This is a rare large seaweed recorded from only one site in Northern Ireland. It grows below low water and has only rarely been recorded anywhere in the British Isles.

In brief

• In Northern Ireland, D. dresnayi has only been recorded from Rathlin Island and the Maidens
• Found in the sublittoral zone in moderate to strong sea currents
• It is an annual species recorded between May and September
• This species is rare, being recorded from only a few sites
• The only threat is any threat to the coast.

Species description: Desmarestia dresnayi is a marine alga, olive brown in colour and of erect habit growing to about 35cm in length with a flattened frond up to 6cm wide. The thin blade rises from a short stipe, a distinct midrib and opposite veins; the blade is somewhat papery in nature.

Life cycle: Unilocular sporangia are scattered over both sides of the blade. Meiosis (reduction division) takes place in the sporangium and haploid zoospores escape. These germinate and give rise to gametophytes bearing oogonia and antheridia. When fertilized each oogonium grows to produce the diploid adult.

Similar species: Similar to young plants of Laminaria saccharina which, however, do not have a midrib. Alaria esculenta does have a midrib; however, the blade of Alaria is much more leathery and may have a cluster of strap-shaped sporophylls near the base. The midrib with opposite veins confirms the identification of Desmarestia. Dictyopteris membranacea has a midrib, but no side veins and is branched dichotomously, similar to Fucus but more delicate.

How to see this species: This species has been recorded at Altacorry Head, Rathlin Island. It is a summer annual with a western distribution. It only grows sublittorally on stones and shells in areas of moderate to strong water current, to depths of at least 18m, between May and September.

The similarity of Desmarestia and young Laminaria saccharina suggests that it could be overlooked.

Current status: The only find of this species from Northern Ireland is from Altacorry Head on Rathlin Island, where it was recorded in the sublittoral by C.A. Maggs (Ulster Museum: F5037). Other records are from the west coasts of Ireland and Scotland, and from France and Spain.

Why is this species a priority in Northern Ireland?

• This species is rare, being recorded from only one site.

Threats: Vulnerable to coastal changes including dumping of illegal waste and changes to current.

Action:

• Maintain the viable population of the species.

Objectives: Report any records to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, cedar.info [at] nmni.com.

Conservation:

• Its only known site is designated as Ramsar site, SPA, SAC, MNR and ASSI.

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

iNaturalist: Desmarestia dresnayi at iNaturalist World Species Observations database.

WoRMS: Desmarestia dresnayi at World Register of Marine Species. Accepted name: Desmarestia dudresnayi J.V.Lamouroux ex Léman, 1819. AphiaID: 295864.

Literature:

Drew, E.A. and Robertson, W.A.A. (1974). Direct observation of Desmarestia dresnayi Lamour. ex Laman in the British Isles and in the Mediterranean. British Phycological Journal 9: 95–200.

Fletcher, R.L. (1987). Seaweeds of the British Isles. Volume 3, Part 1. British Museum (Natural History), London.

Hardy, and Guiry, M.D. (2003). A Check-list and Atlas of the Seaweeds of Britain and Ireland. British Phycological Society.

Morton, O. (1994). Marine Algae of Northern Ireland. Ulster Museum, Belfast.

Morton, O. (2003). The Marine Macroalgae of County Donegal, Ireland. Bulletin Irish Biogeographical Society 27: 3-164.

Classification: Biota; Chromista; Harosa; Heterokonta; Ochrophyta; Phaeophyceae; Fucophycidae; Desmarestiales; Desmarestiaceae; Desmarestia

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Osborne Morton (2026). Desmarestia dresnayi. Lamouroux ex Leman. [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=ZR4980. Accessed on 2026-07-27