Reproductive biology, sexual dimorphism, and population structure of the deep sea hydrothermal vent scale-worm, Branchiplynoee seepensis (Polychaeta : Polynoidae)

The polychaete family Polynoidae (scale-worms) is well-represented at deep sea hydrothermal vents. Most species are free-living in a wide range of hab itats: from high-temperature hydrothermal 'chimney' walls to diffuse ventin g areas. Conversely, species of the genus Branchipolynoe live inside the ma ntle cavity of vent and seep mytilids. Specimens, morphologically close to Branchipolynoe seepensis, have been reported from all the known vent areas on the Mid-Atlantic Ridge (MAR), with varying infestation rates (0-6 indivi duals per host). Reproductive tract, gametogenesis and population structure s were examined for specimens from the Lucky Strike vent field (MAR) in ord er to test whether this species displays dwarf males, protandric hermaphrod itism or differential mortality between males and females. Observations of histological sections reveal the presence of fully developed ovaries in fem ales which originate ventrally in segments 7-9 and of an unusual genital tr act in which both sperm and mature oocytes are stored. Oogenesis is intraov arian and quasi-continuous. The vitellogenic oocytes are only free in the c oelom at their terminal growing stage and are then transferred into an ovis ac through spermathecae. The species displays an external sexual dimorphism in the number of genital papillae and the shape of the pygidial appendages . Sex ratios showed significant deviations from a 1:1 expected ratio, in fa vour of females. The modal decompositions of size-frequency histograms show the occurrence of three modes in females and only two modes in males, indi cating discrete breeding periods. The two first modes were not significantl y different between males and females. These results indicate that B. seepe nsis forms heterosexual pairs and uses internal fertilization to reproduce during discrete spawning periods. Differential mortality between males and females is likely to shape size-histograms as observed by preventing males from reaching the female proportions. Such an observation could be a result of either cannibalism on larger males, small sizes facilitating the male e scape, or natural predation when males move from one bivalve to another to breed.