Species Description: 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.
Regional Occurrence: 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).
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).
Abundance: 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).
Reproduction: 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).
Embryology: 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).
Temperature: 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).
Salinity: Few reports exist on the salinity tolerances of A. nigra. However, the range and habitat of this species suggests is prefers brackish and marine waters.
Trophic Mode: 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).
Predators: 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 2002).
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