Phylum Chordata

    Subphylum Tunicata

    Class Ascidiacea (sea squirts)

    Order Aplousobranchia

    Family Didemnidae

    Genus Didemnum


    The geographical origin of Didemnum is unresolved at this time. Molecular analysis of New Zealand, European, and US populations are currently underway to determine Didemnumís native range (Bullard et al, 2007). In the 1920s a species in the Sea of Japan was described similar to Didemnum colonies found in the US, suggesting a possible Asian origin. However, New Hampshire colonies (Didemnum vestum) and New Zealand colonies (Didemnum vexillum) are thought to be native to their respective regions (Daley and Scavia, 2006). More research needs to be done to clarify and confirm origin.


    Didemnum is a dominant colonial fouling organism of marine habitats, which can form either low undulating mats or ropey chains in ocean depths up to 65 meters (213 ft.). Mats are typically seen where currents are strong, whereas the long ropey morphology appears in areas where there is less wave action (Valentine et al., 2007). Each individual (zooid) is approximately 1 mm (pin size) in size and contributes to a colony of thousands. The zooids are embedded in a tough, leathery, outer covering called a tunic. Before the adults subject themselves to a sessile filter-feeding lifestyle, a short time is spent as a non-feeding, motile larvae (Lambert, 2007). Didemnumís color ranges from an off white or tan to a pinkish orange (Daley and Scavia, 2006). Coastal populations have been found to over-winter, in which the species transforms into an unrecognizable state. Due to the morphological similarities of the Didemnidae family, species identification is difficult, therefore this organism is provisionally being called Didemnum sp.


    Didemnum poses a threat to the ecosystem dynamics wherever it is present. Didemnum smothers natives and overgrows any hard substrata in its path. It can encrust and smother the siphons of bivalves, limiting there ability to feed and grow. Finfish utilize gravel for protection and to disperse their eggs, but when Didemnum is present the larvae of finfish and other organisms can not settle and grow, due to the high acidity of its tissues. This chemical defense also contributes to the resistance to predation. With the exception of the common periwinkle (Littorina littorea), which can only prey on desiccating populations of Didemnum, there are no known predators.


    New England coasts, from New York to northern Maine, are home to Didemnum colonies. Recently, Didemnum sp. has extended out from the New England coastal areas into Georges Bank (Valentice et al., 2007). In addition, Didemnum sp. has been found in the Puget Sound and on various locations along the California and Alaska coasts (Bullard et al., 2007)


    Didemnum can be found colonizing any hard substrate, such as wharf pilings, marinas, docks, ropes, ship hulls, marine debris and gravel beds (Lambert, 2007). Didemnum can even overgrow other ascidians, barnacles, crabs, sponges, anemones, seaweeds, eelgrass, and bivalves, such as oysters, Crassostrea virginica, scallops, Placopecten magellanicus, and mussels, Mytilus edulis (Valentine et al., 2007). Didemnum can not colonize and survive on unstable sandy sediments due to smothering. Georges Bank happens to hold the largest known Didemnum population and is also the only known offshore occurrence (Daley and Scavia, 2006). This suggests the species may prefer the deeper open water due to more stable conditions for growth and survival, whereas in coastal areas desiccation may limit growth to some degree.


    It is thought that Didemnum may have been present in Maine since the 1970s but was not reported until 1988 in the Dimariscotta River and documented in 1993 (Daley and Scavia, 2006). It was also documented in San Francisco Bay, California during this time. In 2002 Didemnum was documented in Georges Bank and further recorded in subsequent years (Valentine, 2007). Didemnum was then recorded in Puget Sound in 2004. It was likely introduced by ballast water, hull fouling, and/or aquaculture.


    Several biological characteristics contribute to the spread and success of Didemnum as an invader. The species is highly adaptable, taking on different morphologies and reproductive strategies to compensate for an ever changing marine environment. For instance, sexual reproduction typically occurs during warm summer months from July to September, while asexual budding occurs throughout the months of May to December. Reproduction and growth of the colony is rapid. Larvae are brooded for a short period and released. Larvae only have 12-24 hrs to find a suitable environment, at which it morphs into adult form (Daley and Scavia, 2006). Didemnum is also very tolerant of a wide range of depths, temperatures and salinities and capable of inhabiting any hard substrata. It has the ability to overgrow and out compete other organisms, while deterring predators with chemical defenses. Healthy colonies do not have any known predators (Valentine, 2007). A unique characteristic that contributes largely to its spread is its ability to fragment off from a colony and reattach elsewhere (Bullard et al., 2007). The ascidian can then be spread around by wave action, tides, storms, and human dispersal, such as aquaculture/fishing gear and activities, ship hulls, and ballast water.


    Antifouling paints can help to prevent hull fouling if reapplied with wear of the vessel. Simple methods such as water blasting or chlorine dosing can also help to limit spread of the species. Other treatment methods include smothering sandy/cobble seabed habitats with uncontaminated dredge soil, smothering artificial rock slopes with geotextile fabric, wrapping wharf pilings with plastic balage wrap, and encapsulation of jetties/pontoons and vessels with plastic silage covers or balage wrap. In addition to encapsulation, 5% acid can be applied in order to accelerate the process. Air drying of equipment and floating docks is also sufficient but time consuming (Pannell and Coutts, 2007). Docks can be lifted right out of the water to allow for desiccation and ultimately death of the colony. Treatment methods should be applied prior to the spawning period to limit spread. Interference and human error contribute to the failure of treatment methods. Methods have been successful in eradicating the species from the treated areas but not completely from the environment (Pannell and Coutts, 2007).


    Genetic analysis is currently in progress to appropriately identify the genus to a species level.


    Very little evidence is available at this time, but the economic threat of Didemnum is expected to be significant. Scallop and finfish fisheries rely heavily in Georges Bank, therefore with the ecosystem potentially affected, the fisheries could suffer dramatically, not only due to the possible decrease in stock but due to management costs. The pearl aquaculture industry is seeing significant operational costs due to the eradication of fouling organisms. Didemnum is reducing the growth and survival causing the industry to resort to manual cleaning of the pearl oysters (Pinctada fucata) (Guenther, 2006). Removal of fouling organisms by manual cleaning, accounts for 30% of operational costs in bivalve farming. Mussel and oyster farmers on Prince Edward Island report costs due to heavy infestations of tunicates, as well.


    Didemnum colonies have dramatically spread across Georges Bank in recent years. An estimated 6 square miles (16 km2) in 2003, has increased to 88 (228 km2) square miles in 2005 (Valentice, 2007). New Bedford, Massachusetts relies heavily on Georges Bank being home to the largest scallop fishery. This rapid spread could potentially damage the fishery, therefore hitting Massachussetts hard at the economic level. There is not enough information available at this time to pinpoint the exact ecological and economic effects of the invasion of Didemnum.


    Brooke A. Gordon, University of Massachusetts Boston, Boston, MA.


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    Daley, B.A., and D. Scavia, 2006. An integrated assessment of the potential for further invasion of the colonial ascidian, Didemnum sp. in large marine ecosystems of the
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    Guenther, J., P.C. Southgate, and R. de Nys, 2006. The effect of age and shell size on accumulation of organisms on the Akoya pearl oyster Pinctada fucata (Gould). Aquaculture, 253 pp. 366-373.

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