Hydroids are a group of animals which are related to sea anemones, corals and jellyfish. Most of them consist of plant-like branching colonies of small zooids, each of which has a ring of tentacles with stinging cells, surrounding a mouth. In some species the branching is very irregular and the zooids are free at the tips of the branches, whilst in other groups the branching is very regular and rather like a feather or a branch of a fir tree, with the zooids enclosed in a protein container. This species is of the latter type, with large numbers of small zooids arranged on branching, two-dimensional feather-like fronds.

In brief

• Populations of this species occur on the east coast of Rathlin and at the Maidens
• In Northern Ireland this species is most abundant on low-lying rock reefs or boulders adjacent to gravel which may contain scallops
• Lives in the sea at depths of 40-100m or more on open coasts
• Scarce in Northern Ireland and almost unknown elsewhere in the UK
• Life cycle unknown, new branches probably arise yearly from a perennial base
• Threatened primarily by bottom fishing with trawls or dredges which roll or remove the boulders.

Species description: A fan-like hydroid with irregularly branching main stems and regular side branches arising in opposite pairs from the main stem. The main stem and secondary branches bear short, regular tertiary branches. The hydrothecae are arranged on the upper edge of the tertiary branches. They are tubular in shape with a smooth outer margin, and are surrounded by four smaller, defensive polyps, one on either side, and one above and below. The gonothecae are arranged on short pedicels. They are roughly oval shaped, and have a wide aperature. The capsule tapers towards the base, and is surrounded by four small, defensive polyps, located in the basal region. Typically 70mm with side branches 10mm in length.

Life cycle: Colonies produce gonothecae in early summer and presumably release planktonic larvae. No other details known.

Similar species: Several other smaller Diphasia species exist in almost the same habitat. Diphasia pinaster has a main stem hardly differentiated from the side branches and Diphasia alata is dark brown in life.

How to see this species: This is a deep water species most likely to be seen by divers visiting the seabed on the east coast of Rathlin Island or the Maidens.

Current status: Populations of this species occur on the east coast of Rathlin. It was always rare in 30-50m depth attached to boulders on a stable gravel seabed. This area has changed considerably since 1986 and when visited in 2005 by divers and a Remote Operated Vehicle (ROV) it was found that the boulders have mostly been removed, presumably by scallop dredging activities. This hydroid was not seen.

Why is this species a priority in Northern Ireland?

• Rapid decline with Northern Ireland being both a UK and Irish stronghold
• Rare - confined to a small population of one or two sites in Northern Ireland.

Threats: The main threat to this species is scallop dredging which, since 1986, has altered the habitat where the main populations live.

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