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General Impact Didemnum spp. can become a nuisance species when they reproduce rapidly and foul marine habitats like shellfish aquacultures and fishing grounds, ship's hulls, and maritime structures. The rapid spread of Didemnum spp. colonies alters marine habitats and threatens to interfere with fishing, aquaculture, and other coastal and offshore activities. They overgrow organisms such as tunicates, sponges, macroalgae, hydroids, anemones, bryozoans, scallops, mussels, oysters, seaweeds, limpets, barnacles, and other species of sea squirts. Where these colonies occur on the seabed, they likely cover the siphons of infaunal bivalves. Didemnum spp. mats choke off bottom-dwellers such as shellfish and may cover grounds needed by fish to lay eggs. (Cohen, 2005; USGS-WHSC, 2005). While Didemnum spp. has been observed primarily colonizing artificial substrates in harbors and manmade structures there are fears that natural reefs may become susceptible. Healthy natural ecosystems such as coral reefs comprise a high biodiversity, with complex interactions among the species, and this is thought to be an important factor in preventing the establishment of Didemnum spp. and other invading species. However, many coral reef areas are becoming degraded due to anthropogenic activities, global warming, natural events like El Nin˜ o, and perhaps other causes. Didemnum spp. is spreading to various temperate and tropical regions of the world. The reasons for this species sudden invasiveness are not known.To add to these fears, Didemnum spp. have not declined with the return of cooler water; on the contrary they continue to proliferate (Lambert, 2002).          Location Specific Impacts: Marlborough Sounds (New Zealand)   Ecosystem change: The New Zealand Mussel Industry Council and the New Zealand Marine Farmers Association are particularly concerned about the possible spread of D. vexillum to marine farming areas in the Marlborough Sounds, where it has the potential to smother mussel lines (Coutts, 2002). Georges Bank (United States (USA))   Competition: Didemnum spp. may profoundly alter communities by outcompeting sessile native species for space (Pederson et al 2003). Habitat alteration: Surveys of Georges Bank documented Didemnum spp. covering about 70% of 70 km2 cobble area (Pederson et al 2003). Modification of natural benthic communities: Didemnum spp. has been observed covering more than 90% of the seabed in some areas and is thought to be changing the benthic communities in the areas it has colonized (Gutierrez et al 2005). Reduction in native biodiversity: Although the potential effect of a Didemnum spp. invasion on theses fisheries is unknown at this time, these colonial tunicates could overgrow hard substrates and smother atlantics sea scallop, which settle on hard substrates as spat and juveniles (Gutierrez et al 2005).