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DescriptionA long, flexible hydroid colony with a prominent main stem and branches. Usually up to 20 cm in length but may reach 35 cm in British waters. Side branches of uniform length but shorter distally giving the colony a tapering outline. Main stem is long, dark and unforked but may become forked in older colonies. The main stem is reddish brown in colour, becoming dark brown to black with age. The segments of the stem, the internodes, are nearly straight, or slightly curved and perfectly tubular. Side branches usually divide into two just after the origin, occasionally into three, with subsequent branches arranged in a zigzag. In young branches the point where the internodes meet, the nodes, are dark, giving a characteristic alternating light and dark pattern. Side branches are usually lighter in colour than the main stem, and decrease in length along the length of the colony.
The polyps are borne in a thin chitinous cup, the hydrotheca. Hydrothecae are elongate (ca 320-500 µm), inverted conical or bell shaped, with a distinctly tapering low portion. The rim of the hydrotheca is either shallow castellate or shallow blunt-cusped but usually rubbed smooth. The base of the hydrothecae attach to the stem by a pedicel composed of up to 20 rings. The reproductive polyps (gonothecae) are elongate and flask shaped, ca 700-1050 µm in length, and release medusae in spring.Obelia longissima may be confused with other Obelia species. For example, Obelia dichotoma may also be elongate but lacks the regular shape and extreme length of Obelia longissima (Cornelius, 1995b). Obelia bidentata has multiple branched stems even when young. Colonies of Obelia dichotoma may be distinguished from Obelia longissima growing in rockpools in spring by its long tubular, nearly straight and darkening internodes (Cornelius, 1995b). No reliable key is available to distinguish between the medusae of Obelia species (for discussion see Cornelius, 1995b).