A large hydroid that occurs in a number of areas around Northern Ireland, but whose status may have deteriorated due to habitat loss.

In brief

• Recorded off Rathlin Island, Maidens, off County Down
• Sublittoral sediments, shells and gravels at depths exceeding 25m
• A conservation priority because of habitat loss
• Main threats — deployment of mobile fishing gear, disruption of seabed.

Species description: A large hydroid, about 300-400mm in height, with tall feathery colonies of unbranched main stems carrying straight side branches about 20mm in length. These are arranged in pairs, almost opposite each other so that the cross-section as viewed from above represents an open ‘V’. The polyps (inhabitants of the colony) face the concave surface of the V. The colonies are somewhat colourless or pale grey. A number of colonies tend to occur in clumps, and seemingly share the same base, or at least abut each other at their bases.

Life cycle: Reproductive polyps are born on modified side branches, which produce medusae (jellyfish-like juvenile forms which may be free swimming or remain attached).

Similar species: Gymnangium montagui and Aglaeophenia spp are similar, but L. myriophyllum is much larger.

How to see this species: L. myriophyllum is found at numerous sites around Rathlin Island, off the Maidens Rocks, off the outer Ards Peninsular, off Kearney Point, St John’s Point, and various locations off south Down. Further afield in Ireland it occurs in Kilkieran Bay and a number of sites in south-west Ireland. It is recorded from a number of sites amongst the islands in south-west Scotland. It occurs in depths exceeding 25m, in semi-sheltered locations experiencing moderate tidal streams.

Current status: The species is known from a number of sites along the Antrim and Down coasts. It has no legal protection.

Why is this species a priority in Northern Ireland? The species is rare and declining. Its habitat has been heavily affected by bottom trawling and dredging fisheries, and it is highly likely that this is the reason for its decline in numbers and distribution.

Threats: Loss of habitat due to impact of bottom fishing with mobile gear.

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

iNaturalist: Lytocarpia myriophyllum at iNaturalist World Species Observations database.

WoRMS: Lytocarpia myriophyllum at World Register of Marine Species. Accepted name: Lytocarpia myriophyllum (Linnaeus, 1758). AphiaID: 117302.

Classification: Biota; Animalia; Cnidaria; Medusozoa; Hydrozoa; Hydroidolina; Leptothecata; Plumularioidea; Aglaopheniidae; Lytocarpia