Colour and growth form are likely related to environmental parameters and substrate orientation. vexillum has seasonal cycles, undergoing regression during the coldest winter months ( Dijkstra et al., 2007a). It grows on both horizontal and vertical surfaces on a wide variety of substrata including intertidal, subtidal rocky surfaces and man-made substrates ( Bullard et al., 2007a Dijkstra et al., 2007a Valentine et al., 2007b). In its reported range, this species has an extremely rapid growth, quickly covering large areas (100 km 2). They range in colour from pinkish, tan or pale orange and can form undulating encrusting mats or rope-like tendrils on vertical surfaces ( Bullard et al., 2007a).
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Zooids appear randomly embedded in a transparent, spiculated tunic (see Kott (2002) and Lambert (2009) for a full description).Ĭolonies exhibit a wide range of morphological variation and within their tunic have calcium carbonate spicules ( Bullard et al., 2007a). It is made up of individual genetically identical zooids that are not connected by an internal vascular system. Asexual reproduction occurs through budding which produces colonies of genetically identical zooids.
vexillum has two types of reproduction: asexual and sexual. New York (NY) to Eastport (ME), Georges Bank, introduced to the Gulf of Maine in the 1970sįound in coast off Alaska, British Columbia to Ensenada, Baja California.īoth global and local population genetics studies are being carried out by a number of institutes in the USA and New Zealand (A Gittengerber, Gimaris, The Netherlands, personal communication, 2009). The Netherlands, England, Ireland, France Threat to native ecosystems, especially in the province of Zeeland, as Didemnum sp. vexillum to successfully occupy new habitats and become a dominant spatial competitor. They have few known predators ( Lambert, 2009) and undergo fast rates of growth ( Valentine et al., 2007b). In addition, didemnids can disperse through larvae or through fragmentation ( Bullard et al., 2007b). Like all ascidians, they produce lecithotrophic larvae that spend less than 24 hours in the water column before settling on suitable substrate and metamorphosing into adult colonies that allow them to build up local populations. Didemnids possess chemical defences ( Pisut and Pawlik, 2002) and an acidic tunic ( Bullard et al., 2007a), and tolerate a wide range of temperatures ( Bullard et al., 2007a Dijkstra et al., 2007a Valentine et al., 2007b, 2009), salinities ( Dijkstra et al., 2007a Bullard and Whitlatch, 2009) and nutrients ( Carman et al., 2007).
The specific vectors for introduction are largely unknown (see Coutts and Forrest, 2007), though international shipping, local boat traffic and transport of aquaculture species are likely sources ( Carlton, 1989 Dijkstra et al., 2007a).
Since its identification in 1988 ( Kott, 2002), sightings of this species have dramatically increased across the globe. vexillum, a compound ascidian (tunicate or sea squirt), belongs to the family Didemnidae ( Kott, 2002 Lambert, 2009 Stefaniak et al., 2009).