Invasion History

First Non-native North American Tidal Record: 1917
First Non-native West Coast Tidal Record: 1917
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Botrylloides diegensis was described by Ritter and Forsyth in 1917 from San Diego, California, and has expanded its known range south to Baja California and north to Newport Bay (Van Name 1945), Mission Bay (Tracy and Reynes 2014), and San Francisco Bay, originally identified as 'Botryllus sp. A,' (Kozloff 1987; Cohen and Carlton 1995). Currently, on the Pacific Coast, it ranges from Bodega Bay to Baja California (Van Name 1945; Lambert and Lambert 2003; Cohen 2005; Sorte and Stachowicz 2011). Although it was described from San Diego Bay, its limited range on the coast, and the lack of other native botryllids on the Pacific Coast, indicates that it is not native to the region. It is likely native to the Indo-West Pacific (Carlton 2009; Lambert, personal communication), but its native region is unknown. Botrylloides diegensis has been collected from northern Europe (Netherlands, British Isles, Brittany), from the Mediterranean (France, Italy), New Zealand, Japan, and South Korea (Faasse 2016; Bishop et al. 2015a; Bishop et al. 2015b; Viard et al. 2019; Lee and Shin 2021; Nydam et all 2021).

North American Invasion History:

Invasion History on the West Coast:

Botrylloides diegensis was described by Ritter and Forsyth in 1917 from San Diego, California, and later found in Newport Bay (Van Name 1945) and Mission Bay (Tracy and Reynes 2014). It was treated as an unidentified species, 'Botryllus sp. A,', when it was photographed by Eugene Kozloff in the 1970s–early 80s, and collected by Cohen and Carlton in San Francisco Bay, in 1993–1994 (Kozloff 1987; Cohen and Carlton 1995; James Carlton, personal communication) and later determined likely to be B. diegensis (Carlton, personal communication). Currently it ranges on the Pacific Coast from Bodega Bay to Baja California (Van Name 1945; Lambert and Lambert 2003; Cohen 2005; Sorte and Stachowicz 2011). Although it was described from San Diego Bay, its limited range on the coast and the lack of other native botryllids on the Pacific Coast, indicates that it is not native to the region.


Invasion History on the East Coast:

Botrylloides diegensis is not known to be established on the East Coast. Early records of this species from the 1970s to the early 2000s actually were misidentifications of Botrylloides violaceus. Around 1972, a scientist who wanted a supply of experimental animals identified a tunicate as B. diegensis in Eel Pond, adjacent to Woods Hole Harbor MA. He reported successful overwintering and reproduction of these animals (Carlton 1989). These organisms may have actually been B. violaceus, or if they were B. diegensis, they may have failed to become established. Literature from the 1990s and early 2000, and some museum records list Botrylloides diegensis from Long Island Sound to the Gulf of Maine (Carlton 1989; Yund and Feldgarden 1992; Berman et al. 1992; Osman and Whitlach 1995; Whitlach and Osman 2000; U.S. National Museum of Natural History; Yale Peabody Museum 2019). However, subsequent examination of Botrylloides sp. along the Northeast Coast of North America indicates that all the specimens found were actually B. violaceus, and are re-identified as such (Gretchen Lambert, personal communication 2000; Harris et al. 2001; Dijkstra et al. 2007).


Invasion History Elsewhere in the World:

Botrylloides diegensis was first found in the Netherlands (Faasse 2006), but the precise location was not given (Faasse 2006). Specimens collected in 2004 at Plymouth, Devon, and Lowestoft, near the western and eastern ends of the English Channel, respectively, were later identified as B. diegensis (Arenas et al. 2005; Bishop et al. 2015a). In surveys on the southern coast of England, it was found from Torbay (Cornwall), and Lowestoft in 2006 to 2012 (Bishop et al. 2015a; Wood et al. 2015). Botrylloides diegensis may be found at many locations on the coast of Brittany (Viard et al. 2019), and in the Channel Islands (Department of the Environment, States of Jersey 2017). It has also been found in Lancashire, on the Irish Sea (Wood et al. 2016), and near Milford Haven, Wales, in the Celtic Sea (NBN Atlas 2019).

Viard et al. (2019) has also genetically identified B. diegensis at several sites on the northern coast of the Mediterranean Sea, in the Ebro Delta, Spain, Sete, France, and Venice, Italy.

Botrylloides diegenisis was first collected and genetically identified on the west coast of Japan in 2005, 2007, and 2009 (Nydam et al. 2021).  Surveys in South Korea have documented a widespread invasion of the Korean Peninsula, supporting an introduced status in the northwest Pacific. This tunicate was collected in 6 Korean ports in 2017, but in 2020 it was collected in 14 ports, on all three coasts (Yellow Sea, Korea Straits, Sea of Japan). Introduced populations include both single-color and two-colored colonies (Lee and Shin 2021).  

Specimens from New Zealand, collected between 2012–2021, have also been genetically identified as B. diegensis (Nydam et al. 2021; Teniz et al. 2021). The Southwest Pacific is considered a possible native region for this species (Carlton 2009. personal communication; Temiz et al. 2021). Alternatively, hull fouling and oyster transplants are possible vectors in the region.


Botrylloides diegensis (Ritter and Forsyth 1917) is a colonial tunicate. Its colonies are flat encrusting, usually ~5 mm thick, and several centimeters across. The tunic is soft and transparent in preserved specimens, but more opaque in living specimens. Zooids are arranged in ladder-like chains, with 10–15 zooids in smaller units, but less distinct in larger colonies. The zooids are cylindrical, about 2–2.5 mm long and 1 mm wide. The oral opening is smooth and circular. The atrial opening is gaping, and the border is extended to form a tongue-like protuberance, called a 'languet,' of varying size and length. There are 16 oral tentacles, of 3 sizes, with the largest on the right and left, larger than the median ones. There are usually 10–12 rows of stigmata and 9–12 stomach folds (Ritter and Forsyth 1917; Van Name 1945; Faasse 2006; Abbott et al. 2007).

Ova (reproductive cells) are located dorsal-posterior to testis, consisting of up to 16 follicles. The larvae of B. diegensis are incubated and nourished by the tunic vascular system, and continue to grow even after the adult zooid dies. The larvae are small (~0.5 mm in length) with 8 lateral ampullae. Fully developed larvae are released from the incubatory pouch (Lambert and Lambert 2003; Faasse 2006; Abbott et al. 2007).

Colors of Botrylloides diegensis and other Botrylloides are variable, contributing to identification confusion in several geographic areas. Botrylloides diegensis is often characterized by having two-colored colonies, with a bright ring of orange, green, or white pigment around the branchial opening, against a background color that is purplish, brown, or suffused with orange, while B. violaceus usually have a single color (Ritter and Forsyth 1917; Van Name 1945; Lambert and Lambert 2003; Faasse 2006). On European coasts, the occurrence of B. diegensis was overlooked due to similarity to the native B. leachi and introduced B. violaceus (Faasse 2006; Gittenberger et al. 2007). Some specimens of B. leachii have a two-colored pattern similar to that of B. diegensis (Viard et al. 2019). However, genetic analysis found some single-color colonies of B. diegensis in the Mediterranean and the English Channel (Viard et al. 2019). In South Korea, genetically identified Botrylloides diegensis of both color morphs were found on a single plate, together with B. violaceus and Botryllus schlosseri (Lee and Shin, 2021). Cohen and Carlton (1995) described it as a "botryllid with chain zooids in San Francisco Bay, but with each zooid twice the size of those in B. aurantius (=Botrylloides violaceus)" and called it Botryllus sp., but later determined that the zooids were not actually larger, but only appeared so due to its color pattern (Cohen, personal communication). A photograph by Eugene Kozloff in his 1983 edition of Seashore Life of the Pacific Northwest of the botryllid is probably B. diegensis and likely the first record of it in San Francisco Bay (Cohen, Carlton, personal communication).  Lambert and Lambert (1998) initially identified all southern California Botrylloides as B. diegensis, before recognizing the presence of B. violaceus (Lambert and Lambert 2003).

On the East Coast, in 1972, a marine biologist placed colonies of tunicates identified as B. diegensis in Eel Pond, adjacent to Woods Hole Harbor, Massachusetts. Botrylloides colonies were found to be widespread from Long Island Sound to the Gulf of Maine, and were initially assumed to be B. diegensis (Cohen 1989; Berman et al. 1992). Subsequent examination of Botrylloides sp. along the Northeast Coast of North America indicates that all the specimens found were actually B. violaceus (Gretchen Lambert, personal communication 2000).



Taxonomic Tree

Kingdom:   Animalia
Phylum:   Chordata
Subphylum:   Tunicata
Class:   Ascidiacea
Order:   Stolidobranchia
Family:   Styelidae
Genus:   Botrylloides
Species:   diegensis


Botrylloides sp. (Kozloff, 1983)
Botryllus sp. (Cohen and Carlton, 1995)
Botrylloides sp. A (Ruiz et al., 2011)
Botrylloides diegense (Van Name 1945, None)

Potentially Misidentified Species

Botrylloides niger
Botrylloides niger is a widely distributed tropical-subtropical species in the Indo-Pacific and Atlantic, with introduced populations in tropical Pacific Mexico, Panama, the Galapagos, and Tahiti (Sheets et al. 2013; NEMESIS 2019)

Botrylloides anceps
Botrylloides anceps is an Indo-Pacific species, native to Australia, but introduced to the Eastern Mediterranean via the Panama Canal (Viard et al. 2019).

Botrylloides giganteus
Botrylloides giganteus is probably native to the tropical Atlantic, and introduced to southern California, the Galapagos Islands, New Zealand, and southern Italy (NEMESIS 2019; Viard et al. 2019).

Botrylloides israeliensis

Botrylloides israeliensis is native to the Mediterranean coast of Israel, and genetically close to one genetically Botrylloides divergent haplotype (BvX-H6) in the English Channel, and to one Italian population (Miseno Lagoon, Viard et al. 2019).

Botrylloides leachii

Botrylloides leachii is native to the Northeast Atlantic and Mediterranean. Zooids are arranged in two parallel chains; Zooids have 8–16 branchial tentacles; branchial sacs with 9–15 stigmata; 9–11 longitudinal stomach folds, colonies are two-colored (de Kluijver et al. 2019). This species is highly variable, and may be a species complex. Viard et al. (2019) made genetic and photographic comparisons of the 'radiata' morph of B. leachi (which has pigment spots radiating around each zooid, resembling a spoked wheel), with B. violaceus and B. diegensis).

Botrylloides violaceus

Botrylloides violaceus is native to the Northwest Pacific, and introduced to the northeast Paciifc (Alaska to Baja California), northwest Atlantic (Newfoundland to Virignia), Northeast Atlantic (Spain to Scotland, Azores), and the Mediterranean (Venice). Colonies range from orange to violet, and are usually a single color. Zooids have 10–11 rows of stigmata and 9–12 stomach folds. Larvae are large are, up to 3 mm (Faasse 2006; Viard et al. 2019; NEMESIS 2019).

Botryllus schlosseri

Colonies of Botryllus schlosseri have zooids arranged in clusters in a star-shaped arrangement around an atrial opening. The zooids have 12–16 oral tentacles, 7–9 rows of stigmata, and 8–10 stomach folds. Larvae are about 0.4 mm in trunk length with 8 ampullae. Color is highly variable, but the star-shaped zooids are usually brighter and distinct from the colony matrix (Van Name 1945; Nishikawa 1991a)



Life History- A colonial (or compound) tunicate consists of many small zooids, bearing most or all of the organs of a solitary tunicate, but modified to varying degrees for colonial life. Colonial ascidians of the genera Botryllus and Botrylloides have small, flattened and curved zooids arranged in systems, with groups of zooids having atrial canals opening into a common cloacal chamber. The zooids are embedded in a mass of test material. Each zooid has an oral siphon and an atrial canal, an opening to a shared cloacal chamber. The thorax contains an elaborate gill basket and below it, a compressed abdomen containing the stomach, intestine, ovaries, testis, and heart. The intestine leads back to the thorax and to the atrial opening. The abdomen ends in stolons which area attached to the substrate and neighboring zooids. Water is pumped through the oral siphon, through finely meshed ciliated gills, where it is filtered and expelled through the oral siphon. Phytoplankton and detritus is filtered by the gills and passed on mucus strings to the stomach and intestines where waste is expelled in the outgoing atrial water (Van Name 1945; Barnes 1983).





Botrylloides violaceus; Botryllus schlosseri; Botrylloides giganteus; Diplosoma listerianum

Trophic Status:

Suspension Feeder



General HabitatMarinas & DocksNone
General HabitatRockyNone
General HabitatOyster ReefNone
General HabitatVessel HullNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone

Life History

Colonial ascidians reproduce both asexually, by budding, and sexually, from fertilized eggs developing into larvae. Buds can form from the body wall of the zooid. Colonies vary in size and can range from small clusters of zooids to huge spreading masses. The zooids are hermaphroditic, with eggs and sperm being produced by a single individual. Eggs may be self-fertilized or fertilized by sperm from nearby animals, but many species have a partial block to self-fertilization. Depending on the life-history of the species, eggs may be externally or internally fertilized. Fertilized eggs hatch into a tadpole larva with a muscular tail, notochord, eyespots, and a set of adhesive papillae. The lecithotrophic (non-feeding, yolk-dependent) larva swims briefly before settlement. Swimming periods are usually less than a day, and some larvae can settle immediately after release, but the larval period can be longer at lower temperatures. Once settled, the tail is absorbed, the gill basket expands, and the tunicate begins to feed by filtering (Van Name 1945; Barnes 1983).

Tolerances and Life History Parameters

Minimum Salinity (‰)18Experimental, 80-100% survival, Smith et al. 2012
Maximum Salinity (‰)35Based on field records
Broad Temperature RangeNoneCold-temperate; Warm-temperate
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

The ecological and economic impacts of Botrylloides diegensis are unknown, as is its distribution and abundance. It is regarded as a possible threat to cultured shellfish (Department of the Environment, States of Jersey 2017).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEP-V Northern California to Mid Channel Islands 1983 Def Estab
P090 San Francisco Bay 1983 Def Estab
NEP-VI Pt. Conception to Southern Baja California 1917 Def Estab
NEA-II None 2002 Def Estab
NEA-IV None 2004 Def Estab
P112 _CDA_P112 (Bodega Bay) 2009 Def Estab
NEA-III None 2004 Def Estab
MED-II None 2016 Def Estab
MED-VI None 2016 Def Estab
NZ-IV None 2012 Crypto Estab
P112 _CDA_P112 (Bodega Bay) 2009 Def Estab
P040 Newport Bay 1945 Def Estab
P030 Mission Bay 2011 Def Estab
P020 San Diego Bay 1917 Def Estab
P053 _CDA_P053 (Santa Monica Bay) 2015 Def Estab
P062 _CDA_P062 (Calleguas) 2015 Def Estab
P050 San Pedro Bay 2003 Def Estab
NWP-3b None 2005 Def Estab
NWP-4b None 2009 Def Estab
P058 _CDA_P058 (San Pedro Channel Islands) 2019 Def Estab
NWP-4a None 2017 Def Estab
NWP-3a None 2017 Def Estab
NZ-VI None 2020 Crypto Estab
MED-IV None 2019 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
767985 Ruiz et al., 2015 2012 2012-08-24 Richmond Marina Bay Yacht Harbor, San Francisco Bay, CA, California, USA Def 37.9134 -122.3523
768084 Ruiz et al., 2015 2012 2012-08-30 Oyster Point Marina, San Francisco Bay, CA, California, USA Def 37.6633 -122.3817
768108 Ruiz et al., 2015 2012 2012-08-29 Coyote Point Marina, San Francisco Bay, CA, California, USA Def 37.5877 -122.3174
768276 Ruiz et al., 2015 2013 2013-08-15 Ballena Isle Marina, San Francisco Bay, CA, California, USA Def 37.7656 -122.2858
768356 Ruiz et al., 2015 2013 2013-08-13 Oyster Point Marina, San Francisco Bay, CA, California, USA Def 37.6639 -122.3821
768380 Ruiz et al., 2015 2013 2013-08-14 Redwood City Marina, San Francisco Bay, CA, California, USA Def 37.5024 -122.2134
768400 Ruiz et al., 2015 2013 2013-08-19 Richmond Marina Bay Yacht Harbor, San Francisco Bay, CA, California, USA Def 37.9138 -122.3522


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