Didemnum vexillum

Invasion History

First Non-native North American Tidal Record: 1982
First Non-native West Coast Tidal Record: 1993
First Non-native East/Gulf Coast Tidal Record: 1982

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General Invasion History:

Since 1990, scientists in several parts of the world have observed the growth and spread of an unidentified species of Didemnum in the temperate marine waters of New Zealand, North America, and Europe. While there have been differing opinions on the identity of these different populations (Kott 2002; Kott 2004), some have suspected that it is a single species (Bullard et al. 2007). Recently, genetic and morphological studies have apparently resolved the great Didemnum species debate. The first description of this tunicate was by Kott (2002). Specimens from New Zealand were intensely reviewed and the study concluded that the correct name is D. vexillum (Lambert 2009; Stefaniak et al. 2009). Didemnum vexillum probably originated from the northwest Pacific, possibly Japan (Lambert 2009). Genetic analysis supports the native status of populations in eastern Japan. Further sampling in the northwest Pacific is needed to determine the full extent of the native range (Stefaniak et al. 2012). It has invaded the West and East Coasts of North America including Mexico, California, Alaska and New York, Massachusetts and Maine.

North American Invasion History:

Invasion History on the West Coast:

Didemnum vexillum was first reported from San Francisco Bay in 1993 and it seemed to be limited to the saltier (>26 ppt) parts of the bay (Cohen 2005; Cohen et al. 2005; Bullard et al. 2007; USGS Woods Hole Science Center 2008; Ruiz et al., unpublished data). North of San Francisco Bay, D. vexillum was found in Tomales Bay (CA) and Humboldt Bay (CA) in 2001, Bodega Harbor (CA) in 2003, Puget Sound (WA) in 2004, Okeover Inlet (British Columbia) in 2003 (Bullard et al. 2007) and Jervis Inlet (British Columbia; USGS Woods Hole Science Center 2008). In 2010, it was found in Sitka (AK), its northernmost site on the West Coast (Cohen et al. 2011). South of San Francisco Bay, D. vexillum was found in Half Moon Bay (CA) in 1997, Monterey Bay (CA) and Elkhorn Slough (CA) in 1998, Morro Bay (CA) in 2000, and Port San Luis (CA) in 2002 (Bullard et al. 2007). In 2007, it was found in Mission Bay (CA) (USGS Woods Hole Science Center 2008). In 2005, it was found fouling oysters (Crassostrea gigas) in Bahia San Quintin, Mexico (Rodriguez and Ibarra-Obando 2008).

On the West Coast, the sequence of occurrences suggests an introduction to San Francisco Bay, followed by dispersal from shipping and aquaculture to the north and south. However, fishermen in British Columbia claim that D. vexillum may have been present for more than a decade before its official discovery (USGS Woods Hole Science Center 2011).

Invasion History on the East Coast:

Didemnum vexillum was first reported in 1982 from the Damariscotta River estuary, Maine (Dijkstra and Nolan 2011). This occurrence has sometimes been attributed to experimental culture of Pacific Oysters (Crassostrea gigas) in the 1970s (Shatkin et al. 1997), but these oysters were hatchery-reared seed, and would not have be fouled with tunicates. Ship fouling from Europe, where fouled Pacific Oysters were planted and reared, is the most probable vector (James T. Carlton, personal communication). In 1996, D. vexillum was collected on Tillies Bank, MA. A few years later in 1998, it was collected on Stellwagen Bank and Georges Bank, MA where it has continued to spread in offshore waters (Bullard et al. 2007). By 1998, D. vexillum was present in the Cape Cod Canal (Sandwich, MA) and had been tentatively identified in photographs taken of floating docks from Woods Hole in 1998 (Bullard et al. 2007; USGS Woods Hole Science Center 2008). In 2000, it was collected in Eel Pond at Woods Hole, MA and was found along the southern side of the Cape and east to Orleans, MA (USGS Woods Hole Science Center 2008). Again in 2000, it was collected in Buzzards Bay, MA and Narragansett Bay, RI (MIT Sea Grant 2008; Osman and Whitlatch 2007, Bullard et al. 2007), as well as in eastern Long Island Sound, NY (Osman and Whitlatch 2007). In 2004, D. vexillum reached its current southern limit of Shinnecock Bay, NY (Bullard et al. 2007; USGS Woods Hole Science Center 2008). There may be a temperature and/or salinity threshold that complicates its spread to warmer (e.g. southern) and low salinity waters, given that its presence seems to be limited to the mouth of estuaries (e.g. Long Island Sound, NY and Narragansett Bay, RI). Currently, D. vexillum ranges from Parrsboro, Nova Scotia to Shinnecock Inlet, NY (Bullard et al. 2007; USGS Woods Hole Science Center 2008; Moore et al. 2014). In recent surveys (2006-2009), it was not found in New Brunswick waters, although it was found in Eastport, Maine, only 2 km from the border (Martin et al. 2011). However, in 2013 it was found on the Bay of Fundy, Nova Scotia (Moore et al. 2014). 

Invasion History Elsewhere in the World:

Northeast Atlantic: The earliest report of Didemnum vexillum from Europe was along the Dutch coastline in 1991. It remained relatively rare until 1996 when the population rapidly expanded, especially in the province of Zeeland, Netherlands (Gittenberger 2007). Subsequently, in 2001, D. vexillum was found growing on the face of a brick quay (wharf or reinforced bank) and overgrowing other invertebrates (barnacles, mussels, tunicates, etc.) in the Port of Le Havre, France. In 2002 it was found growing on ropes and overgrowing other invertebrates in Perros-Guirec, Brittany (USGS Woods Hole Science Center 2011). In 2005, it was found in Ireland’s Malahide Estuary attached to boats, pontoons, ropes, buoys, seaweed and mussels (Minchin and Sides 2006; USGS Woods Hole Science Center 2011). In 2008-2009, D. vexillum was found on the coast of Spain, in the towns of Santander, Baiona, Moana, Corme-Porto, and Gijon (El Nagar et al. 2010).

In 2010, D. vexillum was discovered growing in the Lagoon of Venice, at the head of the Adriatic Sea, the first record from the Mediterranean Sea (Tagliopietra et al. 2012). In 2012, it was found growing on oysters in Fangar Bay, in the Ebro Delta, on the Spanish Mediterranean coast. Genetic analysis suggested the likely source was oyster cultures on the French Atlantic Coast. This invasion suggests that D. vexillum has greater capacity, than previously thought, to invade warm waters (Ordonez et al. 2015).

Southwest Pacific: In October 2001, An unidentified species of Didemnum was discovered fouling mooring posts and boats in Whangamata Harbour, New Zealand. Patricia Kott described it as D. vexillum, and considered it a native species, previously unnoticed in New Zealand (Kott 2002). In December 2001, this organism was found overgrowing a barge in Shakespeare Harbour near Picton, New Zealand. In 2003, eradication of the fouling on the barges, recreational vessels, moorings, salmon cages and wharf pilings was attempted, using a variety of methods (Coutts and Forrest 2007). Fouled areas were wrapped in plastic and treated with chlorine, moorings were water-blasted, boats were cleaned and repainted with antifouling paints, the seabed was covered with fresh dredge spoil, and riprap was covered with Geotextile fabric and treated. However, some tunicates survived in joints in the riprap and eventually repopulated the area (Coutts and Forrest 2007).

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Description

Didemnum vexillum is an aggressive and rapidly spreading colonial tunicate species forming extensive thin sheets which often overgrow rocks, shells, other sessile organisms (e.g. sponges, hydroids, oysters), and even itself ultimately forming large sponge-like masses. These masses often have long, flexible leaf or flag like projections that are cylindrical and branched. Large D. vexillum colonies may appear to be folding in on themselves and neighboring surfaces. Colonies are yellowish-cream in color with the yellow pigment observed in the gut, eggs, and embryos. The thorax of its zooids are white in color. Star shaped calcareous spicules are patchily distributed in the surface layer and in the cloacal cavity; patchiness increases closer to the tunic surface. Spicules are approximately 58 µm in diameter with 9-11 conical rays. The oral siphon is short with only six lobes. The atrial siphon is surrounded by large clumps of spicules around the opening. There are 8-9 stigmata in an anterior row of the oral cavity. The post-stomach gut forms a double loop in the abdomen. Zooids are about 1 mm long and are arranged along the common cloacal canals that extend through the colony. Didemnum vexillum has nine coils of vas deferens around the testis. Embryos are approximately 600 µm (with tail wrapped around body) and are incubated in the core of the tunic, and not within each zooid (Kott 2002, as D. vexillum, New Zealand).

Morphological identification criteria for Didemnum species, particularly in the USA, have not been re-evaluated since Van Name published his monograph in 1945. Since that time, there has been a turbulent taxonomic evolution in the identification of Didemnum species. The near simultaneous discovery of outbreaks of similar didemnids in New Zealand, the East and West Coast (North America), and Europe raised questions of whether a single or multiple species existed, and whether the outbreaks were true invasions or simply population booms of native species (Bullard et al. 2007). Kott (2002) named the New Zealand form D. vexillum, but found morphological differences in specimens from the northwest Atlantic (New Hampshire) and therefore named these as a separate species, D. vestum. Gretchen Lambert investigated the possibility that this species was conspecific with D. lahillei, described from Brittany in 1909. However, she found that the type material (physical representative specimen) was a mixture of several species (Lambert 2009). In 2005, others began a molecular (18S rDNA) study using the samples from New Hampshire, New Zealand and Japan, and found that all of the samples were the same species, for which D. vexillum was the valid name (Lambert 2009; Stefaniak et al. 2009). Genomic sampling of populations worldwide indicates high genetic diversity, but also genetic differentiation among populations. Of two major groups with different COI (Cytochrome oxidase I) groups, only one was widley introduced, with three major colonization events (Casso et al. 2018).

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Taxonomy

Ecology

General:

Life History- A colonial tunicate consists of many zooids, bearing most or all of the organs of a solitary tunicate, but modified to varying degrees for colonial life. Colonial tunicates of the family Didemnidae have small zooids, completely embedded in an encrusting and thin tunic. Each zooid has an oral siphon and an atrial aperture which opens to a shared cloacal chamber. Water is pumped into the oral siphon, through finely meshed ciliated gills on the pharynx, where phytoplankton and detritus is filtered, and passed on mucus strings to the stomach and intestines. Excess waste is expelled in the outgoing atrial water (Van Name 1945; Barnes 1983).

Colonial tunicates reproduce both asexually by budding and sexually from fertilized eggs that develop into larvae. Buds can form from the body wall of the zooids. Colonies vary in size ranging from small clusters of zooids to huge spreading masses. The zooids are hermaphroditic, which means both eggs and sperm are released into the atrial chamber. Eggs may be self-fertilized or fertilized by sperm from nearby animals, but some species have a partial block to self-fertilization. Fertilized eggs are brooded within the tunic until they hatch into lecithotrophic (non-feeding, yolk-dependent) tadpole larvae. The larva has a muscular tail and a notochord, eyespots, and a set of adhesive papillae. The larvae are expelled upon hatching and swim briefly before settlement. Swimming periods are usually less than a day, but some larvae settle immediately after release or swim for longer periods if the water temperature is low. In experiments, 10% of larvae remained viable after a delay of 36 h. In field experiments, most settlement occured within 250 m of the parent population, but some settlement is possible at distances of 1 km or more (Fletcher et al. 2012). On settlement the tail is absorbed, the gill basket expands, and the tunicate begins to feed by filtering (Van Name 1945; Barnes 1983).

Colonies of D. vexillum can also disperse by fragmentation and zooids are capable of reproduction while suspended in the water column. Fragments of colonies can remain suspended for up to three weeks, and can reattach to surfaces and grow. However, the apparent health of the colony declines, the longer it is suspended in the water column. The viability of fragments is a concern for attempts to control or eradicate this species (Morris and Carman 2012). Fragments will reattach on eelgrass and artificial surfaces at water temperatures of 6-10 °C, but at a lower rate than at summer temperatures (16-22 °C) (Carman et al. 2014). Exposure to high temperatures results in increased DNA methlylation, and decreased growth rates, which may 'buy survival time' as a stress resonse (Hawes et al. 2018). The rapid spread of D. vexillum may be due to the ability of genetically different colonies to fuse, forming rapidly growing ramets, forming transient chimeras, stretching and dispersing across a substrate, and later resegregating (Fidler et al. 2018).

In all part of its native and introduced range, D. vexillum is more frequently reported from anthropogenic stuctures than from natural surfaces, (Simkanin et al. 2012). Dock floats are especially favored habitats, probably because their motion provides rapid water exchange, and a fresh supply of food-laden water (Glasby 2001). Other colonized man-made structures include pilings, piers, aquaculture structures, and boat hulls (Carman et al. 2010; Davidson et al. 2010; Simkanin et al. 2012). Natural habitats include rocky reefs, gravel bottoms, bivalve colonies, seaweeds, and eelgrass (Valentine et al. 2007; Carman and Grunden 2010 Simkanin et al. 2012; Carman et al. 2016).

Food:

Phytoplankton, bacteria, detritus

Consumers:

Starfish, urchins

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Coarse Woody Debris	None
General Habitat	Unstructured Bottom	None
General Habitat	Oyster Reef	None
General Habitat	Marinas & Docks	None
General Habitat	Rocky	None
General Habitat	Bedrock	None
General Habitat	Vessel Hull	None
General Habitat	Grass Bed	None
Salinity Range	Polyhaline	18-30 PSU
Salinity Range	Euhaline	30-40 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Vertical Habitat	Epibenthic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	-2	Field data, Bullard et al. 2007
Maximum Temperature (ºC)	24	Field data, Bullard et al. 2007, reduced growth at 23 C, experimental data, McCarthy et al. 2007
Minimum Salinity (‰)	19	Field and laboratory data (Bullard et al. 2007; Gröner et al. 2011; Hawes et al. 2018). Growth and survival at medium (15-28 PSU) and low (10-26 PSU) sites was greatly reduced compared to a high-salinity site (26-30 PSU) in the Thames estuary, Connecticut (Bullard and Whitlach 2009; Auker 2019).
Maximum Salinity (‰)	35	Highest observed?
Minimum Reproductive Temperature	14	Onset of spring recruitment, but fall reproduction ceases at 9-11 C (Valentine et al. 2009). In New Zealand, recruitment was not detected when temperatures dropped below 12 C, but some larvae were present in the the tissues of colonies (Fletcher et al. 2013).
Maximum Reproductive Temperature	20	end of spring recruitment (Valentine et al. 2009)
Minimum Duration	0	Larvae can settle immediately on release (Fletcher et al. 2012)
Maximum Duration	1.5	In experiments, 10% of larvae remained viable after a delay of 36 h (Fletcher et al. 2012)
Broad Temperature Range	None	Cold temperate
Broad Salinity Range	None	Polyhaline-Euhaline

General Impacts

Didemnum vexillum is widely considered to be an invasive species with potentially important economic and ecological impacts. As a recent invader in many parts of the world, the extent of its impacts have only just begun to be studied. Didemnum vexillum is unique in that, unlike many marine invasive organisms, it is not only common in confined, disturbed, and polluted harbors and estuaries, but is also common in the more ‘pristine’ waters of Georges Bank (MA), British Columbia and New Zealand. Consequently, its invasions potentially have major implications on industries that take place in 'cleaner' waters, such as fisheries and aquaculture (Bullard et al. 2007; Valentine et al. 2007), and may impact natural ecosystems by significantly altering the local habitat (Bullard et al. 2007; Valentine et al. 2007).  

Economic Impacts

Fisheries: Economic impacts on 'wild' fisheries (e.g. bottom fishes, scallops, lobsters, mussels, etc.) are expected due to D. vexillum altering habitat and food resources on Georges Bank and elsewhere in the world (Bullard et al. 2007; Valentine et al. 2007). Didemnum vexillum is considered a major threat to New Zealand's mussel industry because of its demonstrated invasiveness on artificial structures, and its ability to over-grow and smother mussels (Coutts and Forrest 2007). To help save the shellfish industry in New Zealand, $650,000 NZ dollars were spent on eradication of D. vexillum (Coutts and Forrest 2007). However, the attempts to eradicate D. vexillum were unsuccessful and it was soon seen spreading to mussel farms in the region, resulting in significant crop losses (Coutts and Forrest 2007).

Shipping and Industry: Since New Zealand relies on sea-borne shipping for over 90% (by volume) of its international commerce (Hewitt et al., 2004), D. vexillum is considered a serious and persistent fouling pest to their commercial shipping industry and ports (Coutts and Forrest 2007).


Ecological Impacts

Competition: Rapid population explosions are known to reduce the abundance of previously established benthic species and cause significant changes in benthic community structure (Whitlatch et al. 1995; Bak et al. 1996; Lambert 2001; Castilla et al. 2004). Since D. vexillum can attach and encrust nearly every substrate it encounters, competition for resources (e.g. suitable attachment substrates, food, etc.) becomes a problem. In many locations, D. vexillum overgrows benthic biota, such as seaweeds, scallops, mussels, and other invertebrates (Bullard et al. 2007; Auker and Oviatt 2008; Gittenberger 2007; Valentine et al. 2007; Dijkstra and Harris 2009) which further exacerbates competition pressures. However, its competitiveness is partly related to environmental conditions. In laboratory and field conditions, D. vexillum was most competitive at sites with cooler temperatures (approx. 15-21°C), where it outgrew other tunicates, such as Botrylloides violaceus, Botryllus schlosseri, and Ascidiella aspersa (McCarthy et al. 2007). However, D. vexillum was less dominant on plates where other colonies of species had been established (Osman and Whitlatch 2007). In experiments at Eel Pond (Woods Hole), D. vexillum began to recruit later (October) than other species, however, it rapidly expanded its coverage on plates from October-December (Agius 2007). In Narragansett Bay (MA), D. vexillum dominated plates in the fall, and overgrew Blue Mussel (Mytilus edulis) recruits (Auker and Oviatt 2008). Didemnum vexillum was one of several invasive fouling species which showed increased growth (% coverage) at temperatures 3.5 and 4.5°C above the ambient temperature in Bodega Harbor (13.5°C), while the native Distaplia occidentalis showed reduced survival (Sorte et al. 2010).

In field experiments with a diversity of competitors and predators, D. vexillum was less successful in the presence of other species, suggesting that it is not a stronger competitor compared to other non-native colonial ascidians (e.g. Botrylloides violaceus, Botryllus schlosseri) (Stefaniak 2007). However, the ability of colonies to form tendrils for dispersal, to fuse, to form extensive colonies on pebble and cobble surfaces, and to resist grazing, due to calcareous spicules in the tunic, enable D. vexillum to form extensive colonies (Stefaniak 2017).

Habitat Change: Didemnum vexillum overgrows gravel, seaweeds, scallops, mussels, and other invertebrates thereby greatly altering the structure of the pre-existing habitat (Bullard et al. 2007; Auker and Oviatt 2008; Gittenberger 2007; Valentine et al. 2007). These D. vexillum mats are thought to reduce the amount of seabed surface suitable for larval settlement of other benthic species, such as sea scallops, on Georges Bank, MA. Additionally, these mats likely reduce the amount of suitable shelter available for juvenile fish and other prey organisms (Valentine et al. 2007). Sea Scallops overgrown by D. vexillum have reduced swimming speeds, and may be less able to escape predators (Dijkstra and Nolan 2011). By 2003-2006, colonial tunicates, including D. vexillum, replaced mussels (M. edulis) as the dominant species in fouling communities in Portsmouth Harbor, NH. (Dijkstra and Harris 2009). This is a major functional habitat change because while mussels provided a year-round substrate available to other organisms for settlement, tunicates, such as D. vexillum, are more resistant to secondary settlement by these other organisms. However, D. vexillum dies off seasonally and creates large areas of bare substrate available for colonization by other organisms (Dijkstra and Harris 2009). Of course this bare substrate is only available to those organisms with settlement periods that overlap with D. vexillum seasonal die offs, thereby excluding some benthic species from settlement opportunities.

Herbivory: Experiments by Byrne and  Stachowicz (2009) in Bodega Harbor (CA) indicate that D. vexillum has a lower filtration rate than the native Distaplia occidentalis. Similar results were obtained for other exotic/native pairs. It has been suggested that the cumulative effect of increased invasions in filter feeding fouling communities may increase seasonal consistency of filtration. This is probably due to spreading out of recruitment times of filter feeding organisms rather than increases in filtration rates.

Food/Prey: On Georges Bank (MA), Valentine et al. (2007) suggest that dense mats of D. vexillum colonies possibly form a physical barrier between fish and benthic prey resources, such as worms and bivalves. In Long Island Sound (NY),  predation studies by Osman and Whitlatch (2007) found that some predation did occur on D. vexillum recruits by the gastropod Mitrella lunata. Additionally, their study suggests that there was possibly a fish predator that preyed on their suspended uncaged D. vexillum treatments. However, Osman and Whitlatch (2007) state that the growth and dominance of juvenile and adult colonies of D. vexillum at Pine Island suggests that their abundance is not significantly reduced by predation.

Toxicity: Many species of Didemnum are chemically defended by a variety of compounds and for most species, including D. vexillum, this results in a lower surface pH (2-3) (Bullard et al. 2007).

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Regional Impacts

NEA-II	None		Ecological Impact	Competition
Overgrowth of epibenthic flora and fauna, killing some species (Netherlands, Gittenberger 2007).
NEA-II	None		Ecological Impact	Habitat Change
Overgrowth of epibenthic flora and fauna, killing some species (Netherlands, Gittenberger 2007).
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Competition
Didemnum vexillum overgrows benthic fauna, including scallops, mussels, and other sessile species, on the Georges Bank (Valentine et al. 2007). On bare experimental plates in Fishers Island Sound, Connecticut, Didemnum was most competitive at sites with cooler temperatures, where it outgrew other tunicates, such as Botrylloides violaceus, Botryllus schlosseri, and Ascidiella aspera. However, Didemnum was less dominant on plates in which colonies of other species were already established (Osman and Whitlatch 2007). Invasion success of D. vexiullum in experimental fouling communities was dependent on the presence of large amounts of unoccupied space (Janiak et al. 2013). In experiments at Eel Pond, Woods Hole, Didemnum began to recruit later than other species, in October, and rapidly expanded its coverage on plates in October-December (Agius 2007). In Narragansett Bay, Didemnum dominated plates in the fall, and overgrew recruits of the Blue Mussel (Mytilus edulis) (Auker and Oviatt 2008). In field experiments with a diversity of competitors and predators, Didemnum vexillum was less successful than when predators were excluded. However, the ability of colonies to form tendrils for dispersal, to fuse, to form extensive colonies on pebble and cobble surfaces, and to resist grazing, due to calcareous spicules in the tunic, enable D. vexillum to form extensive colonies in open areas (Stefaniak 2017). On Georges Bank, the abundance of Didemnum vexillum had a negative asssociation with that of both juvenile and Sea Scallops (Placopecten magellanicus. Didemnnum vexillum was more abundant i areas open to trawling than in areas closed to fishing, indicating that it may be favored by disturbance (Kaplan et al. 2017)
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Habitat Change
Didemnum vexillum overgrows gravel and benthic fauna, including scallops, mussels, and other sessile species. The surface of D. vexillum appears to repel settlement of secondary fouling organisms, suggesting that large areas of D. vexillum may reduce the amount of seabed suitable for settlement of larvae of other benthic species, including sea scallops.  The mats of Didemnum sp. may also reduce the amount of suitable shelter available for juvenile fish and other organisms (Valentine et al. 2007). On Georges Bank, the abundance of Didemnum vexillum had a negative asssociation with that of both juvenile and Sea Scallops (Placopecten magellanicus. Didemnum vexillum may inhibit the settling of Sea Scallops, because of the acidity of its tunic (Kaplan et al. 2017).
NA-ET3	Cape Cod to Cape Hatteras		Ecological Impact	Food/Prey
Dense mats of Didemnum vexillum colonies possibly form a barrier between fish and their benthic prey such as worms and bivalves, on the Georges Bank (Valentine et al. 2007). Some predation occurred on Didemnum, by the snail Astyris lunata and probably by fish on uncaged plates, but this tunicate appears less limited by predation than other tunicates, whose recruits are less likely to survive in open, deeper waters than Didemnum (Osman and Whitlatch 2007). Many benthic invertebrates on Georges Bank were positively associated with D. vexillum, and many of these invertebrates were important components of 5 important species of fishes (Winter Skate (Leucoraja ocellata), Little Skate (Leucoraja erinacea), Haddock (Melanogrammus aeglefinus), Winter Flounder (Pseudopleuronectes americanus), and Longhorn Sculpin (Myoxocephalus octodecemspinosus) (Smith et al. 2014).
NA-ET3	Cape Cod to Cape Hatteras		Economic Impact	Fisheries
Economic impacts on 'wild' fisheries, such as bottom fishes, scallops and lobsters are expected due to habitat change and replacement of food resources. Didemnum vexillum was found fouling aquaculture gear at 11 sites, and cultured Bay Scallops (Argopecten irradians) at three sites, of 26 aquaculture sites surveyed on Marthas Vineyard (Carman et al. 2010). This tunicate was also reported at aquaculture sites in Rhode Island (Carman et al. 2010). On Georges Bank, a major concern was possible negative impacts on the commercially important Sea Scallop (Placopecten magellanicus). Experiments indicate that can negatively affect larval settlement (Morris et al. 2009) and escape responses of the scallops (Dijkstra et al. 2011; Kaplan et al. 2017). However, the commerically important fishes Haddock Melanogrammus aeglefinus and Winter Flounder (Pseudopleuronectes americanus) fed heavily on benthic invertebrates positively associated with D. vexillum (Smith et al. 2014).
M040	Long Island Sound		Ecological Impact	Competition
On bare experimental plates in Fishers Island Sound, Connecticut, Didemnum was most competitive at sites with cooler temperatures, where it outgrew other tunicates, such as Botrylloides violaceus, Botryllus schlosseri, and Ascidiella aspera. However, Didemnum was less dominant on plates in which colonies of other species were already established (Osman and Whitlatch 2007). Invasion success of D. vexillum in experimental fouling communities was dependent on the presence of large amounts of unoccupied space (Janiak et al. 2013). In field experiments with a diversity of competitors and predators, Didemnum vexillum was less successful than when competitors and predators were excluded. However, the ability of colonies to form tendrils for dispersal, to fuse, to form extensive colonies on pebble and cobble surfaces, and to resist grazing, due to calcareous spicules in the tunic, enable D. vexillum to form extensive colonies (Stefaniak 2017).
M040	Long Island Sound		Ecological Impact	Food/Prey
Some predation occurred on Didemnum, by the snail Mitrella lunata and probably by fish on uncaged plates, but this tunicate appears less limited by predation than other tunicates, whose recruits are less likely to survive in open, deeper waters than Didemnum (Osman and Whitlatch 2007).
N195	_CDA_N195 (Cape Cod)		Ecological Impact	Competition
In experiments at Eel Pond, Woods Hole, Didemnum began to recruit later than other species, in October, and rapidly expanded its coverage on plates in October-December (Agius 2007).
M020	Narragansett Bay		Ecological Impact	Competition
In Narragansett Bay, Didemnum dominated plates in the fall, and overgrew recruits of the Blue Mussel (Mytilus edulis) (Auker and Oviatt 2008).
NEP-III	Alaskan panhandle to N. of Puget Sound		Economic Impact	Fisheries
Didemnum vexillum was found fouling shellfish cages and other aquaculture structures at Taylor Shellfish Farms, Quilcene, WA, and in Gallagher Cove, WA, Puget Sound and on oyster trays in Deep Bay, Baynes Sound, and mussel cages in Okeover Inlet, British Columbia (USGS Woods hole Science Center 2008). Switzer et al. (2011) tested chemical treatments (hydrated lime), mechanical treatments (manual scrubbing) and biological control (sea urchins. Strongylocentrotus droebachiensis, native) for cleaning fouled oysters. The mechanical and chemical treatments reduced fouling by Didemnum, but created free pace for botryllid tunicates to settle.
P290	Puget Sound		Economic Impact	Fisheries
Didemnum vexillum was found fouling shellfish cages and other aquaculture structures at Taylor Shellfish Farms, Quilcene, WA, and in Gallagher Cove, WA, Puget Sound (USGS Woods Hole Science Center 2008).
NZ-IV	None		Economic Impact	Fisheries
Didemnum vexillum is considered a major threat to New Zealand's mussel industry (Coutts and Forrest 2007). It was also found overgrowing salmon aquaculture cages (USGS Woods Hole Science Center 2008). As much as $650,000 NZ dollars were spent on an unsuccessful eradication attempt (Coutts and Forrest 2007).
NZ-IV	None		Economic Impact	Shipping/Boating
Didemnum vexillum is considered a serious fouling pest to recreational boating and commerical shipping. In New Zealand, attempts at eradicating and preventing the spread of Didemnum vexillum required costly cleaning of boats, pilngs, floats, etc.(Coutts and Forrest 2007).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Competition
In Portsmouth Harbor, by 2003-2006, colonial tunicates including D. vexillum replaced the mussel Mytilus edulis (1979-1982) as the dominant species in fouling communities (Dijkstra and Harris 2009).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Habitat Change
In Portsmouth Harbor, by 2003-2006, colonial tunicates including D. vexillum replaced the mussel Mytilus edulis (1979-1982) as the dominant species in fouling communities (Dijkstra and Harris 2009). A major functional change is that while mussel shells provided a year-round structure on the substrate, available to settlement by other organisms, colonial tunicates are more resistant to secondary settlement, and die off seasonally, creating large areas of bare substrate which can be colonized by other organisms (Dijkstra and Harris 2009). Didemnum vexillum can make Sea Scallops (Placopecten magellanicus) more vulnerable to predators by fouling their shells and decreasing their swimming speed (Dijkstra and Nolan 2011). On Georges Bank, extensive cover of D. vexillum is negatively correlated iwth abundance of P. magellanicus, barncles, tube anemones, Cerianthus, to sea urchins (Strongylocentrotus droebachiensis). Didemnum vexillum is considered a major driver of biodiversity decline on Georges Bank. Its effects are strongest on areas disturbed (by trawling (Kaplan et al. 2017).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Competition
Didemnum vexillum was one of several invasive fouling species which showed increased growth (% coverage) at temperatures 3.5 and 4.5⁰C above the ambient temperature in Bodega Harbor (13.5⁰C), while the native Distaplia occidentalis showed reduced survival (Sorte et al. 2010). Didemnum vexillum was one of a group of seven non-native species, most of which were rare or absent in 1970-1971, but were among the eight most abundant species in 2005-2009. Spawning periods and abundance of species in this group appeared to be favored by a 1⁰C increase in average temperatures at this site over a 30-year period (Sorte and Stachowicz 2011). Didemnum vexillum overgrowth had a modest effect on the growth of eelgrass (Zostera marina) blades in mesocosms filled with water from Tomales Bay (Long and Grosholz 2015).
P112	_CDA_P112 (Bodega Bay)		Ecological Impact	Competition
Didemnum vexillum was one of several invasive fouling species which showed increased growth (% coverage) at temperatures 3.5 and 4.5⁰C above the ambient temperature in Bodega Harbor (13.5⁰C), while the native Distaplia occidentalis showed reduced survival (Sorte et al. 2010). Didemnum vexillum was one of a group of seven non-native species, most of which were rare or absent in 1970-1971, but were among the eight most abundant species in 2005-2009. Spawning periods and abundance of species in this group appeared to be favored by a 1⁰C increase in average temperatures at this site over a 30-year period (Sorte and Stachowicz 2011).
N195	_CDA_N195 (Cape Cod)		Economic Impact	Fisheries
Didemnum vexillum was found fouling aquaculture gear at 11 sites, and cultured Bay Scallops (Argopecten irradians) at three sites, of 26 aquaculture sites surveyed on Marthas Vineyard (Carman et al. 2010). This tunicate was also reported at aquaculture sites in Rhode Island (Carman et al. 2010).
NA-ET2	Bay of Fundy to Cape Cod		Economic Impact	Fisheries
Didemnum vexillum was reported fouling aquaculure sites in Maine (Carman et al. 2010; Bullard et al. 215).
N130	Great Bay		Ecological Impact	Competition
In Portsmouth Harbor, by 2003-2006, colonial tunicates including D. vexillum replaced the mussel Mytilus edulis (1979-1982) as the dominant species in fouling communities (Dijkstra and Harris 2009).
N130	Great Bay		Ecological Impact	Habitat Change
In Portsmouth Harbor, by 2003-2006, colonial tunicates including D. vexillum replaced the mussel Mytilus edulis as the dominant species in fouling communities (Dijkstra and Harris 2009). A major functional change occured because while mussel shells provided structure, which other organisms can settle upon, colonial tunicates are more resistant to secondary settlement.  However, colonial tunicates die off seasonally, creating large areas of bare substrate which can be colonized by other organisms (Dijkstra and Harris 2009).
NEA-II	None		Economic Impact	Shipping/Boating
Because of potential economic and ecological impacts, eradication was attempted in Holyhead Harbour, Wales, in 2009, by enclosing floats in plastic bags, creating a stagnant environment. This was temporarily effective, but the regrowth of new colonies was seen by 2011 (Holt 2011). Fouling impacts fave been reported for the British Isles (Minchin et al. 2013).
N135	_CDA_N135 (Piscataqua-Salmon Falls)		Ecological Impact	Food/Prey
Increasing abundance of the introduced colonial tunicates Didemnum vexillum and Diplosoma listerianum has resulted in population growth of the native Bloodstar starfish Henricia sanguinolenta (Dijkstra et al. 2012).
N135	_CDA_N135 (Piscataqua-Salmon Falls)		Ecological Impact	Trophic Cascade
Increased abundance of the Bloodstar starfish (Henricia sanguinolenta), supported by growing populations of Didemnum vexillum and Diplosoma listerianum, has resulted in increased predation and near-disappearance of the cryptogenic sponge Halichondria panicea (Dijkstra et al. 2012).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Food/Prey
Increasing abundance of the introduced colonial tunicates Didemnum vexillum and Diplosoma listerianum has resulted in population growth of the native Bloodstar starfish Henricia sanguinolenta (Dijkstra et al. 2012).
NA-ET2	Bay of Fundy to Cape Cod		Ecological Impact	Trophic Cascade
Increased abundance of the Bloodstar starfish (Henricia sanguinolenta), supported by growing populations of Didemnum vexillum and Diplosoma listerianum, has resulted in increased predation and near-disspapearance of the cryptogenic sponge Halichondria panicea (Dijkstra et al. 2012).
P290	Puget Sound		Ecological Impact	Habitat Change
The introduced isopod Ianiropsis serricaudis occurred in high abundance with D. vexillum at Taylor Shellifish Frams, Puget Sound, but not found at other sites in the Sound (Cordell et al. 2012).
NEP-III	Alaskan panhandle to N. of Puget Sound		Ecological Impact	Habitat Change
The introduced isopod Ianiropsis serricaudid occurred in high abundance with D. vexillum at Taylor Shellifish Frams, Puget Sound, but not found at other sites in the Sound (Cordell et al. 2012).
P110	Tomales Bay		Ecological Impact	Competition
Didemnum vexillum overgrowth had a modest effect on the growth of eelgrass (Zostera marina blades in mesocosms filled with water from Tomales Bay (Long and Grosholz 2015).
P110	Tomales Bay		Ecological Impact	Habitat Change
Didemnum vexillum faciltated growth of invertebrates (polychaetes and tanaids) on eelgrass blades (Long and Grosholz 2015).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Habitat Change
Didemnum vexillum faciltated growth of invertebrates (polychaetes and tanaids) on eelgrass blades (Long and Grosholz 2015)
NEA-III	None		Economic Impact	Fisheries
Fouling of cultured mussels by a variety of non-native tunicates was reported beginning in 2013, and was a serious problem by 2016 (Palanisamy et al. 2018).
N070	Damariscotta River		Economic Impact	Fisheries
Didemnum vexillum was reportedly fouling aquaculture sites in Maine (Carman et al. 2010; Bullard et al. 2015)
CA	California		Ecological Impact	Competition
Didemnum vexillum was one of several invasive fouling species which showed increased growth (% coverage) at temperatures 3.5 and 4.5⁰C above the ambient temperature in Bodega Harbor (13.5⁰C), while the native Distaplia occidentalis showed reduced survival (Sorte et al. 2010). Didemnum vexillum was one of a group of seven non-native species, most of which were rare or absent in 1970-1971, but were among the eight most abundant species in 2005-2009. Spawning periods and abundance of species in this group appeared to be favored by a 1⁰C increase in average temperatures at this site over a 30-year period (Sorte and Stachowicz 2011). Didemnum vexillum overgrowth had a modest effect on the growth of eelgrass (Zostera marina) blades in mesocosms filled with water from Tomales Bay (Long and Grosholz 2015)., Didemnum vexillum overgrowth had a modest effect on the growth of eelgrass (Zostera marina blades in mesocosms filled with water from Tomales Bay (Long and Grosholz 2015)., Didemnum vexillum was one of several invasive fouling species which showed increased growth (% coverage) at temperatures 3.5 and 4.5⁰C above the ambient temperature in Bodega Harbor (13.5⁰C), while the native Distaplia occidentalis showed reduced survival (Sorte et al. 2010). Didemnum vexillum was one of a group of seven non-native species, most of which were rare or absent in 1970-1971, but were among the eight most abundant species in 2005-2009. Spawning periods and abundance of species in this group appeared to be favored by a 1⁰C increase in average temperatures at this site over a 30-year period (Sorte and Stachowicz 2011).
CA	California		Ecological Impact	Habitat Change
Didemnum vexillum faciltated growth of invertebrates (polychaetes and tanaids) on eelgrass blades (Long and Grosholz 2015), Didemnum vexillum faciltated growth of invertebrates (polychaetes and tanaids) on eelgrass blades (Long and Grosholz 2015).
WA	Washington		Ecological Impact	Habitat Change
The introduced isopod Ianiropsis serricaudis occurred in high abundance with D. vexillum at Taylor Shellifish Frams, Puget Sound, but not found at other sites in the Sound (Cordell et al. 2012).
MA	Massachusetts		Ecological Impact	Competition
In experiments at Eel Pond, Woods Hole, Didemnum began to recruit later than other species, in October, and rapidly expanded its coverage on plates in October-December (Agius 2007).
MA	Massachusetts		Economic Impact	Fisheries
Didemnum vexillum was found fouling aquaculture gear at 11 sites, and cultured Bay Scallops (Argopecten irradians) at three sites, of 26 aquaculture sites surveyed on Marthas Vineyard (Carman et al. 2010). This tunicate was also reported at aquaculture sites in Rhode Island (Carman et al. 2010).
ME	Maine		Economic Impact	Fisheries
Didemnum vexillum was reportedly fouling aquaculture sites in Maine (Carman et al. 2010; Bullard et al. 2015)
NH	New Hampshire		Ecological Impact	Food/Prey
Increasing abundance of the introduced colonial tunicates Didemnum vexillum and Diplosoma listerianum has resulted in population growth of the native Bloodstar starfish Henricia sanguinolenta (Dijkstra et al. 2012).
NH	New Hampshire		Ecological Impact	Trophic Cascade
Increased abundance of the Bloodstar starfish (Henricia sanguinolenta), supported by growing populations of Didemnum vexillum and Diplosoma listerianum, has resulted in increased predation and near-disappearance of the cryptogenic sponge Halichondria panicea (Dijkstra et al. 2012).
MED-V	None		Economic Impact	Fisheries
Interferes with tuna aquaculture, requires extra work in cleaning (Cinar and Ozgul 2922)

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