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DescriptionAglaophenia tubulifera is a colonial hydroid that is infrequently branched at the stolon and has singular, erect, feather-like plumes with brown, monosiphonic stems reaching up to 6 cm in height. The plumes have numerous yellowish, straight or slightly curved branches (hydrocladia) that alternate on either side of the stem and absent towards the base. The upper surface of each branch has a single row of 4 polyps; a feeding hydranth that is nearest the stem and 3 inconspicuous mouth less nematophores. Hydranths are short and cylindrical with an encirclement of 10 short, round-tipped tentacles around the conical hypostome (mouth region) that is encased by a protective chitinous, elongated cup (hydrothecae). The hydrothecae are longer than the hydranths and have 9 projecting cusps, the outer one longest. Nematophores are smaller than the hydranths and encased by nematothecae, those furthest from the stem are horizontally level with the rim of hydrothecae on the same branch. Individual plumes have up to 12 groups of reproductive gonothecae that are protected by basket-like structures of yellowish branches (corbulae) and vary from 0.19 cm to 0.35 cm in length.Colonies of Aglaophenia tubulifera are dioecious. In Roscoff, reproduction has been known to take place from June-September (Svoboda & Cornelius, 1991). The brownish colour of the stem is due to symbiotic zooxanthellae (Svoboda & Cornelius, 1991). The spacing between the branches increases with water depth.