Sargassum muticum Wireweed

What is it?

Sargassum muticum is an invasive brown seaweed that has recently found its way to the shores of Ireland. In Japanese waters, where it originates, it is a relatively small, innocuous seaweed (1-2 m in length). However, outside its original range it is found to be highly invasive, and can reach impressive lengths of 16 m. It is now widespread in Strangford Lough, many bays in Connemara and Galway Bay, Co's Donegal, Sligo and Mayo, Co's Kerry, Cork and Wexford. It is now believed that this species has a wider distribution than previously thought and that it will spread to all coastal areas of Ireland within a short time.

How did it get to Ireland?

Sargassum muticum was first found and identified in Europe in 1973 by Bill Farnham at Bembridge Ledges in the Isle of Wight, U.K., when plants were about two years' old, and it may have arrived on the north coast of France as early as 1966. Since then, it has spread to most coastal countries in Europe, from Norway to Italy. In 1995, it was discovered in Strangford Lough, Co. Down, near an oyster farm. This was the only known location in Ireland until September 2001, when it was found at Cashel Bay, Co. Galway by Ciarán Loughnane from NUI Galway. The transplantation of oyster seed from infected regions of Europe and the transportation of fertile fronds by currents or by boats or ships are believed to be the most likely sources of inoculation to new areas.

Biology and life history

Sargassum muticum is usually 1-3 m in length, but can grow up to an incredible 16 m in length in certain habitats, and may form floating mats on the sea surface. It can grow at up to 10 cm each day, and it also has a relatively long life-span of 3-4 years. It reproduces both sexually and via floating fragments. Fertilisation usually takes place on the surfaces of the reproductive bodies (receptacles) and thousands of "germlings" are released. These settle rapidly, usually near the parent alga. Dense mats of Sargassum can form very quickly. Fronds, if detached, can continue to shed germlings as they drift, making for a very powerful dispersal mechanism. The loss of an entire frond is not fatal to the weed, as a new frond can grow from the primary axis that remains, often buried in sand or detritus.

The alga has two distinct parts: the perennial, dark brown basal axes, and the lighter coloured annual primary laterals. The latter are shed or torn off in late summer. During the summer months, the number of small round vesicles (air bladders, below and click on picture for a close-up) increases. Receptacles are most abundant in early autumn, just prior to the shedding of annual primary laterals. Because of its strongly seasonal growth pattern, Sargassum muticum may be most conspicuous on the shore during the summer months.

Habitat

Sargassum is highly tolerant to environmental parameters such as desiccation, full sunlight and variations in salinity and temperature. This enables it to occupy a broad range of habitats from the upper intertidal (mainly rock pools) to the subtidal and substrata from exposed rock to Zostera marina (Eel-grass) beds.

What problems are associated with it?

In the European countries where it has already become established, there are several effects reported to be associated with this weed. Its growth on the surface of the water can impede boat traffic and swimmers; it can also cur down light penetration to underwater communities. The reduction in light and space on the sea floor may lead to localised reduction in native species such as Saccharina latifolia (formerly Laminaria saccharina; sugar kelp), Himanthalia elongata (thongweed), and Zostera marina. Large mats of weed that break off in late summer can accumulate on the shore to form dense clumps of slowly rotting weed. However, Sargassum can also populate areas of sparse algal development, thereby providing additional habitats for marine life, such as small fish and crustaceans.

Is there anything that can be done?

It is now believed that with the presence of this species at a number of localities, its spread to the remainder of the coastline is now inevitable. Through the close monitoring of affected areas, and increased public awareness and vigilance, the speed at which it spreads could be reduced, but this is a highly invasive species that is currently common from Norway south to Spain, and in the western Mediterranean, and is is likely to spread all along the Irish south, west, north and nor-west coast.

What can I do to help?

Please report any sightings of Sargassum muticum to Mike Guiry at the Martin Ryan Institute, NUIG. Importers of shellfish should be aware of the dangers associated with the importation of foreign spat and be vigilant to the presence of similar alien marine species. In addition, boat owners should also be aware of importing such species in ballast water or attached to hulls. To report a sighting of Sargassum muticum, please contact:

Mike Guiry
Martin Ryan Institute
National University of Ireland Galway
e-mail: mike.guiry@nuigalway.ie

Recent reports

• Janny Levin reports a mass invasion at Ard East, Carna, Co. na Gaillimhe in September 2009
• Katie O'Dwyer and Kevin Flannery found plants in Dingle Harbour, Co. Kerry in June 2010
• Lin Baldock reports plants at Oysterbed Pier, Sneem, Co. Kerry in July 2010, where she had been diving for many years.
• Aisling Nic an tSithigh reports mature plants at An Caladh Mór, Inis Meáin, Oiléain Árann (Middle Island, Aran Islands), on 17 July 2010.
  
  

A close-up picture taken with a digital camera would help with correct identification. No more than 5 Mb, and attach to the e-mail, please.