Northern Ireland's Priority Species

Mertensia maritima – oyster plant


Distribution map

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Mertensia maritima (L.) Gray
Family: Boraginaceae

Oyster plant, Mertensia maritima, is a northern coastal species found in the British Isles, Scandinavia, Greenland, and northern North America. A closely-allied species, Mertensia asiatica, occurs on coasts of east Asia. The British and Irish populations are the most southerly in Europe, but there has been a loss of sites from southern parts of Ireland and Great Britain over the past century or so.

In brief

  • Found on some of the shores of south County Down and north County Antrim

  • Grows on shingle, or occasionally sand, on the upper sea shore

  • Best seen in flower, June to August

  • Northern Ireland holds the majority of the Irish sites

  • Threatened by climate warming and habitat damage, especially shingle removal.

Species description
An attractive perennial herbaceous plant with the shoots lying flat against the ground. A long spirally-twisted taproot anchors the plants in the substrate and one to many shoots arise from the crown of this root. Leaves are thick, oval, a silvery blue-green, reputedly tasting of oysters. Flowers, starting pink but turning blue, are formed in clusters from June onwards.

Life cycle
Flowers are probably normally self-pollinated, although some insect visitors have been observed in non-Irish populations. The fruits ripen from the end of July to September and normally germinate during the following season from spring onwards, with germination rates being improved by exposure to cold temperatures of 2°C. Seeds can be dispersed by sea water; immersion in sea water for up to 18 days has no adverse effect on viability. Germination produces young plants with a rosette of leaves, with the first flowering shoots produced the year following germination. During winter the above-ground parts die back, and fresh shoots arise each spring from dormant buds at or below ground level. After about five years, when maximum growth of individual plants has been reached, they enter senescence, with death following in autumn. Vegetative reproduction from fragments of root systems broken off during winter storms and shingle movement may occur.

Similar species
There are no similar species in Ireland.

How to see this species
Formerly, the best site to see this plant was at Glassdrumman Port north of Annalong in County Down, but the site was severely damaged recently by illegal shingle removal.

The plant is well established in some of the coves in the cliffs to the east of the Giantís Causeway.

Current status
Oyster plant is currently recorded from two main areas in Northern Ireland: the Giantís Causeway area of Co. Antrim and on Rathlin Island, and the shingle beaches between Bloody Bridge and Glasdrumman port in Co. Down. It occurs sporadically in small numbers at a few sites around Dundrum Bay. It is protected under the Wildlife (NI) Order, 1985.

Why is this species a priority in Northern Ireland?

  • Northern Ireland holds the majority of the Irish sites

  • It is listed as rare in the Irish Red Data Book.

Threats/Causes of decline
The principal threat at present is habitat destruction caused by illegal removal of shingle for use as aggregate or other purposes. This recently occurred at the prime County Down site, Glassdrumman Port, with severe consequences to the population. Losses may also result from storms, recreational pressures or coastal development.

Over the past century or more there has been a shift of the latitudinal range of oyster plant northwards, associated with climatic warming and retreat of glaciers. This is reflected in Great Britain and Ireland with the loss of sites in southern England, Wales and the southern half of Ireland.

Conservation of this species

Current action

  • Oyster plant is protected under the Wildlife Order (NI) 1985

  • Removal of shingle from beaches is illegal

  • There is a UK Habitat Action Plan for Coastal Vegetated Shingle

  • There is a Northern Ireland Habitat Action Plan for Coastal Vegetated Shingle.

Proposed objectives/actions

  • Prevent further net loss of existing vegetated shingle structures. (However local gains and losses due to storm events occur sporadically and should be accepted provided that the regional resources are maintained overall)

  • Prevent, where possible, further exploitation of, or damage to, existing vegetated shingle sites through human activities

  • Maintain the current extent of coastal vegetated shingle at 50ha

  • Maintain the area of coastal vegetated shingle in favourable condition at 25ha

  • By 2015, restore to favourable condition as much as is practical of the remainder of the resource, that is, 25ha.

What you can do
Records of the sites where you see this species are important, especially if you find it in one of the old sites from which it has not been recorded recently. Send any records to either Botanic Society for the British Isles (BSBI), c/o Botany Department, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU or to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5256, [at]

Also important are reports of damaging activities such as removal of shingle. Report any such observations to Environment & Heritage Service.

Further information

UK Habitat Action Plan for Coastal Vegetated Shingle

Northern Ireland Action Plan for Coastal Vegetated Shingle

Flora of Northern Ireland

Curtis, T.G.F and McGough, H.N (1988). The Irish Red Data Book – 1 Vascular Plants. Stationery Office, Dublin.

Day, G. and Hackney, P. (2004). County Down Scarce, Rare & Extinct Vascular Plant Register and Checklist of Species. National Museums and Galleries of Northern Ireland.

Preston, C.D., Pearman, D.A. and Dines, T.D. (2002). New Atlas of the British & Irish Flora. Oxford University Press/DEFRA.

Scott, G.A.M. (1963). Biological Flora of the British Isles – No. 89 Mertensia maritima (L.) Gray. Journal of Ecology 51: 733-742.

Text written by:
Paul Hackney