Description: The main stem of the colony is thick and sparsely branched with alternately arranged side branches. Often the main stem and side branches terminate in long, thin tendrils. The hydrothecae are short and are arranged in opposite pairs. They are attached to the side branches for approximately 3/4 of their length, the upper 1/4 being unattached with a flared outer margin and a S-shape curved inner margin. There is a single opercular flap attached to the inner margin. The gonothecae are relatively large and are arranged on short pedicels. The male capsule is elongate, and tapers towards the base. There are four thick spines that surround the males raised aperture. The female gonotheca is more ovate in shape and four long, leaf-like processes surround the aperture. Typically individual fronds are 60 mm in length.

Habitat: This hydroid often grows on other hydroids. It is normally found in moderate tidal streams and may be northern in its distribution.

Similar Species: This is one of the smaller Diphasia species and should be compared with Diphasia attenuata and Diphasia rosacea.

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

iNaturalist: Diphasia fallax at iNaturalist World Species Observations database.

WoRMS: Diphasia fallax at World Register of Marine Species. Accepted name: Diphasia fallax (Johnston, 1847). AphiaID: 117879.

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