Description: The main stem is straight but not as robust as Diphasia alata, sometimes it is extended, forming a long thin tendril that divides at its apex, into two short, thickened processes. The side branches are alternate and support oppositely paired hydrothecae. The hydrothecae are attached to the side branches for approximately 1/2 of their length. They are tubular in shape and bend outwards from the side branch in 1 plane at an angle of approximately 45 degrees. The outer rim of the hydrotheca is smooth, and on the inner face there is a notch and the opercular flap adjoins this. Colonies typically 40-50mm in size.

Habitat: This hydroid often grows on the stems of other hydroids, and the tendrils are probably used to help secure attachment.

Distribution: Reported from scattered localities all around the British Isles, but appears to be more frequent on the south western coasts.

Similar Species: This is a smaller Diphasia species and is most likely to be confused with Diphasia rosacea or Diphasia fallax.

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

iNaturalist: Diphasia attenuata at iNaturalist World Species Observations database.

WoRMS: Diphasia attenuata at World Register of Marine Species. Accepted name: Diphasia attenuata (Hincks, 1866). AphiaID: 117877.

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