Description: Colony pinnate with the main stem straight and thick and the side branches alternate. The side branches increase in length towards the colonies base, giving a triangular outline to the apex of the colony. The hydrothecae are arranged in opposite pairs and are attached to the side branches for about 2/3 of their length. The outer rim of the hydrotheca is smooth and flared and there is a single, rounded opercular flap. Both the male and female gonothecae are small, quadrangular in shape and taper towards the base. Each 'corner' of the capsule forms a projection which curves inwards towards the centre of the theca. The colour of the colony in life is a distinctive chocolate-brown. Colony typically 100-130 mm in height by 60 mm in width.

Habitat: A deep-water species rarely reported from diving depths in the British Isles. On Rathlin Island, Co Antrim, this species is quite common on the wave sheltered east coast below 30 metres attached to bedrock and stable boulders in tidal streams of 2-4 knots.

Distribution: This species is known from scattered localities around the British Isles, and from Norway to the Azores.

Similar Species: On Rathlin Island this species occurs in the company of several other rather rare hydroids, namely Diphasia nigra, Polyplumaria flabellata, Diphasia fallax.

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

iNaturalist: Diphasia alata at iNaturalist World Species Observations database.

WoRMS: Diphasia alata at World Register of Marine Species. Accepted name: Diphasia alata (Hincks, 1855). AphiaID: 117876.

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