Science Advisory Report  2012/049

Science advice from a risk assessment of five sessile tunicate species

Summary

• Sessile tunicates (ascidians) are filter feeders whose diets consist primarily of phytoplankton, suspended particulate matter, diatoms, invertebrate larvae and suspended bacteria.
• Ascidians colonize a variety of natural hard substrates such as pebbles, gravel, cobbles, boulders, and rock outcrops, as well as anemones, barnacles, bryozoans, hydroids, limpets, macro algae, mussels, other ascidians, oysters, polychaetes, scallops, and sponges. They also colonize artificial hard substrates (concrete, metal, plastic, and wood) including automobile tires, buoys, chains, docks, floats, jetties, moorings, pilings, ropes, and boat hulls.
• A major factor in ascidians’ establishment is the composition of marine epifaunal communities and the availability of suitable substrate. Typically, diverse natural communities are resistant to invasion, while newly introduced artificial substrates provide ideal habitat and ascidian populations can take advantage.
• Larval dispersal away from the parent colony is limited by the short free-swimming stage (minutes to hours) and is affected by angle of the sun, currents, water temperature, wave action, and wind exposure.
• Adult dispersal is limited to the movement of the substrate they inhabit. Fouling of aquaculture equipment, boat hulls, fishing gear, oyster and shellfish stock as well as colony fragments in ballast, dredges, fishing trawls, and floating debris are vectors of dispersal.
• In general, adult ascidians have communal, parasitic, and symbiotic organisms associated with them. Algae, amphipods, ciliates, copepods, decapods, hydroids, molluscs, nematodes, pea crab, polychaetes, protozoans, and shrimps have been found living on, or in, ascidians.
• Adults have relatively few predators due to the low nutritive value of the tunic. Larvae are susceptible to predation while planktonic and during early settlement. Ascidian predators include fishes, flatworms, polychaete worms, echinoderms (mainly sea stars), gastropods, seals, and cetaceans.
• Overall ecological risk posed by each of these species was classified as high, generally with moderate uncertainty. The exception was Vase Tunicate on the west coast where the risk was moderate with high uncertainty.
• Overall risk posed by parasites, pathogens, or fellow travelers of the evaluated tunicates was found to be low with high or very high uncertainty with the exception of the Club Tunicate. Club Tunicate is known to carry a large variety of epibiont fauna including the other tunicate species evaluated in this risk assessment. As the evaluated tunicates were found to pose a high risk, the Club Tunicate, therefore, poses a high risk for its fellow travelers.
• Since natural long distance dispersal appears very limited, increased management of human-mediated dispersal vectors could substantially reduce spread of these nonindigenous tunicate species.
• Best practice advice should be developed in collaboration with aquaculture groups, small craft operators, and other stakeholders. These efforts should be aimed at educating marine users on these species, including how they can spread, to determine suitable practices to limit further spread and associated ecological and genetic consequences.
• This risk assessment was undertaken in 2007 and is based on the best available information at that time. If vector movement patterns or global climate conditions change significantly, a re-assessment may be required.

This Science Advisory Report is from the March 13-14, 2007 tunicate risk assessment. Additional publications from this process will be posted as they become available on the Fisheries and Oceans Canada Science Advisory Schedule .

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