Northern Ireland's Priority Species

Desmarestia dudresnayi – a brown seaweed

Desmarestia dudresnayi

Desmarestia dudresnayi Lamouroux ex Leman
Family: Desmarestiaceae

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

  • 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 one site

  • 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/Causes of decline
Vulnerable to coastal changes including dumping of illegal waste and changes to current.

Conservation of this species

Current action

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

Proposed objectives/actions

  • Maintain the viable population of the species.

What you can do
Report any records to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, [at]

Further information



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: 95200.

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.

Text written by:
Osborne Morton