Ciona Intestinalis Mussel

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Ascidians can grow in a variety of forms ranging from purely solitary individuals to true colonies (Ruppert & Barnes 1996). All types of ascidians may be invasive, but the characteristics of their invasions differ. Invasive solitary species grow in dense monospecific aggregations (e.g., Bourque et al. 2003, Blum et al. 2007, LeBlanc et al. 2007, Figures 1, 2 & 3). Because each individual holds a significant amount of water inside its body, stands of solitary individuals can possess tremendous mass. For example, fouling clusters of Ciona intestinalis can weigh ~2 kg m-1 of line on mussel cultures and can add ~20 kg of mass to a single buoy (Daigle & Herbinger 2008, MacDonald personal communication). In contrast, invasive colonial species form blanket-like mats (e.g., Bullard et al. 2007a, Figures 4, 5 & 6). These mats are relatively thin, but can form impenetrable sheets that prevent exchanges of water, food and larvae between the benthos and water column and can prevent predators from foraging in benthic habitats (e.g., Lengyel et al. 2008).

Ciona Intestinalis Mussel

Figure 1. Dense aggregation of the solitary ascidian Ciona intestinalis covering a mussel aquaculture cage in Prince Edward Island, Canada. These types of cages are traditionally pulled from the water by hand, but when heavily fouled with ascidians, as in this photograph, a crane is required to lift them. Photograph by Garth Arsenault.

Figure 1. Dense aggregation of the solitary ascidian Ciona intestinalis covering a mussel aquaculture cage in Prince Edward Island, Canada. These types of cages are traditionally pulled from the water by hand, but when heavily fouled with ascidians, as in this photograph, a crane is required to lift them. Photograph by Garth Arsenault.

For a comprehensive review of ascidian ecology and natural history see Lambert (2005a). However, certain aspects of ascidian biology need to be addressed here as they play an important role in invasions success. These traits include ascidian reproductive capabilities, competitiveness and defense mechanisms.

Ascidians reproduce sexually and produce short-lived tadpole larvae (Svane & Young 1989). Reproduction differs depending on growth form. Solitary ascidians have external fertilization and their eggs develop into larvae in the plankton. Colonial ascidians have internal fertilization and release well developed brooded larvae. Once hatched or released, tadpole larvae swim for minutes to days (e.g., Olson and McPherson 1987, Stoner 1990, Bingham & Young 1991, Marshall & Keough 2003). Given this short larval period, ascidians possess limited dispersal capabilities and generally have very localized recruitment (Grosberg 1987, Davis & Butler 1989, Havenhand 1991). Colonial ascidian colonies increase in size through the asexual production of zooids and some colonial species can also disperse asexually through fragmentation (Stoner 1989, Lambert 2005b, Bullard et al. 2007b). Fragmentation may be a particularly important ecological characteristic for invasive species because disturbance events (e.g., storms, boats impacts on the benthos, dredging, etc) can create viable fragments that are transported by currents and can reattachment to the bottom.

Hunter Logo Styela
Figure 2. Heavy fouling of Styela clava on a mooring line, Great Harbor, Woods Hole, Massachusetts. Other ascidians present include Botrylloides violaceus, Didemnum vexillum and Ciona intestinalis. Photograph by Anne Goodwin.
Mussel Cultures Fouling Styela Clava
Figure 3. Close up of Styela clava fouling; this photograph is of the same mooring line as Figure 2.
Figure 4. Dense mats of the colonial ascidian Didemnum vexillum on the bottom of Long Island Sound, Connecticut at a depth of 113 feet. Photograph by Dave Cohen.
Ascidian Didemnum Vexillum
Figure 5. Colonies of Didemnum vexillum encrusting pilings and rocks in Narragansett Bay, Rhode Island. The visible colonies are growing into the intertidal zone and are exposed during low tide. Photograph by Chris Deacutis.

Ascidians are strong spatial competitors that are frequently at the top of competitive hierarchies (Grosberg 1981, Nandakumar et al. 1993, Rajbanshi & Pederson 2007). They aggressively compete for space in several ways. First, ascidian life history characteristics facilitate rapid reproduction that allows larvae to quickly colonize open surfaces (e.g., Bourgue et al. 2007, Arsenault & Davidson 2008). Second, ascidians grow quickly to monopolize space and overgrow sessile species (Grosberg 1981, Russ 1982, Bullard et al. 2007a). Finally, they frequently possess noxious chemicals that they use to attack other species and prevent the attachment of epibionts and spatial competitors (Jackson & Buss 1975, Davis 1991, Teo & Ryland 1995, Morris et al. 2008). These advantages give invasive ascidians a significant edge when they reach new habitats and allow them to invade even highly diverse communities.

Ascidians frequently possess potent anti-predator defenses. Chemical defenses are common in ascidians and include the production of secondary metabolites and inorganic acids and the concentration of heavy metals (Stoecker 1980, Lindquist et al. 1992, Pisut & Pawlik

2002, Tarjuelo et al. 2002, Jimenez et al. 2003). In addition, many species possess thick, protective tunics (Ruppert & Barnes 1996). Given these defenses, invasive ascidians are often relatively immune to predation.

Figure 6. Botrylloides violaceus colonies overgrowing rocks and the algae Ascophyllum nodosum, Sandwich, Massachusetts. Photograph by Dann Blackwood.

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  • teodros
    How rapid does ascidian grow?
    8 years ago
  • willow
    Are there ciona in Long island sound?
    8 years ago

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