There is a great deal more to be learned about invasive ascidians. In terms of basic biology, we need to gain a better understanding of factors that lead to the spread of invasive ascidians and the ecological impacts they have on invaded communities. In particular, more research should be focused on invasive ascidian dispersal ranges and rates, especially in the case of dispersal of colony fragments. Basic questions that remain unanswered include: What is the rate of fragment production? How long can fragments survive under different environmental conditions (e.g., in ballast water, free-floating in the water column, in benthic habitats, etc)? What proportion of fragments successfully reattach? How do fragment dispersal rates and distances compare to larval dispersal (Worcester 1994)? Additionally, detailed information about the types of ecological conditions that favor the growth of invasive ascidians would facilitate the development of predictive models of ascidian spread. For example, are there correlations between successful ascidian invasions and levels of nutrient enrichment, pollution, shoreline development or coastal disturbance? Do invasive ascidians have adaptive abilities that native species lack? More work is also needed to assess the biological and ecological impacts that invasive ascidians have on marine communities and aquacultured organisms. How much of an impact do invasive ascidians have on biodiversity? Can invasive ascidians change large scale ecosystem functions? Are fouling-related decreases in shellfish growth rates different depending on the type of aquaculture gear used (e.g., cages versus lines)? What are the mortalities rates for shellfish during heavy ascidian outbreaks? What densities of ascidians become prohibitively detrimental to the aquaculture industry?
Additional management efforts and strategies are also needed. It would be of great value to develop comprehensive datasets of regional ascidian diversity and abundance. Baseline datasets allow researchers to quickly determine whether newly observed ascidians are recently arrived invasive species or blooming populations of cryptogenic native species. This question is not merely academic as in some cases government agencies handle the control native species differently than invasive species. Active management responses may be delayed until the "residency status" of a newly observed ascidian can be determined, but by the time control efforts begin the species may be firmly established. This is exactly what happened when Didemnum vexillum first appeared in Shakespeare Bay, New Zealand (Coutts & Forrest 2007).
Dedicated monitoring programs in at-risk areas would also be helpful and would facilitate the discovery of newly arrived invasive ascidians (e.g., Cohen et al. 2005). Currently most ascidian invasions are detected either when an expert fortuitously discovers an unfamiliar species or, more commonly, when the invasive population has gotten so large that its presence is noticed by the general public. For example, Didemnum vexillum was first observed in Long Island Sound, USA by researchers at Groton, CT in 2000 (Bullard et al. 2007a). By the time it was noticed, the species was already well established at numerous sites along the Connecticut coastline. The following year extraordinarily large populations (100s m in extent) were discovered in deep areas of Long Island Sound (Figure 4). This was the first time these sites had been surveyed for ascidians, so the species had likely been there for some time, possibly years. Had a dedicated coastal monitoring system been in place the species may have been detected much earlier.
Perhaps the most important area of work is developing and refining, ecologically friendly methods to control invasive ascidians. At present, most control efforts remain localized in scope (e.g., scraping ascidians off individual boat hulls, power washing infected mussel lines, etc), though larger comprehensive control efforts have been attempted (e.g. Coutts & Forrest 2007). The "holy grail" of invasive ascidian management would be the development an effective, inexpensive, easy to use, non-toxic control method that targets ascidians but does not harm other organisms.
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