Description: A fan-like hydroid with irregularly branching main stems and regular side-branches arising in opposite pairs from the main stem. The hydrothecae are arranged on the upper edge of the side branch. They are tubular in shape with a smooth outer margin, and are surrounded by 4 smaller, defensive polyps, one on either side, and one above and below. The gonothecae are arranged on short pedicels. They are roughly oval shaped, and have a wide aperature. The capsule tapers towards the base, and is surrounded by four small, defensive polyps, located in the basal region. Typically 70 mm with side-branches 10 mm in length.

Habitat: A deep water species rarely found in less than 50 m and thus not often encountered by divers. Found at sites with moderate to strong tidal streams and scour from mobile gravel.

Ecology: Grows in company of Diphasia spp. especially Diphasia alata, in sites with a high diversity of hydroids.

Distribution: The east coast of Rathlin below 40 m; the Maidens, Co Antrim; North Channel 45m; east side of Ushant 50 m. Also reported from Shetland, western Scotland, North Sea, Plymouth, Scillies and Roscoff.

Similar Species: This is a distinctive species, unlikely to be confused with any other apart from Polyplumaria frutescens.

Key Identification Features:

• Side branches opposite, bearing short third-order branches

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

iNaturalist: Polyplumaria flabellata at iNaturalist World Species Observations database.

WoRMS: Polyplumaria flabellata at World Register of Marine Species. Accepted name: Polyplumaria flabellata Sars, 1874. AphiaID: 117827.

Classification: Biota; Animalia; Cnidaria; Medusozoa; Hydrozoa; Hydroidolina; Leptothecata; Plumularioidea; Halopterididae; Polyplumaria