Bythograea laubieri, B. vrijenhoeki and alvinellid polychaetes dominate the vent fauna<br/>of the Pacific-Antarctic Ridge. We studied gonadal development in females of both crabs. They have<br/>ovary microstructures characteristic of brachyuran decapods. The ovaries are paired organs overlying<br/>the hepatopancreas beneath the carapace. Oogonia proliferate from the germinal epithelium and<br/>develop into previtellogenic oocytes that grow to 60 Mm before undergoing vitellogenesis. The<br/>observed maximum sizes of mature oocytes were 209 Mm in B. laubieri and 138 Mm in B. vrijenhoeki.<br/>No ovigerous females were found in the samples, which agrees with the segregation behaviour of<br/>ovigerous females away from the direct influence of the active chimney in some related species. In<br/>contrast to other species of bythograeid crabs, the oocyte size-frequency data suggest that B. laubieri<br/>and B. vrijenhoeki lack synchrony in reproduction of the population as a whole. Synchrony and seasonality<br/>in reproduction of B. thermydron have been linked to the formation of phytoplankton blooms<br/>in surface waters. We suggest that the biogeography of bythograeid crabs is determined by contrasting<br/>oceanographic regimes that influence the reproductive patterns observed in different species
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