Description: This hydroid is somewhat reminiscent of Diphasia alata in that the main stems are straight, robust and taper towards the apex of the colony. In life it is usually of a pale grey to red coloration, but when dried out it turns black, hence the specific name. The side branches are alternate and support alternate hydrothecae. The hydrothecae are tubular in shape and are attached to the side branch for approximately 4/5 of their length. The upper 1/5 being unattached with a slightly flared, smooth outer margin. The gonothecae are arranged on short pedicels. The female capsule is ovate in shape, and has four longitudinal lines that meet at the aperture of the capsule. The male gonothecae are much smaller than the females, and have four short, blunt spines surrounding the aperture, which are absent in females. A large, robust hydroid typically 150-200 mm in height.

Habitat: A deep water circalittoral species found in moderate to strong tidal streams attached to bedrock or stable hard substrata.

Distribution: A rare species which occurs frequently on the east coast of Rathlin Island. Also known from south-west England, north-west France, the western English Channel with a single record from Northumberland.

Key Identification Features:

• Regularly branched, pinnate colonies
• Hydrothecae in opposite pairs, free of the stem for their outer 1/5

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

iNaturalist: Diphasia nigra at iNaturalist World Species Observations database.

WoRMS: Diphasia nigra at World Register of Marine Species. Accepted name: Diphasia nigra (Pallas, 1766). AphiaID: 117881.

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