SAMS news room

Smothering sea squirt reaches Scotland

A highly invasive marine species, the sea squirt Didemnum vexillum, has appeared in Scottish waters for the first time. The colonial ascidian has been spreading around the world in recent years, colonizing marine structures, ships’ hulls, and natural marine habitats.

The Scottish Association for Marine Science (SAMS) has an ongoing six-year invasive marine species programme.
Dr Liz Cook
of SAMS coordinates the UK Marine Aliens Programme and is member of the UK Didemnum Group. Her group has been on the look-out for the invasive sea squirt for some time, following reports that it had been found in Holyhead Harbour, North Wales, in 2008.

Dr Cook said, “unfortunately, this marine alien brings with it potentially high economic impacts. It is highly invasive, spreading and reproducing rapidly, and can affect marinas, fisheries and aquaculture as it tends to smother any substrate on which it establishes.”

Dr Kenny Black of SAMS said, “this tunicate could have considerable economic impact in the aquaculture industry, fouling salmon farm infrastructure, reducing water flow and weighing down nets and lines. Shellfish farmers suffer as the sea squirt colonies, resembling large, spongy blankets, impede growth and make the smothered shellfish potentially unsellable”  

Chris Beveridge of SAMS spotted the small colony of what she suspected to be the carpet sea squirt during a routine survey of Largs Yacht Haven in October 2009. “We carry out surveys of west coast marinas every year, on the lookout for a variety of non-native species that have the potential to reach Scotland,” she said. “This is one of our target species, so when I spotted a mussel on a pontoon, covered with a fawn coloured growth, I immediately suspected the invasive sea squirt Didemnum.” She subsequently found a larger sample on mooring lines during a follow-up visit in November. The species has now been confirmed as the invasive sea squirt.

A survey of other marinas in the Clyde and on the Argyll coast will be carried out from mid-January by SAMS to investigate how widespread the invader has become. Funded by Scottish Natural Heritage (SNH) and Marine Scotland, the information gathered will be used to help formulate a management strategy for the species.

Didemnum, the invasive smothering sea squirt, is most likely to spread by attaching itself to the hulls of boats. It is a distinctive mustard, pale orangey-yellow or beige colour and often appears as pale flat patches. Larger growths may look like wax dripping from artificial structures just below water level. Its surface has leaf-like veins with tiny pores and it has a spongy texture and a leathery feel to it.


Recommendations to prevent further spread of the organism include:

•    Keep hulls of boats clean, free of fouling and treated with anti-fouling paint
•    When hulls are cleaned make use of a closed-loop or filtered wash down facility and/or steam clean
•    Clean fishing gear and other equipment on a regular basis
•    Dispose of any fouling including weed carefully so that it doesn’t go back into the water.

There will be more information on the following website: www.nonnativespecies.org from 1 February.



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