Outer Hebrides Biological Recording  Outer Hebrides Biological Recording

Invasive Non-native Marine and Coastal Species

Bonnemaisonia hamifera

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Bonnemaisonia hamifera

Bonnemaisonia hamifera is a branched red seaweed, growing to 20-30 cm high with an erect main axis 1 mm in diameter and characteristic curved hooks. A small (2-3 cm) filamentous tetrasporophyte phase (Trailliella) also occurs.
It is found in the shallow subtidal waters, to depths of 15-20 m where it grows attached to rocks, or epiphytically on other seaweeds attached by its characteristic hooks. The filamentous form occurs in shaded rock pools on the lower shore and in the subtidal zone.
Native in the Pacific around Japan, it was first recorded in the UK in the late 19th century. It is now so widespread and abundant in Europe that it can be regarded as fully established, with no possibility of eradication. However, there have been no negative impacts on native marine flora or marine based activities.

Darwins Barnacle

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Darwin's Barnacle Austrominius modestus

A small sessile barnacle, 5-10 mm in diameter, Darwin's barnacle, Austrominius modestus is characterised by having four shell plates which form a low, conical body shape with a diamond shaped opening. This barnacle is distributed around most coasts of England and Wales, a few areas of Scotland and some Scottish islands including the Outer Hebrides. It is native to Australasia and was first recorded in Britain in the 1940s.
It is most common in the mid-shore to shallow subtidal areas of estuarine and sheltered marine habitats, and tolerates a wide range of temperature and salinities. It attaches to a variety of substrates including rocks, stones, hard-shelled animals and artificial structures including ships.
Darwin's barnacle competes with native species for space and appears to have entirely displaced native barnacle species in some places. Economically its main impact is the costs arising from the fouling of vessels, equipment and interference with aquacultural activities.

Green Sea-fingers

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Green Sea-fingers Codium fragile fragile

Green Sea-fingers Codium fragile fragile is a spongy green seaweed with numerous Y-shaped, branching, cylindrical fronds extending to about 25 cm. The fronds have a felt-like texture and a disc-shaped holdfast formed from many fine filaments. It can only be distinguished from native Codium tomentosum by microscopic examination.
Native to the Pacific around Japan and Korea it first appeared in Devon in 1939 growing on oyster shells. This species is found on rock and coralline algae in pools and on open rock from the mid to lower shore, and subtidally to depths of fifteen metres. On sandy or muddy shores it attaches to bivalve shells, rocks or artificial structures. It mainly inhabits protected bays and estuaries but also occurs on semi-exposed shores.
Its most significant impact on native marine flora has occurred where algal diversity in the invaded area is low. In Britain algal diversity is high and green sea fingers has not yet occurred in densities high enough to cause problems. At high densities it can be a fouling nuisance in shellfish beds, smothering mussels and scallops, clogging scallop dredges and interfering with harvesting. It also fouls boats, fishing nets, wharf pilings and jetties.

Harpoon Weed

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Harpoon Weed, Asparagopsis armata

The life cycle of Harpoon Weed Asparagopsis armata, has two morphologically distinct phases. The sexual (gametophyte) plant is rosy, yellowish pink or whitish pink, erect and spreading, with many feathery branches; up to 30 cm tall with some branches developing as conspicuous harpoon-like barbed structures up to 10 mm long. This stage of the harpoon weed is only common in south western locations in the UK.
The asexual (tetrasporophyte) plant, previously known as known as Falkenbergia has spread north as far as Shetland. It is rosy pink, filamentous, and forms fine woolly balls 10 - 20mm in diameter and occurs all year round. It is epiphytic or sometimes free living, typically found subtidally and sometimes tangled up in other seaweeds.
Harpoon weed was first recorded in Britain and Ireland in the 1940s probably spread from alien populations already established in Europe. It is reported to dominate algal assemblages in some locations; forming bloom-like outbreaks during winter in the NW Mediterranean and at such times economic losses to fisheries have been reported due to harpoon weed clogging fishing nets. In Ireland, harpoon weed has recently been identified as a commercially important species for the production of cosmetics.

Oyster Thief

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Oyster Thief, Colpomenia peregrina

The Oyster thief Colpomenia peregrina is a non-gelatinous, olive-green coloured alga which forms a thin-walled hollow sphere, usually 3 – 9 cm. The young are brown and balloon-like, becoming yellow-brown, contorted and collapsed with age. It may be confused with the native Leathesia difformis which is lobed with a gelatinous surface in contrast to Colpomenia peregrina which is dry and papery and can be torn easily. The oyster thief is usually epiphytic, growing on a variety of seaweeds in mid to lower shore rock pools and in the shallow subtidal region, particularly in sheltered areas.
It was introduced to France from the USA with imports of oysters at the end of the 19th century. It was first recorded in southern England in 1907 either as a result of natural spread or introduced with commercial oyster imports. By the 1940s it had spread to the Orkneys and has been recorded in the Outer Hebrides since the 1970s.
No significant impacts have been reported on native species and although economic losses were reported from French oyster beds in the early 20th century there have been no recent reports.


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Wireweed Sargassum muticum

A non-native seaweed Wireweed, Sargassum muticum, was initially introduced to France on oysters either from Canada or Japan, and it was first found on the Isle of Wight in 1973. It is a fast-growing species producing large numbers of spores and can spread at the rate of approximately 30 km per year. It prefers warmer waters, but will tolerate a wide range of temperatures and salinity including estuarine conditions.
Wireweed can be a pest in harbours, shallow waters and on beaches, interfering with recreational use of waterways with detached plants forming large floating masses which can block propellers and intakes. It can also foul oyster beds and fishermens nets and competes with native plants such as sea grasses.
It was first recorded in south-west Scotland in 2005 and subsequently found in surveys conducted from 2010-2012. It was found on North Uist in August 2013. This was the first record for the islands and for Scotland beyond the Firth of Clyde and Loch Fyne.