| Summary: | Gametogenesis and reproductive periodicity of the solitary scleractinians Flabellum alabastrum (from the Rockall Trough) and F. angulare (from the Porcupine Seabight) were investigated. Samples were collected between depths from 1370 to 2190 m for F. alabastrum and 2412 to 2467 m for F. angulare. Both species showed gonochorism with a 1:1 sex-ratio and broadcast spawning of gametes is inferred from the lack of brooded planulae. Oocyte sizes were large in both species (925 µm in F. alabastrum and 1015 µm in F. angulare), suggesting lecithotrophic larval development. Fecundity and periodicity of oocyte development differed between the two species. Flabellum alabastrum produced a maximum of 2800 oocytes per polyp quasi-continuously, whereas the deeper species F. angulare produced a maximum of 550 oocytes per polyp either seasonally or periodically. Both species showed size-dependent fecundity. The data show a decrease in oocyte size and fecundity with depth, in concordance with other deep-water scleractinian species.
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