Taxonomic note Species identification and synonymization has been very
confusing for this species. For many years Molgula manhattensis has
been considered the senior synonym of M. tubifera (Ørsted, 1844),
M. socialis Alder, 1848, M. siphonata Alder, 1850, and M.
simplex Alder & Hancock, 1870 (see discussion by Berrill, 1931).
However, more recent studies indicate that some or all of these species may
be valid separate species. It is not possible without actually examining
specimens or studying descriptions in detail to determine which of the
synonymized taxa occur in which localities, and there is a possibility that
the true M. manhattensis does not occur in European waters (see e.g.
Haywood & Ryland, 1995). Arenas et al. (2006) found M. socialis, but
not M. manhattensis in southern England. World Register of Marine
http://www.marinespecies.org) lists 7 synonyms for M. manhattensis,
and only M. tubifera is identical to the species listed above. M.
macrosiphonica Kupffer, 1872, occurring in the western Baltic, has also
been included in the synonymy of M. manhattensis (Lützen, 1967), but
WoRMS lists it as a synonym of M. provisionalis Van Name, 1945, which
only occurs in the western North Atlantic. Molecular studies are currently
undertaken in the Netherlands to determine the true identity of M.
manhattensis, but until the results are known, it must be assumed that
the species is the one found in Nordic waters.
Almost spherical, 20-30 mm
in diameter (up to 40 mm), externally covered by densely set, unbranched
“hairs” and often with sand-grains, algal filaments, or shell fragments
attached. The siphons are close together and relatively long, but usually
contract when the animals are out of water or preserved. The atrial siphon
has a squarish opening and is longer than the branchial (oral) siphon, which
has 6 marginal indentations. The body color is greenish grey and, if it is
not too covered in debris, the pale pink reproductive organs may be visible
through the tunic. It is often attached to man-made structures or mussels in
There are several other
species of Molgula found in Nordic waters, e.g. Molgula citrina
Alder & Hancock, 1848, M. occulta Kupffer, 1875, M. oculata
Forbes, 1848 and M. complanata Alder & Hancock, 1870.
East coast of North America from Maine to Texas (Lützen, 1967). Some authors
consider M. manhattensis to be cryptogenic (Jewett et al., 2005). Until
synonymy has been clarified this remains unresolved.
In Danish fjords and brackish waters a species identified as Molgula
tubifera (Ørsted, 1844) has been recorded since its original description
from Øresund (the Sound). Later this was synonymized with M. manhattensis
(Lützen, 1967), which has been recorded from several fjords (Rasmussen,
1973; Randløv & Riisgård, 1979). It was first recorded from Ireland in 1998
(Minchin, 2007). It is not considered introduced in Germany or Belgium (Gollasch
& Nehring, 2006; Kerckhof et al., 2007). Hayward & Ryland (1995) consider
M. tubifera and M. manhattensis different species, and hence
claim that M. manhattensis does not occur in British waters. In the
Netherlands M. manhattensis/ tubifera has been known since 1762
(Wolff, 2005) when it was illustrated, but not named, from sluice-gates. In
Norway it has been found only a few places on the central west coast
(Hopkins, 2001). In Sweden it is found on the west coast (Petersen & Svane,
2002), but is also included in the “alert list” (http://www.frammandearter.se/)
for species not yet found in Swedish waters. No ascidians occur in the
eastern and northern Baltic Sea.
has also been introduced to the west coast of the USA where it was first
found in 1984 in southern California (Lambert & Lambert, 1998).
It has also been introduced
to Australia, where it was first recorded in 1967 (Hewitt et al., 2004), and
it has been recorded from the southern coasts of China (Huang et al., 1993)
and Japan, where the first record was from 1972 (Iwasaki, 2006).
is often associated with low salinity, e.g. in estuaries (Lambert & Lambert,
1998). This is in accordance with its occurrence in Danish waters (Lützen,
1967; Rasmussen, 1973). It may also be relatively tolerant to hypoxia,
though it may delay reproduction under these circumstances (Sagasti et al.,
2003). In its native area it lives in a sort of symbiosis with a fungus-like
microbial organism, Nephromyces sp. (Saffo, 1982; Saffo & Davis,
1982). Whether this also occur in European strains is unknown. The function
of this symbiont is not fully understood but is involved in metabolism of
nitrogen waste (Saffo, 1988).
Like other ascidians it is a
suspension feeder and it efficiently retains particles down to 1-2 µm (Randløv
& Riisgård, 1979). Filtration rate seems to be lower than for other solitary
species, though the branchial basket has a more complex structure (Petersen
& Svane, 2002).
It may be host for the
commensal copepod Lichomolgus albens Thorell, 1859 (Rasmussen, 1973).
Reproduction Ascidians are generally hermaphroditic, and this is also
the case for this species. Molgula manhattensis has been shown to be
able to self-fertilize, and although success of embryonic development was
lower in a few specimens, usually normal embryos developed. Eggs are usually
shed during the night and fertilized eggs develop into a tadpole larva in
about 24 h (Grave, 1933). The free swimming tadpole stage last only a few
days (Saffo & Davis, 1982). Sexual maturity is reached in 3 weeks, though
fertility increases after one month (Grave, 1933). A free swimming larval
stage may be by-passed and metamorphosis completed in situ (Morgan,
1942), but this may also indicate that there are cryptic species. In Danish
waters M. manhattensis has small eggs and free swimming larvae.
Reproduction begins when water temperature reaches about 10° C (Lützen,
1967). Reproductive plasticity, high fecundity and short generation time may
explain the success of this species as a colonizer.
is part of the fouling community and has the same impacts as other fouling
organisms. It has a high biodeposition rate relative to several other
shallow water suspension feeders (Haven & Morales-Alamo, 1966). The state of
Washington on the west coast of the USA has implemented a program to manage
invasive tunicates in ports and marinas. M. manhattensis has been
given low priority because its impacts are considered relatively minor (LeClair
et al., 2009), but it is also listed as a priority species on the “watch