||Ascidia nigra Savigny, 1816
||Black Solitary Tunicate
||Phallusia nigra Savigny, 1816
The solitary black tunicate,
Ascidia nigra, is a conspicuous species that is readily
distinguished from other fouling organisms such as sponges and other
tunicates (eg. da Rocha et al. 1999, Goodbody
1962, Kaplan 1988, Voss 1980). The body, or test, is bluish-black
to brownish-black with a leathery texture (Voss 1980). The sloughing
of the outer layer and toxicity of the tissues aid in keeping the
tunicate clean and unfouled by other organisms (Goodbody 1962).
The test forms a long tube or sac shape, called a tunic, with two
siphons at the top of the body. The openings of both siphons are
round with fringed edges (Kaplan 1988). Water and food is drawn
into the body via the tall incurrent (buccal) siphon, while the
shorter excurrent (atrial) siphon excretes water and waste.
Potentially Misidentified Species
The black tunicate is conspicuous,
and no other local fouling organisms have characteristics that could
potentially confuse identification.
HABITAT AND DISTRIBUTION
The range of A. nigra
extends from Florida to Brazil and Bermuda in the western Atlantic
Ocean (Goodbody 1962), the Red Sea, Gulf of Aden and the Gulf of
Guinea (Millar 1958, Van Name 1945). Populations are usually confined
to sheltered bays and lagoons to a depth of about 25 feet (Goodbody
1962). Like many other fouling organisms, the black tunicate is
found on hard submerged surfaces such as rocks, seawalls, buoys,
ship hulls, dock pilings and mangrove prop roots (Goodbody 1962,
Voss 1980). Most individuals are found on vertical or inclined surfaces
over horizontal areas (da Rocha et al. 1999).
The black tunicate can
be found throughout the IRL on a variety of surfaces, including:
seawalls and jetty rocks; bridge and dock pilings (Bingham &
Walters 1989); buoys and floating docks; and the submerged prop
roots of the red mangrove, Rhizophora
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
The tunics of A. nigra can grow to a height
of about 15 cm (Kaplan 1988), but most specimens do not exceed 5cm
(da Rocha et al. 1999, Voss 1980). Most ascidians have
a lifespan of 1-3 years (Ruppert & Barnes 1994), varying among
species and with environmental conditions. The lifespan of A.
nigra populations in Jamaica has been recorded at 19-22 months
(Goodbody 1962), and black tunicates in Brazil typically live 1-2
years (da Rocha et al. 1999). The greatest mortality occurs
during the first three weeks of life, and is affected by density
and growth of nearby organisms (Goodbody & Gibson 1974).
Goodbody (1962) considered the black tunicate
a primary colonizer, settling in large numbers on new or clean surfaces
before ultimately being replaced by other organisms. No abundance
estimates are available for populations in the IRL, but tunicates
in estuaries of Brazil have been recorded at densities of up to
12 individuals per square meter (da Rocha et al. 1999).
Most tunicates are
hermaphrodites, but self-fertilization is rare (Ruppert & Barnes
1994). Instead, individuals reproduce via cross-fertilization. Some
solitary tunicates brood their young until hatching, but others
release eggs and sperm through the excurrent atrial siphon. Fertilization
occurs in the water column, and a single larva hatches from each
egg. Reproduction occurs throughout the year in A. nigra (Goodbody
1962, Goodbody & Gibson 1974), but peaks in some locations according
to season. In Brazil, the largest abundance of tunicates can be
found in April to June, suggesting larval release in September through
March (da Rocha et al. 1999). Studies have found that breeding
begins in individuals about 85 days old, and spawning occurs at
60 day intervals thereafter (Goodbody 1962).
Ascidians produce tadpole
larvae with a visible notochord. Hence, they are included in the
phylum Chordata, along with mammals, birds and fishes. Larvae are
lecithitrophic, obtaining nutrients from yolk reserves as opposed
to feeding on other organisms (Ruppert & Barnes 1994). Because
of this life-history pattern, larvae must find a suitable habitat
to settle before food reserves are exhausted. The planktonic period
of most ascidian larvae is less than 36 hours (Ruppert & Barnes
1994). Locomotion throughout the water column facilitates this search.
Larvae swim in a similar fashion to fishes, bending at the junction
between the trunk and the tail to undulate through the water (McHenry
2005). After finding a suitable habitat, larvae attach themselves
to the substrate via a series of adhesive-producing structures at
the front of the trunk called papillae. When attachment is complete,
larvae metamorphose into juveniles. Like many other fouling organisms,
larvae of A. nigra appear to be cued to settle near other
black tunicates (Grave 1935). The black pigment is not developed
in juveniles until about 20 days after settlement, and most recruits
are visible to the naked eye within four weeks (Goodbody 1962).
Little information exists
on temperature tolerances of the black tunicate, but populations
are usually found in warm tropical and sub-tropical waters. Colonization
and reproduction appear to be somewhat seasonal in certain locations,
and are most likely linked to water temperature. Densities of A.
nigra in Brazil were recorded at 3-4 individuals m-2
in September to March, rising to 10-12 m-2 in April to
June. These populations experienced a seasonal temperature range
of 19 to 29°C (da Rocha et al. 1999).
Few reports exist on the
salinity tolerances of A. nigra. However, the range and
habitat of this species suggests is prefers brackish and marine
The black tunicate is a
sessile, benthic filter-feeder. The incurrent siphon takes water
into a sieve-like pharyngeal basket that filters out food of the
appropriate size class before water is pumped from the animal via
the excurrent siphon. Filtration rates for ascidians can be extremely
high, allowing them to obtain large quantities of plankton from
the water column. A single, average-sized A. nigra can
pass 173 liters of water through its body in 24 hours (Ruppert &
Barnes 1994). Gut content analyses on black tunicates form the IRL
have revealed several prey items, including: barnacle nauplii and
cyprid larvae; copepod nauplii; eggs; bivalve and gastropod veligers;
and setigers (Bingham & Walters 1989).
Properties of the tunic
and interior fluids make A. nigra unpalatable to many predators
and reduce fouling on the outside surface. Vanadium has been found
in high concentrations on the surface of the tunic (Stoecker 1978,
1979, 1980), and sulfuric acid concentrations in the tissues and
interior fluids reduce the pH to less than 3.0 in some cases (Hirose
et al. 2001, Pisut & Pawlik 2002). Even with these
defenses, some organisms prey on A. nigra, including the
bluehead wrasse, Thalassoma bifasciatum (Pisut & Pawlik
As a component of coastal
and estuarine fouling communities, A. nigra is found alongside
several species of invertebrates, namely sponges and other ascidians.
In addition, a few species are commonly found living on or inside
the tunic, including: the amphipods, Erichthonius brasiliensis
and Leucothoe spinicarpa; the copepod, Janstockia phallusiella;
the angular brittlestar, Ophiothrix
angulata; and the crab, Tunicotheres moseri (Boxshall
& Marchenkov 2005, Goodbody 1962, Hernandez et al.
2008, Thiel 1999). For extensive lists of other species found throughout
the ecosystems in which A. nigra occurs, please refer to
the "Habitats of the IRL" link at the left of this page.
No information is available at this time
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analyses and plankton samples. J. Exp. Mar. Biol. Ecol.
Boxshall, GA & A Marchenkov. 2005. A new
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from a compound ascidian host collected in the Suez Canal. Zoosyst.
da Rocha, RM, da Cruz Lotufo, TM & S de
Almeida Rodrigues. 1999. The biology of Phallusia nigra
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acids? J. Exp. Mar. Biol. Ecol. 270: 203-214.
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defense and ecology in benthic ascidians. Mar. Ecol. Prog. Ser.
Theil, M. 1999. Host-use and population demographics
of the ascidian-dwelling amphipod Leucothoe spinicarpa:
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Report by: LH Sweat,
Smithsonian Marine Station at Fort Pierce
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Page last updated: 20 August 2009