Aquatic Invasions (2009) Volume 4, Issue 1: 71-79 This is an Open Access article; doi: 10.3391/ai. 2009.4.1.7 © 2009 The Author(s). Journal compilation © 2009 REABIC Special issue “Proceedings of the 2nd International Invasive Sea Squirt Conference” (October 2-4, 2007, Prince Edward Island, Canada) Andrea Locke and Mary Carman (Guest Editors) Research article A screening procedure for potential tunicate invaders of Atlantic Canada Andrea Locke Gulf Fisheries Centre, Fisheries and Oceans Canada, P.O. Box 5030, Moncton, New Brunswick, E1C 9B6 Canada E-mail: [email protected] Received 18 March 2008; accepted for special issue 13 May 2008; accepted in revised form 23 December 2008; published online 16 January 2009 Abstract The conditions for a successful invasion involve the intersection of a species, its vector, and an appropriate receiving environment. Species distribution (biogeography) and availability of a shipping vector were used as filters to reduce a list of 57 tunicates with a history of invasion in marine or estuarine waters worldwide, to a more manageable basis for a “watch list” or “trigger list” for non-indigenous invasive tunicates in Atlantic Canada. Seven species from the worldwide invasives list were already present in Atlantic Canada: the non-indigenous Styela clava, Ciona intestinalis (cryptogenic in southern Nova Scotia but non-indigenous in northern Atlantic Canada), Botryllus schlosseri, Botrylloides violaceus and Molgula manhattensis, and the native Aplidium glabrum and Didemnum candidum. Nine species, not currently present in Atlantic Canada, were removed from the analysis due to insufficient distribution data. All of the remaining 41 species co-occurred in one or more bioregions with species presently found in Atlantic Canada. Examination of distributions relative to shipping patterns eliminated eight species not present in the areas with the most shipping traffic to Atlantic Canada: the eastern seaboard of the USA, the Caribbean Sea, northern Europe and the Mediterranean Sea. A climate zone filter to remove species found only in subtropical or tropical waters eliminated 21 species. Applying both the shipping and climate zone filters resulted in a “watch list” of 17 tunicate species considered the most likely to successfully invade Atlantic Canada: Ascidia sydneiensis, Ascidiella aspersa, Botrylloides leachi, Clavelina lepadiformis, Cnemidocarpa irene, Corella eumyota, Cystodytes dellechiajei, Didemnum vexillum, Diplosoma listerianum, Perophora japonica, Perophora multiclathrata, Phallusia mammillata, Polyandrocarpa zorritensis, Polyclinum constellatum, Styela canopus, Styela plicata, and Symplegma brakenhielmi. Key words: tunicate, ascidian, biogeography, prediction, invasion, Atlantic Canada Introduction invasion has been regarded as an enormously complex process, some phases of the process are Prediction of species invasions that are likely to highly predictable; for example, modeling the occur in a particular bioregion or ecosystem is ecological niche characteristics of a species can considered a prerequisite for developing a predict its potential geographic distribution at a “watch list” for early detection (to determine level sufficient for management (Peterson 2003). where and how to monitor species of concern) or One problem with ecological niche modeling, “trigger list” for rapid response (to identify what from the viewpoint of developing a list of likely risk assessments and control methods should be invaders to a region, is the time required to prepared ahead of time) (Ricciardi and obtain sufficient point distribution and environ- Rasmussen 1998; Locke and Hanson 2009). In mental information for a large number of recent years, there have been numerous attempts potential invasive species (Peterson and Vieglais to model and predict species invasions 2001). The effort required to obtain this detailed (e.g.,Williamson and Fitter 1996; Ricciardi and information may be justified in the study of a Rasmussen 1998; Smith et al. 1999; Kolar and few high-risk species, but there is also a need for Lodge 2001; Peterson and Vieglais 2001; a rapid screening tool that can narrow down a list Peterson 2003; Reusser and Lee 2005; Nyberg of potential invaders from an extensive list of and Wallentinus 2005; Miller et al. 2007). These candidates (e.g., Ricciardi and Rasmussen 1998). models have been based either on species The conditions for a successful invasion attributes associated with invasion success, or on involve the intersection of a species, its vector, the niche requirements of species. While and an appropriate receiving environment. In 71 A. Locke terrestrial environments, for example, the proba- Methods bility of successful introduction is positively correlated with the climate/habitat similarity The starting point for this analysis was a list of between donor and recipient regions as well as 57 tunicate species with a history of invasion propagule supply (Hayes et al. 2004). Marine somewhere in the world; distribution records biogeographical classification systems have been were supplemented for the 40 species listed in based on climate, ocean basins, oceanography, the database of Hayes et al. (2004) and records bathymetry and biotic distributions (Kelleher et of another 17 species were obtained from a al. 1995; Lourie and Vincent 2004), and can be survey of the scientific literature and invasive used as a surrogate of climate/habitat similarity. species websites (Table 1). A regional code Here, I use the biogeographic classification corresponding to one of the 208 IUCN adopted by the International Union for the bioregions (Kelleher et al. 1995) was assigned to Conservation of Nature (IUCN) (Kelleher et al. each location where each tunicate occurred. 1995) which has been used to compile ballast Five non-indigenous species are presently water transit data by the Global Ballast Water found in Atlantic Canada (Botrylloides violaceus Programme and Lloyds Maritime Intelligence Oka, 1927, Botryllus schlosseri (Pallas, 1766), Unit, and to match potential invaders with Ciona intestinalis (Linnaeus, 1767), Molgula shipping from their donor regions in previous manhattensis (DeKay, 1843) and Styela clava AIS hazard analyses (Hayes et al. 2004; Reusser Herdman, 1882). Ciona intestinalis is and Lee 2005). Carlton (1996) suggested that an cryptogenic in southern Nova Scotia but non- analysis of shipping traffic from regions indigenous in the Gulf of St. Lawrence. Two containing known invaders could help predict other species present in Atlantic Canada, where those species would next invade. Hull Aplidium glabrum (Verrill, 1871) and Didemnum fouling (including sea chests and other candidum Savigny, 1816 are native, but have a structures) has been proposed as a vector for history of invasion in other regions. These seven many invasive tunicates (e.g., Coutts et al. 2003; species occur in numerous regions worldwide. Minchin and Sides 2006). The assumption on which this analysis is based In this paper I propose using species is that environmental conditions in those regions distributions (biogeography) and vector traffic were similar enough to those of Atlantic Canada patterns to develop a screening tool for potential for components of the biological community to tunicate invaders of Atlantic Canada. The survive in both areas. Thus, the regions outside starting point was to update and expand upon an of Atlantic Canada where these seven species existing list of tunicates with a history of occurred were treated as potential donor regions invasion of marine and estuarine waters (Hayes for new tunicate invasions. The other invasive et al. 2004) and remove species already present tunicates occurring in those regions were treated in Atlantic Canada. This is consistent with the as species with the potential to survive in approach of Ricciardi and Rasmussen (1998), Atlantic Canada. This assumption is graphically who recommended a focus on species with a represented by Venn diagrams illustrating the known invasion history when predicting future range of possibilities from complete or near- invaders. To these known invaders, I then complete overlap of species tolerance (Figure applied a biogeographic filter (based on co- 1(a)) to species for which this assumption occurrence with invasive tunicates presently represents the region of overlap despite largely found in Atlantic Canada) followed by a different habitat requirements (Figure 1(b)). The shipping vector filter to screen potential invaders strength of the predictions developed here will of Atlantic Canada from the worldwide invasive be greater if the situation corresponds to Figure tunicate biota. Conceptually, this process is 1(a), although this assumption has not been comparable to Stages I (transport vector survival tested. and release filter) and II (environment survival The availability of shipping vectors was used and reproduction filter) in an invasion to further refine the list of potential invaders. framework developed by Colautti and MacIsaac Vessel movement tracks were obtained from (2004). Finally, I applied a second environmental Kelly (2004), and patterns in international ship- filter to eliminate species currently found only in ping traffic to Atlantic Canada were examined in tropical and subtropical waters. order to identify likely sources of primary 72 A screening procedure for potential tunicate invaders Table 1. List of invasive tunicate species, screened by vector and climate filters. In bioregion(s) with heavy Found in temperate or Species