Species Description: The star tunicate, Botryllus schlosseri is a colonial tunicate not native to the U.S., but now occurs on both North American coasts. Colonies grow in flat sheets or lobes on hard substrata and are composed of many (hundreds to thousands) clonal, asexually produced zooids arranged in approximately star- or flower-shaped clusters called systems. Each system is comprised of approximately 20 oval- to tear-shaped zooids.
Color is variable but all zooids within a colony are the same color. Zooids are often orange, yellow, black, red, gray-green, or white and are embedded in a firm fleshy matrix (the test) that is often purple or brown to colorless. Each zooid possesses its own incurrent siphon while all zooids within a colony share a single, large common excurrent siphon (NIMPIS 2002, Cohen 2005).
Potentially Misidentified Species: Colonial Botryllus schlosseri is similar in appearance to other colonial tunicates such as those belonging to the genus Botrylloides. The systems (zooid clusters) of Botrylloides species tend to be elongate in shape, often arranged in meandering double rows, but only occasionally exhibiting the star pattern typical of B. schlosseri (Cohen 2005).
Regional Occurrence: Botryllus schlosseri was introduced from Europe and is now found on both coasts of North America as well as Australia and the Asian Pacific (NIMPIS 2002).
On the Atlantic coast of North America, it is occurs from Maine (occasionally north to the Bay of Fundy) south to Florida and the Gulf of Mexico (Cohen 2005). Occurrence down the Atlantic seaboard may be disjunct, possibly due to discontinuity of hard substrates, but collection records indicate its presence in New Jersey and the North Carolina (NIMPIS 2002, Hiscock 2007, Salem Sound Coastwatch undated).
IRL Distribution: Mook (1993) identified Botryllus schlosseri as established and seasonally abundant in the IRL (Carlton and Ruckelshaus 1997).
Age, Size, Lifespan: Individual Botryllus schlosseri zooids are usually 2.5-5 mm in length, system clusters around 5-10 mm in diameter and colonies are typically 10 cm or less across (Cohen 2005).
Chadwick-Furman and Weissman (1995) reported that colonies in the field populations they studied had lifespans ranging from an average of 82-247 days and that colony life expectancy was seasonally dependent. This is in sharp contrast to colonies that have been maintained in the laboratory for more than 7 years (Rinkevich and Shapira 1998).
B. schlosseri colonies grew exponentially as juveniles attaining colony sizes of up to 1400 zooids within 69 days (Chadwick-Furman and Weissman 1995).
Abundance: Carlton and Ruckelshaus (1997) indicate that Botryllus schlosseri is seasonally common in Florida fouling communities, as is the co-occurring non-native ascidian Styela plicata and the cryptogenic (native range uncertain) B. niger (= Botrylloides nigrum).
Reproduction: Reproduction in B. schlosseri, a sessile hermaphrodite, includes a sexual and an asexual component. Sexual reproduction involves the release of male gametes into the water followed by uptake in the incurrent siphons of nearby colonies and interanal fertilization of eggs (Phillippi et al. 2004). Asexual reproduction involves a synchronized budding process that occurs on an approximately weekly basis to increase the size of the clonal colony until it becomes large enough to reproduce sexually. Closely genetically related colonies may fuse with one another when they come into contact, resulting in larger "chimera" (made up of genetically distinct individuals) colonies that may reach sexual maturity more rapidly than smaller unfused colonies. Sexual maturity in field populations in Monterey, CA was attained in 49 days, corresponding to 7 asexual replication cycles (Chadwick-Furman and Weissman 1995, Salem Sound Coastwatch undated).
Reproductive seasonality appears quite variable across the broad distribution range of the species.
Embryology: Internal gestation and development to a free-swimming "tadpole" larval stage is followed by release to the water column and a short (up to 24-36 hours) planktonic duration that is probably capable of only local (1-10 km) dispersal (Berrill 1950, 1975, Hiscock 2007).
Temperature: Broad latitudinal distribution of Botryllus schlosseri (from Florida north to the Bay of Fundy on the east coast of North America, for example), indicates a eurythermal condition. Sexual reproduction is likely to be seasonally restricted to a brief summer season at higher latitudes. Brunetti et al. (1980) report a lower reproductive temperature of 11ºC for the European population examined by these authors.
Salinity: Hiscock (2007) indicates Botryllus schlosseri is relatively euryhaline, tolerating salinities ranging from 18-40 ppt.
Trophic Mode: Botryllus schlosseri is a suspension feeder whose diet includes suspended phytoplankton, zooplankton and suspended organic matter (Millar 1971, NIMPIS 2002).
Associated Species: Various invertebrate species such as flatworms, crustaceans, and gastropods have been reported to feed on Botryllus schlosseri colonies (Cohen 2005).
Invasion History: The European native range of Botryllus schlosseri is believed to encompass the Mediterranean, and may also include the Adriatic and Black seas, Great Britain and France, Norway, and the Faroe Islands (Cohen 2005, Salem Sound Coastwatch undated).
Ruiz et al. (2000) indicate that the first records of B. schlosseri on the east coast of North America date to 1841 in Massachusetts, while the earliest reported occurrence in the Gulf of Mexico appears to be 1921. On the U.S. west coast, the earliest reports of B. schlosseri include reports from San Francisco Bay dating to the mid 1940s, San Diego Bay and Mission Bay dating to the early 1960s, and from a Puget Sound oyster farm in the late 1960s or early 1970s. Broader distribution up and down the Pacific coast of North America from British Columbia to Mexico was only documented starting in the mid-1990s (Lambert and Lambert 1998, Cohen 2005).
The species has been broadly introduced elsewhere as well, and can now be found in Australia (since 1905), Tasmania and New Zealand (since 1928), Japan, and Hong Kong. Ship hull fouling is probably the most common introduction vector for B. schlosseri, but accidental introduction in shipments of live oysters or other wild-harvested or cultured organisms has probably also played a role in expanding the range of this organism (Cohen 2005).
Potential to Compete With Natives: B. schlosseri competes with other attached benthic filter feeders for space and possibly food as well. Fast-growing B. schlosseri colonies may overgrow neighboring organisms and alter community dynamics (NIMPIS 2002, Cohen 2005). Harms and Anger (1983) report barnacles and mussels as among the most important space competitors with B. schlosseri.
Schmidt and Warner (1986) examined spatial competition among B. schlosseri and three other encrusting colonial ascidian species and reported no significant overgrowth among competing species.
Possible Economic Consequences of Invasion: An abundant harbor fouling organism, Botryllus schlosseri is a nuisance species that fouls boat hulls, marine equipment, aquaculture gear, and other submerged structures. It can also overgrow and compete for space with cultured oysters and mussels (NIMPIS 2002, Cohen 2005).
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