Botryllus schlosseri

History of Spread

Botryllus schlosseri (Golden Star Tunicate) now has a worldwide distribution, including harbors and ports on both sides of the North and South Atlantic (Argentina, South Africa), and both sides of the North and South Pacific (China to Russia, Mexico to Alaska; Australia, New Zealand, Chile)The region of origin of B. schlosseri is unknown, but James T. Carlton (2000, personal communication) suspects the Indo-Pacific as a possible native region. Recent genetic work supports a Mediterranean origin (e.g., Rinkevich et al. 1995; Ben-Shlomo et al. 2001; Ben Shlomo et al. 2006).

Botryllus schlosseri was first reported from North America near Boston MA by Couthouy (1838). It was later recorded from (Brooklyn, Long Island NY; Boston, Salem MA) as B. gouldi by Verrill (1871). Later accounts gave wider ranges: 'Portland, Maine, southward, and is locally abundant from the vicinity of Boston to New Jersey, inclusive. It is also abundant on the west coast of Florida, including the Tortugas, but the only record from intermediate points that has thus far come to notice is the probable one of Pearse, Humm and Wharton....who report a Botryllus sp. from Beaufort, North Carolina (Van Name 1945). 'Boston (or less regularly Bay of Fundy) south at least to Chesapeake Bay....Sporadically south to Gulf of Mexico' (Gosner 1978). The range of B. schlosseri has been extended northwards in recent years, to the Atlantic coast of Nova Scotia and the Gulf of St. Lawrence (Prince Edward Island, reached in 2001, Locke et al. 2007). Van Name (1945) wrote: 'It is, however, very probably an introduced species brought here on the bottom of ships'.

A recent genetic analysis indicates that 'B. schlosseri' is a complex of at least 5 cryptic species (A-E), but only one, clade A, had a widespread distribution in the Northeast and Northwest Atlantic, and Northeast Pacific. The other clades had restricted distributions in Europe (Bock et al. 2012). However, Yund et al. (2015) have identified at least one subclade which appears to be native to the Northwest Atlantic. This subclade is most genetically diverse in that region, and 9 of its 12 haplotypes are unique to that region. The divergence between the prevailing Northwest Atlantic haplotype those from European waters is too great to be accounted for by evolution (Yund et al. 2015). We cannot exclude the occurrence of European genotypes or crytpic species in East Coast water, and so we will treat B. schlosseri as a single cryptogenic species on the East Coast.

Botryllus schlosseri was first collected on the West Coast in 1947, at Mare Island, San Francisco Bay (Carlton 1979). It now occurs in harbors from Ensenada, Mexico (Lambert and Lambert 2003) to Sitka, Alaska (2001, Ruiz et al. unpublished data).

In Chesapeake Bay, B. schlosseri was not reported in the course of an extensive survey of Chesapeake benthos in 1915-1922 (Cowles 1930) or in shoreline surveys near Norfolk by Ferguson et al. (1949). However, it was reported in fouling on the dredge Chinook in Hampton Roads in 1923 (Visscher 1928). It was listed as a rare species in deeper waters of the lower Bay by Wass (1965). A colony was collected at Virginia Institute of Marine Science, Gloucester Point in 1962 (Calder 1972), and it was common to abundant on piers at Norfolk in 1964-1965 (Calder and Brehmer 1967). 'During the drought years of the mid-sixties, B. schlosseri suddenly appeared at Gloucester Point on tray oysters and eventually erupted to cover nearly all tufts of eelgrass (Zostera marina) and (Ruppia (R. maritima , Widgeon Grass) in shallow water in the lower York River. It is a fast-growing pernicious pest on trays in the cool months of spring and fall but barely survives hot summers in Virginia....B. schlosseri was not vigorous in the wet year of 1971, but it was still present on trays of oysters in the spring of 1972. After Agnes it disappeared and no trace has been found at any fouling stations. It may not recover its distribution of the 1960's until another series of droughts occurs' (Andrews 1973). B. schlosseri was abundant on fouling panels in Lynnhaven Bay in 1977 (Otsuka and Dauer 1980) and common on' settling plates (1994-95) in all of the major lower Bay regions sampled except Norfolk (15-18 ppt), and not at all sites. B. schlosseri is much more common on spring-early summer plates than summer-fall plates (Ruiz et al. unpublished data).

The spread of B. schlosseri in the lower Bay in the 1960s is suggestive of a recent introduction, either from Europe or from further north along the East Coast, but this pattern could also represent population fluctuations in response to long-term salinity or temperature changes.

History References - Andrews 1973; Ben-Shlomo et al. 2001; Boyd et al. 1990; Calder 1972; Calder and Brehmer 1967; Carlton 1979; Cowles 1930; Couthouy 1838; Ferguson et al. 1949; Gosner 1978; Gould 1841; Otsuka and Dauer 1980; Rinkevich et al. 1995; 2001; Ruiz et al. unpublished data; Stoner et al. 2001; Verrill 1871; Visscher 1928; Van Name 1945; Wass 1963

Invasion Comments