with other more mobile symbionts like the above-mentioned T. ferruginea and G. histrio. Accordingly, the second (i.e. growth in parallel) and third (i.e. fast symbiont growth) hypotheses seem more likely to explain the observed trends in host/symbiont size relationships.
The nature of the observed differences in host/symbiont size relationships of females versus that of males/juveniles must also be explained. The maximum male length, about 17 mm, corresponds to a 45–50 mm host length, which represents the upper limit in the positive sector of male/host size regression (Fig. 5B). Accordingly, all (or nearly all) males have to leave their hosts (or die) at, or just before, this critical size. The resulting emptied mollusks may be re-infested by juveniles, leading to a disruption in the host/symbiont size relationship when the scale worms become adults (Fig. 5B, right side of the graph). In the case of juveniles, the upper limit of the positive size relationship is around 7 and 35 mm length for the symbionts and hosts, respectively. Larger worms have already started to develop secondary sexual features (e.g. gonoduct papillae), so that they are considered either as males or females. The associations at the right side of the graph (Fig. 5C), which are far from the levels of confidence for the positive regression line, are thus considered as the result of secondary colonization/recruitment of empty large mytilids.
The destiny of large males still remains an open question. Pair formation and pseudo-copulation with internal fertilization was suggested as the more reliable reproductive behavior in this species (Van Dover et al. 1999; Jollivet et al. 2000). Coupling in Branchipolynoe seepensis is indirectly supported by the fact that the observed male/female pairs were more numerous than expected (Fig. 7D). We may infer from this that adult males leave their hosts searching for females. This is a critical behavior, probably leading to a high level of mortality, either before or after spawning. Mortality associated with spawning may be an intrinsic characteristic of males (males are semelparous, in contrast with the semi-continuous iteroparous females). This agrees with the ability of females to store sperm and may explain the observed deviation in sex ratio in favor of females among mature worms.
The combination of all above features allowed us to propose a life cycle scenario for Branchipolynoe seepensis. The symbionts (as larvae or as juveniles) infest mytilids all along the host size range, provided they are not already colonized. Most settlers infest small empty mytilids (i.e. 3.6 to 25–35 mm length), as they are substantially more numerous than larger ones (Comptet & Desbruyères 1998; E. M. Krylova & A.V. Gebruk personal observations), and virtually all larger mytilids are infested (Fig. 2). Juveniles then live permanently inside the mantle cavity, growing with their hosts. As growth progresses, they become males or females. Mature males leave mytilids for pairing and then die. We assume that pairing must be temporary, as this species has a much lower frequency of pairs compared with other symbiotic polychaetes with pair formation (e.g.Ruff 1991; Martin et al. 1992; Britayev & Zamyshliak 1996). In turn, females continue life as permanent residents inside the hosts. As they are able to store sperm, females may be able to produce several generations of fertilized eggs from a single coupling.
Among mytilids longer than 25–35 mm, the infestation rate exceeds 80–100% so that they are a limited resource for symbionts looking for hosts. Empty mytilids in this size group may occur as a result of symbiont death or male reproductive migrations, or lack of initial colonization. Then, they again became available either to be colonized as a first host by settling larvae (secondary colonizers) or to be occupied by migrating males as a temporal shelter. Although the secondary colonizers will again grow together with their hosts, the wide range of host sizes will prevent the detection of any significant host/symbiont size relationship.
As a result of both our data and the previous studies (Desbruyères et al. 1985; Fisher et al. 1994; Jollivet et al. 2000; Van Dover 2002; Britayev et al. 2003; Ward et al. 2004), it has been possible to point out some peculiarities of the Branchipolynoe seepensis life strategy. These include the establishment of parasitic (i.e. kleptoparasitic) relationships with their hosts, the starting of the association from the smallest mytilid size classes, sexual size dimorphism, a sex ratio deviated from 1:1 in favor of females, the occurrence of temporal pairing and pseudocopulation, the sperm storage in females, the production of large eggs (likely lecithotrophic or with direct development), a life span longer for females than for males, the possible reproductive migrations of the adult males, a short period of pairing followed by males death (semelparity), and the contrasting semi-continuous iteroparous reproductive cycle in females.
Branchipolynoe seepensis shares some of these features with shallow-water scale worms, particularly those that have also a symbiotic behavior (Martin & Britayev 1998). Several characteristic traits (infestation of hosts early in their ontogeny, sperm storage, extremely large eggs) strongly suggest either the existence of phylogenetic constraints in this particular group of scale worms or the development of specific adaptations to the hydrothermal environment.