Life-history traits of the symbiotic scale-worm Branchipolynoe seepensis and its relationships with host mussels of the genus Bathymodiolus from hydrothermal vents.

13 páginas, 10 figuras, 2 tablas.Associations between scale-worms and giant mussels are common constituents of hydrothermal vent and cold seep ecosystems, but very little is known about their nature and ecology. Here, we analyze the ecological characteristics of the associations between Branchipolynoe seepensis, an obligate symbiotic polychaete, and their host mytilid mussels Bathymodiolus puteoserpentis and B. azoricus inhabiting hydrothermal vent fields on the Mid-Atlantic Ridge. Infested mussels generally harbored a single symbiont (<4% had two to six worms). Infestation rate varied from 7.2% to 76.5%, increasing with mussel size, and was significantly lower for B. puteoserpentis. Symbiont density at Lucky Strike ranged between 1071 and 1191 individuals m)2. Female symbiont size was always positively correlated with host size, while only males and juveniles from small mussels showed the same trend. This suggested a relatively long-lasting host/symbiont association for females and short-lasting association with successive reproductive migrations for adult males. The sex ratio of symbionts was always biased in favor of females. Males were smaller and more slender than females and had one mode in their size distributions, whereas females typically had three or more modes, suggestive of a longer life span in females. Between 59.1% and 72.2% of mussels had damaged soft tissues with substantially higher incidence of trauma in infested ones, suggesting that symbionts may cause trauma. The symbionts also induce tunnel-like structures among the ctenidia, indicating fidelity to a particular location inside the host. Based on our data, together with the fact that infested mussels became relatively wider than non-infested ones, this association is considered parasitic (likely kleptoparasitic). Our data, together with those from previous studies, allowed us to define the main life-history traits of B. seepensis: (i) the relationship with their host is parasitic, (ii) the association begins at the smallest mytilid size classes, (iii) there is sexual dimorphism in body size, (iv) sex ratio deviates from 1:1 in favor of females, (v) fertilization occurs through temporal pairing and pseudocopulation, (vi) sperm are stored by females, (vii) eggs are large (likely lecithotrophic or with direct development), (viii) females have a longer life span than males, (ix) adult males may be semalparous, undertaking reproductive migrations followed by a short period of pairing and then death, and (x) females have a semi-continuous iteroparous reproductive cycle.This study was supported by the Russian Foundation for Basic Research (project no. 05-04-48350)Peer reviewe

Symbiont–host interactions in the association of the scale-worm Branchipolynoe aff.seepensis (Polychaeta: Polynoidae) with the hydrothermal mussels Bathymodiolus spp. (Bivalvia: Mytilidae)

4 páginas, 4 figuras, 1 tabla.Introduction Symbiotic polynoids of the genus Branchipolynoe associated with mussels are a common component of hydrothermal vent and coldwater seep communities. Recent detailed studies carried out on the Atlantic scale-worm B. aff. seepensis Pettibone, 1986 (Jollivet et al., 2000) revealed some interesting features of its reproductive biology: external sexual dimorphism, internal fertilisation and dominance of females in the population. Nevertheless, the role of symbiotic polychaetes in the communities and their relationships with their hosts remain virtually unknown. In fact, the association of the different species of Branchipolynoe with their host mytilids Bathymodiolus spp. has been usually regarded as commensalism (Pettibone, 1986; Chevaldonne et al., 1998). However the finding of bits of mussel gills in the gut of B. symmyitilida from the Galapagos Rift (Desbruyères et al., 1985) pointed to a parasitic behaviour. The present paper reports preliminary results based on material collected mainly during a recent cruise of the R/V Akademik Mstislav Keldysh (2002) at the hydrothermal vent fields along the MAR and complimentary data of previous French and Russian expeditions which allowed to better understand the true nature of the relationships between the Atlantic scale worm B. aff. seepensis with their mytilid hosts Bathymodiolus puteoserpentis and B. azoricus.This study was supported by the Russian Foundation for Basic Research (project no. 01-04- 49022).Peer reviewe