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 A small hydroid rarely exceeding 7 cm in height. Erect stems rise from a branching stolon. Stems occasionally branched. Branched stems reach greater heights than non branched stems with a maximum of ca 7.5 and 3 cm respectively (Cornelius, 1995b). Hydrothecae are small and variable in shape. Gonothecae are large and pear shaped.Dynamena pumila is found on wave sheltered to moderately wave exposed shores down to the shallow sublittoral. The microhabitat that Dynamena pumila occupies relates to the level of wave exposure of the habitat (Cornelius, 1995b). In sheltered habitats, it may be found on the tips of Ascophyllum nodosum fronds. In slightly more wave exposed habitats, the hydroid may be found lower down on the stipes of Fucus vesiculosus and, on wave exposed coasts, between the holdfasts of Fucus serratus (Cornelius, 1995b). Dynamena pumila is tolerant of brackish conditions.

Dynamena pumila may be confused with Amphisbetia operculata or Tridentata distans, both of which also have paired hydrothecae. However, both species are less common than Dynamena pumila. Amphisbetia operculata is a more delicate species with longer cusps, while the stem of Tridentata distans bears several oblique or triangular nodes/joints (Cornelius, 1995b). Dynamena pumila have no free swimming medusae and planula larvae are brooded in an extension of the gonothecae (acrocyst).


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©  The Marine Biological Association of the United Kingdom

Source: Marine Life Information Network

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