Description: This hydroid consists of upright, unbranched or sparsely branched main stems arising from a branching rhizome. The stems bear triangular hydrothecae and are constricted between each pair to a narrow waist.

Habitat: Typically this is a shore species, normally growing on fucoids, especially Fucus serratus. It is most abundant on shores subject to strong water movement from tidal streams or wave action. It may also occur in brackish water, below low water mark, but is otherwise not usually found in the sublittoral zones.

Distribution: A common and widespread species in the British Isles.

Similar Species: Most likely to be confused with Diphasia species, due to its opposite, paired hydrothecae.

Key Identification Features:

• Hydrothecae in opposite pairs, with narrow constriction of main stem between each pair
• Normally on the low shore, attached to fucoids such as Fucus serratus or Ascophyllum nodosum.

Distribution Map from NBN: Dynamena pumila at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Dynamena pumila at iNaturalist World Species Observations database.

WoRMS: Dynamena pumila at World Register of Marine Species. Accepted name: Dynamena pumila (Linnaeus, 1758). AphiaID: 117888.

Classification: Biota; Animalia; Cnidaria; Medusozoa; Hydrozoa; Hydroidolina; Leptothecata; Sertularioidea; Sertulariidae; Dynamena