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Invasion of the Sea Squirts

Sea squirts may have a comical name, but there's nothing funny about them. Just ask mussel farmers in Prince Edward Island whose businesses are being impacted by the invasion. Sea squirts, more properly known as tunicates because their thick skin resembles a tunic, are small marine animals that spend most of their short lives attached to any underwater surface that they can find. Wharf pilings, rocks, seaweeds and the undersides of boats are all favourites. They lead a simple life, filtering water for nutrients and excreting waste. Their effect on the environment and the economy, though, can be devastating.

In the past decade, in the waters off Canada's Atlantic provinces, tunicates have found a desirable new home – namely, farmed mussels and all the gear associated with their cultivation. A tunicate infestation on the three-metre-long socks, in which the mussels grow to saleable size, can be so dense as to make the socks too heavy to be pulled up for harvesting of the mussels. Or worse, it can break the sock off its mooring to be lost at sea, along with all its mussels. Even if the tunicate-infested sock can be retrieved, stripping the slimy creatures off the mussels is a costly, time-consuming process. The effect of the tunicates on the mussels themselves is not yet clearly understood, but they may be competing for food with them, and possibly reducing yields.

At present there are five tunicate species on Canada's “Most Wanted” list: the solitary Clubbed (Styela clava) and Sea Vase (Ciona intestinalis) tunicates, and the colonial Violet (Botrylloides violaceus), Golden Star (Botryllus schlosseri), and Didemnum sp. The Clubbed, Violet and Didemnum sp. are recent introductions to Canadian waters in the past decade, although Didemnum sp. is not yet present on the East Coast. The Sea Vase and Golden Star tunicates have been present since the early 1900s, but recently have started to exhibit the explosive population growth typical of invasive species. The tunicates have come in as hitchhikers on boats, fishing gear, or have been released in ballast waters. Now that they are established in Canadian waters, the tiny tunicates may even be tagging along with mussel seed from an infested area, for grow-out in uninfested areas. Prince Edward Island is the site of the most devastating infestations, but there is evidence that the tunicates are spreading to other locations in the Atlantic provinces.

Three invasive tunicate species of concern in Nova Scotia's marine waters: top and bottom left: Sea Vase tunicate; top, centre-right: Violet tunicate; and bottom, centre-right: Golden Star tunicate

Three invasive tunicate species of concern in Nova Scotia's marine waters: top and bottom left: Sea Vase tunicate; top, centre-right: Violet tunicate; and bottom, centre-right: Golden Star tunicate

In September 2005, the Canadian and Prince Edward Island governments earmarked close to one million dollars to support research and monitoring that would respond to the tunicate invasion in the Atlantic provinces. As part of the offensive, Fisheries and Oceans Canada (DFO) biologist Dawn Sephton is conducting a tunicate monitoring project for sixty sites around Nova Scotia. Colleagues at DFO offices in New Brunswick and Prince Edward Island are doing similar work. Researchers in Newfoundland are keeping a watching brief, as, so far, there are no known outbreaks there.

Sephton, who works out of the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, describes tunicates as “the perfect competitive organism”. They can withstand many unfavourable environmental conditions, have few known predators, and, worst of all, are frighteningly prolific. A single Sea Vase tunicate can produce up to 10,000 larvae per season!

Sephton says her objective is a simple, but urgent, matter, “We need to better understand invasive tunicates so we can develop ways to manage them and reduce their impact.” The research is also proactive, as the tunicate invasion has not gained much of a foothold in Nova Scotia – yet. The first challenge is to look at places where there may be a problem developing, identify which tunicates are present, and gain an understanding of their basic biology in that location. This information will lay the groundwork for developing strategies to mitigate and manage them.

To monitor different sites, Sephton uses low-cost, low-tech, yet effective, tunicate collectors that were developed in-house. The collectors hang, from May to October, at least one metre below the surface in a sheltered spot in a potential site of infestation. Some of the collectors are pulled out mid-season, while others remain for the entire season. All are examined to determine the absence/presence of tunicates. In conjunction with researchers at Dalhousie University, other collectors are removed weekly at sites with ongoing infestation and examined under a stereo microscope to document the life cycle of the tunicates, determine their numbers, and for genetic analysis.

Tunicate monitoring collector with a heavy infestation of Sea Vase tunicate.

Tunicate monitoring collector with a heavy infestation of Sea Vase tunicate.

A vital component of the battle is input from the public. People, whose livelihoods are linked to the sea, are keen observers of the marine environment. The Nova Scotia Department of Fisheries and Aquaculture conducts an annual mail-out survey to holders of mussel aquaculture leases to tap into that valuable knowledge base, and the responses are providing solid information about tunicate distribution. Going hand in hand with this is the pressing need to increase public awareness about tunicates. Toward this end, Sephton plans to use all her monitoring sites as places of public education, with information brochures, posters and community talks. A tunicates reporting line and email have also been established in Nova Scotia (1-888-435-4040 or email XMARinvasive@mar.dfo.gc.ca ). Groups, such as the Fishermen and Scientists Research Society, are helping get the information out through their quarterly newsletter.

Owners of boats, whether commercial or recreational, are a particularly crucial group to target. In fact, these people are on the very front lines of slowing the spread of tunicates. Tunicates are highly opportunistic and can easily spread to an uninfected area by hitching a ride. To limit their spread, it is vitally important that any boats travelling into new areas be cleaned meticulously. This entails carefully examining the boat bottom, motor, trailer and gear for the presence of tunicates. If found they must be thoroughly scraped off, or, even more effective, sprayed with vinegar, which kills the tunicates, and left to dry completely.

The research is still at early stages, but the results are supporting suspicions that the tunicates are affecting more areas in Nova Scotia than were previously known. This is critical information, as the earlier an infestation is known, the better it is in terms of managing it. Unfortunately, once a tunicate population becomes established, eradication is virtually impossible and becomes increasingly expensive. Sephton's hope is that by learning more about the tunicates, we will be able to find their “Achilles heel and use it to limit and manage them.”

For more information, visit:
http://www.dfo-mpo.gc.ca/media/back-fiche/2005/hq-ac83a-eng.htm