Reproductive ecology of Vestimentifera (Polychaeta: Siboglinidae) from hydrothermal vents and cold seeps
Giant siboglinid tubeworms (formerly Vestimentifera) are ecologically important members of deep-sea chemosynthetic communities, including hydrothermal vents and cold seeps. Species of siboglinids are community dominants, primary colonists of new vent sites, and the longest living and fastest growing marine invertebrates. Their mechanisms of propagation, dispersal and genetic exchange have been widely discussed. Direct sperm transfer from males to females has been documented in two species, Ridgeia piscesae and Tevnia jerichonana, but others were believed to spawn primary oocytes. Brooding of embryos have never been observed in any species. These observations have led to the logical supposition that fertilization must be external in all but two species. Here I report sperm storage at the posterior end of the oviduct in 5 species, including tubeworms from both vents and seeps. I show experimentally that fertilization is internal, that meiosis is completed after eggs are released from the female, and that the dispersal phase includes the entire embryonic period. The contrasting physical and chemical conditions of hydrothermal vents and cold seeps are reflected in the life history patterns of the dierent species of vestimentiferans. Image analysis of histological sections show that vestimentiferans from hydrothermal vents have signicantly less percentage of trophosome than species from cold seeps. The reproductive condition, however, was found to be irrespective of the surrounding environmental conditions. Previous studies of the lipid class composition suggested that vestimentiferans store lipids in the gonad. However, because the gonadal tissue has never been dissected and compared with the other tissues of the trunk, it was not possible to determine whether the gonad comprises the only lipid store in vestimentiferans, or if different lipid classes are stored in different tissues. Here I show that two main storage lipids are found in the trunk of female vestimentiferans. Substantial reserves of wax esters are found in the gonad, and triacylglycerols in other tissues of the trunk. Due to the close association between the gonadal tissue and the trophosome, the only way to determine the reproductive effort in vestimentiferan tubeworms has been, to date, the study of histological sections of the trunk of the individual. The results presented here show a linear relationship between the amount of gonad and the concentration of wax esters in the trunk of female vestimentiferans. The determination of the concentration of wax esters is a biochemical technique that allows comparison of the amount of gonadal tissue with the amount of trophosome, and should be considered as a new and simple method for the determination of the reproductive condition in this group.