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.

Action:

• Increase the number of viable populations of the species
• Maintain the range of the species
• Establish appropriate management of known sites
• Establish measures emerging from the Northern Ireland inshore fisheries review to ensure recovery in key areas.

Objectives: Records of new sites for this species, with details of habitat and population are always valuable. Send these to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257 or e-mail cedar.info [at] nmni.com. If you are a diver, and interested in becoming involved in recording marine life in Northern Ireland through the Seasearch project, contact Claire Goodwin at claire.goodwin [at] gmail.com or look at the web site www.seasearch.org.uk (Northern Ireland web pages).

Conservation: Survey work is being conducted throughout Northern Ireland waters (including the north coast) by Northern Ireland Environment Agency for this and other species and habitats generally.

Rathlin Island waters have been designated a Special Area for Conservation under the EU Habitats Directive.

There is no Species Action Plan for Lytocarpia myriophyllum, however the relevant Habitat Action Plan is Inshore Sublittoral Sediment which carries the following measures:

• maintain the extent and quality of sublittoral sediment habitats in the UK, including the full diversity of communities.

Measures to be considered further include:

• protecting sites of conservation importance from damage through contamination and physical disturbance or excessive use (for example, nutrient enrichment, dredging and development)
• requiring EIAs for coastal developments to examine potential effects on nearshore areas
• monitoring any impact of dump sites on inshore sublittoral sediment habitats, communities and wildlife, and take action as appropriate
• developing and implementing strategies for the conservation and management of the wider marine environment at local, regional and national levels. For example, integrated Coastal Management Plans, water quality objectives, pollution control and avoidance measures. Species recovery and habitat restoration programmes should be included.

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.

Literature:

Picton, B.E. and Morrow, C.C. (2007). [In] Encyclopedia of Marine Life of Britain and Ireland.

http://www.habitas.org.uk/marinelife/species.asp?item=D5660.

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

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Allen & Mellon Environmental Ltd. (2026). Lytocarpia myriophyllum. (Linnaeus, 1758). [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=D5660. Accessed on 2026-07-28