This is a rare large seaweed recorded from only Rathlin or The Maidens 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 35 cm in length with a flattened frond up to 6 cm 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 Saccharina latissima 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 polypodioides 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 from Rathlin Island and The Maidens. 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 18 m, between May and September. The similarity of Desmarestia and young Saccharina latissima suggests that it could be overlooked.

Current status: Rare. It was first recorded from Northern Ireland at Altacorry Head on Rathlin Island, where it was recorded in the sublittoral by C.A. Maggs (Ulster Museum: F5037). It is now known to be found at Rathlin and The Maidens, both in the North Channel. Other records are from the west coasts of Ireland and Scotland, and from France and Spain. It is thought to be a climate-sensitive northern species.

Why is this species a priority in Northern Ireland?

• Criterion 3: Rare or restricted range – rare breeding species confined to 5 hectads or less (based on expert judgement).
• Criterion 5: Climate Change Sensitive.

Threats:

• Vulnerable to coastal changes including dumping of illegal waste and changes to current.
• Since this species is thought to be an indicator of good water quality it may be especially sensitive to changes.

Action: Records of new sites for this species, with details of habitat and population are always valuable. Observations with a photograph can be recorded using iNaturalist, find the MBDP-NI iNaturalist project here.

Alternatively, records can be sent to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257, e-mail cedar.records@nationalmuseumsni.org, or via the online portal.

Objectives:

• Maintain the viable population of the species.

Conservation:

• Rathlin Island is a Marine Protected Area with multiple designations.

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.

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

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Adapted from text by 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 2026-09-13