Description: This is a tiny hydroid consisting of clusters of unbranched stems bearing hydrothecae in a line on one side of each stem. The stems are typically greenish-yellow in colour. Hydrothecae are well-separated on the stems, with a jointed section between each hydrotheca-bearing one. Typically 30mm in height, but white colonies larger than the normal form were found at Lundy Island in 1984 in abundance.

Habitat: Normally grows on bedrock, boulders or cobbles with some nearby sand or gravel causing scouring of the substratum. Usually in weak tidal streams.

Distribution: Widely distributed in the British Isles, but more common in the south.

Similar Species: The overall appearance of this species is similar to that of individual side-branches of Halopteris catharina but this species has unbranched stems arising from a basal rhizoid mass.

Key Identification Features:

• Hairlike colonies with hydrothecae arranged in a single series

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

iNaturalist: Antennella secundaria at iNaturalist World Species Observations database.

WoRMS: Antennella secundaria at World Register of Marine Species. Accepted name: Antennella secundaria (Gmelin, 1791). AphiaID: 117628.

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