Description: The main stems are straight and unbranched and arise from a tangled basal mass of rhizoids, usually in clumps of 3 or more. In strong tidal streams but shelter from wave action these clumps may consists of 50 or more stems. Each upright branch is reported to be a separate individual (Hughes, 1977). The main stems bear whorled side-branches in groups of 6, each bearing approximately 6 small hydrothecae. The hydrothecae and defensive polyps have a similar structure and arrangement to Nemertesia ramosa. The reproductive gonothecae are ovoid and clustered around the stem on short pedicels in mature colonies. Colonies normally 200-300 mm in height when full-grown.

Habitat: This is a common species in the sublittoral environment and appears to prefer stable rock surfaces in the circalittoral zone, in areas subjected to moderate to strong currents. It is scarce at sites exposed to the Atlantic swell.

Ecology: The nudibranchs Lomanotus marmoratus, Doto pinnatifida and Doto fragilis all feed on this hydroid.

Distribution: This species is found all around the British Isles.

Similar Species: Unlikely to be confused with any species other than Nemertesia ramosa.

Key Identification Features:

• 3-dimensional whorled secondary branches.
• Main stems unbranched and in clumps.

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

iNaturalist: Nemertesia antennina at iNaturalist World Species Observations database.

WoRMS: Nemertesia antennina at World Register of Marine Species. Accepted name: Nemertesia antennina (Linnaeus, 1758). AphiaID: 117809.

Classification: Biota; Animalia; Cnidaria; Medusozoa; Hydrozoa; Hydroidolina; Leptothecata; Plumularioidea; Plumulariidae; Nemertesia