Invasion History

First Non-native North American Tidal Record: 1992
First Non-native West Coast Tidal Record: 1992
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

The native range of Bostrichobranchus pilularis is from at least from the Gulf of St. Lawrence to the west coast of Florida and Texas (Van Name 1945). Collections at the US National Museum of Natural History include specimens from the Arctic Ocean (e.g. USNM 19974 from the Beaufort Sea), and Colombia (USNM 25097; US National Museum of Natural History 2011). This wide geographical range suggests that multiple species may be grouped under this name. This tunicate inhabits sand and mud and is usually collected by dredging (Van Name 1945).

North American Invasion History:

Invasion History on the West Coast:

There is only one known record of Bostrichobranchus pilularis from the West Coast, from dredge collections in April 1992, in San Dieguito Lagoon, a small estuary north of San Diego. It was not found in subsequent collections there or elsewhere (Lambert and Lambert 1998). Since this tunicate is not usually found in fouling, or with oysters, and since the San Dieguito Lagoon receives only small-boat traffic, the mechanism of introduction is not clear.


Bostrichobranchus pilularis is a solitary tunicate, which occurs unattached in soft sediments. Its globular shaped body is covered with a thin layer of mud, and, when its siphons are retracted, it looks like a small soft ball of mud. The tunic of the body, when cleaned, is very thin, soft, nearly transparent, thickly covered with minute granules, and composed of minute fibers, usually concealed by the adhering particles of mud and fine sand. The siphons are naked, nearly transparent, slender, tapered, and as long as the diameter of the body. They originate close together and are slightly divergent, with both of them being nearly straight. The siphons are completely retractable, and their bases are surrounded and connected by a narrow, naked, oval or oblong band, which is usually conspicuous when the tubes are contracted. The oral siphon is a little shorter than the atrial, the aperture surrounded by six acute, conical papillae, and twelve small dark brownish spots. The atrial siphon has a small square aperture, surrounded by four small lobes and four small, reddish brown eyespots. In life, when cleaned, the body is transparent and grayish, with the dark intestine showing through very distinctly, and the siphons with greenish coloring at their base. The diameter is usually about 5 mm, and seldom more than 6-8 mm (Verrill 1871; Van Name 1945).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Chordata
Subphylum:   Tunicata
Class:   Ascidiacea
Order:   Stolidobranchia
Family:   Molgulidae
Genus:   Bostrichobranchus
Species:   pilularis


Bostrichobranchus manhattensis (Traustedt, 1883)
Caesira pellucida (Hartmeyer, 1911)
Eugyra glutinans (Sumner, Osburn & Cole, 1913)
Eurgyra pilularis (Verrill, 1872)
Molgula producta (Whieaves, 1874)
Bostrichobranchus molguloides (Metcalf, 1900)
Eugyriopsis manhattensis (Pizon, 1898)
Herdmania bostrichobranchus (Metcalf, 1900)
Molgula pellucida (Verrill, 1872)
Molgula pilularis (Verrill, 1871)

Potentially Misidentified Species



Life History- A solitary tunicate is ovoid, elongate or vase-like in shape, with two openings or siphons. Most solitary tunicates attach to substrates by their side or base, but some attach with a conspicuous stalk. They are sessile filter feeders with two siphons, an oral and an atrial siphon. Water is pumped in through the oral siphon, where phytoplankton and detritus is filtered by the gills, and passed on mucus strings to the stomach and intestines. Waste is then expelled in the outgoing atrial water.

Solitary ascidians are hermaphroditic, meaning that both eggs and sperm are released to the atrial chamber. 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 species, eggs may be externally or internally fertilized. In external fertilizers, eggs and sperm are released through the atrial siphon into the surrounding water column were fertilization takes place. In internal fertilizers, eggs are brooded and fertilized within the atrial chamber and then released into the water column upon hatching. 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 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 (Barnes 1983).


Phytoplankton, detritus

Trophic Status:

Suspension Feeder



General HabitatUnstructured BottomNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEndobenthicNone

Tolerances and Life History Parameters

Broad Temperature RangeNoneCold temperate-Tropical
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

Bostrichobranchus pilularis was briefly common in San Dieguito Lagoon, Southern California, but then apparently disappeared. No ecological impacts are known from this short-lived invasion.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NA-S3 None 0 Native Estab
NA-ET1 Gulf of St. Lawrence to Bay of Fundy 0 Native Estab
NA-ET2 Bay of Fundy to Cape Cod 0 Native Estab
NA-ET3 Cape Cod to Cape Hatteras 0 Native Estab
CAR-VII Cape Hatteras to Mid-East Florida 0 Native Estab
CAR-I Northern Yucatan, Gulf of Mexico, Florida Straits, to Middle Eastern Florida 0 Native Estab
NEP-VI Pt. Conception to Southern Baja California 1992 Def Unk
NA-S2 None 0 Native Estab
CAR-III None 0 Native Estab
P022 _CDA_P022 (San Diego) 1992 Def Unk

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Barnes, Robert D. (1983) Invertebrate Zoology, Saunders, Philadelphia. Pp. 883

Gosner, Kenneth L. (1978) A field guide to the Atlantic seashore., In: (Eds.) . , Boston. Pp. <missing location>

Lambert, C. C.; Lambert, G. (1998) Non-indigenous ascidians in southern California harbors and marinas., Marine Biology 130: 675-688

Van Name, Willard G. (1945) The North and South American ascidians, Bulletin of the American Museum of Natural History 84: 1-462

Verrill, A. E. (1871) Brief descriptions of new and imperfectly known ascidians from New England, American Journal of Science Ser. 3 1: 211-212