colonial; fixed gonophores

Fluorescent.

 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).

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

Arctic Ocean, Atlantic, Belgian Exclusive Economic Zone, Blankenberge, British Isles, Canada, Cobscook Bay, Dutch Exclusive Economic Zone, East North Atlantic, European waters (ERMS scope), Gulf of Maine, Gulf of St. Lawrence, Irish Exclusive economic Zone, Mediterranean Sea, Nieuwpoort, North West Atlantic, Oostende, St. Lawrence Estuary, Swedish Exclusive Economic Zone, United Kingdom Exclusive Economic Zone, USA, Wimereux, Zeebrugge

Arctic to Cape Hatteras

A common and widespread species in the British Isles.

Known from rocky shores.

intertidal and infralittoral of the Gulf and estuary

Depth range based on 135 specimens in 1 taxon.
Water temperature and chemistry ranges based on 16 samples.

Environmental ranges
  Depth range (m): -3 - 79
  Temperature range (°C): 10.036 - 12.348
  Nitrate (umol/L): 4.729 - 7.121
  Salinity (PPS): 34.621 - 35.363
  Oxygen (ml/l): 5.952 - 6.151
  Phosphate (umol/l): 0.336 - 0.511
  Silicate (umol/l): 2.448 - 5.452

Graphical representation

Depth range (m): -3 - 79

Temperature range (°C): 10.036 - 12.348

Nitrate (umol/L): 4.729 - 7.121

Salinity (PPS): 34.621 - 35.363

Oxygen (ml/l): 5.952 - 6.151

Phosphate (umol/l): 0.336 - 0.511

Silicate (umol/l): 2.448 - 5.452
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

 Dynamena pumila is an intertidal hydroid found primarily on kelp and fucoid species where it can form dense populations. It can also be found growing on rock.

Typically this is a shore species, normally growing on the brown seaweed 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.