Management of Biological Invasions (2012) Volume 3, Issue 1: 15–23 doi: http://dx.doi.org/10.3391/mbi.2012.3.1.02 Open Access © 2012 The Author(s). Journal compilation © 2012 REABIC Research Article Effect of colonial tunicate presence on Ciona intestinalis recruitment within a mussel farming environment S. Christine Paetzold1, Donna J. Giberson2, Jonathan Hill1, John D.P. Davidson1 and Jeff Davidson1 1 Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island C1A 4P3, Canada 2 Department of Biology, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island C1A 4P3, Canada E-mail: [email protected] (SCP), [email protected] (DJG), [email protected] (JH), [email protected] (JDPD), [email protected] (JD) *Corresponding author Received: 4 November 2011 / Accepted: 7 August 2012 / Published online: 15 December 2012 Handling editor: Elias Dana, University of Almeria, Spain Abstract Aquatic invasive species decrease yields and increase costs in aquaculture operations worldwide. Anecdotal evidence from Prince Edward Island (PEI, Canada) estuaries suggested that recruitment of the non-indigenous solitary tunicate Ciona intestinalis may be lower on aquaculture gear where colonial tunicates (Botryllus schlosseri and Botrylloides violaceus) are already present. We tested this interspecific competition hypothesis by comparing C. intestinalis recruitment on un-fouled settlement plates to those pre-settled with Botryllus schlosseri or Botrylloides violaceus. C. intestinalis occurred on all plates after 2 month, but it was much more abundant (~80% coverage) on unfouled plates than on pre-settled plates (<10% coverage). However, C. intestinalis showed higher individual growth on pre-settled plates than on unfouled plates. High reproductive potential for C. intestinalis appears to result in rapid recruitment to control plates, but this may be impeded on pre-settled plates due to competition for space, negative settlement cues produced by the colonial tunicates, allelopathy or overgrowth. Key words: tunicates; ascidian; aquaculture; Botryllus schlosseri; Botrylloides violaceus species: Ciona intestinalis (L., 1758), Styela Introduction clava (Herdman, 1881), Botryllus schlosseri (Pallas, 1766) and Botrylloides violaceus Oka, Introductions of aquatic invasive species are 1927 (Locke et al. 2007). Tunicates (class becoming more and more common, especially Ascidiacea) are sessile filter feeders that settle those mediated by human actions (Carlton and on hard substrates (e.g. wharf pilings, buoys, Geller 1993; Ruiz et al. 1997; Cohen and Carlton aquaculture gear, hard-bodied biota) after a brief 1998; Ruiz et al. 2000). Often, invasive species period of a few hours to several days as free- preferentially settle on artificial substrates swimming larvae (Berrill 1950; Millar 1952; (Tyrrell and Byers 2007) such as ropes, nets, Dybern 1965; Yamaguchi 1975; Saito et al. cages and other gear used in aquaculture 1981; Jackson 2005). The blue mussel (Mytilus operations worldwide (Lambert 2007). The edulis L., 1758) aquaculture industry in Atlantic impacts, both on the native populations also Canada has been adversely affected by these living on these artificial substrates (K. Ellis, non-indigenous tunicates, especially by the pers. comm.) and the aquaculture economy, can solitary species C. intestinalis (Carver et al. be detrimental (e.g. Carver et al. 2003; Lutz- 2003, 2006; Daigle and Herbinger 2009). For Collins et al. 2009). In recent years, Prince example, since C. intestinalis consists of 95% Edward Island (PEI; Canada) estuaries have water (calculated from data in Carver et al. experienced the introduction of 4 tunicate 2006), it adds considerable weight to mussel 15 S.C. Paetzold et al. socks (>3 kg m–1; Ramsay et al. 2008b), leading surface compared to the finite shape of a solitary to increased labour and crop losses. Especially in tunicate, though solitary species can also often recent years, research efforts have focused on be found in large groups of individuals. Studies preventing or mitigating C. intestinalis fouling on competitive superiority of one species over on mussel socks (e.g. Bakker et al. 2011; another have had differing outcomes. Botryllus Paetzold and Davidson 2011; Parent et al. 2011). schlosseri was a dominant competitor whose While mussel growers on PEI consider colonial presence deterred some species from settling on tunicates a nuisance species, research has not the same substrate in a fouling community in one shown any adverse effects of Botryllus schlosseri study (Grosberg 1981). However, established and Botrylloides violaceus on cultured mussels Botryllus schlosseri colonies had no effect on (Arens et al. 2011; N. McNair, PEI Department larval settlement by other species in a different of Fisheries, Aquaculture and Rural study (Bullard et al. 2004), leading those Development, pers. comm.), possibly because researchers to hypothesize that invertebrate the colonial tunicate fouling levels on assessed larvae do not avoid settling near superior mussel socks were low (maximum of ~0.36 kg competitors (Bullard et al. 2004; see also Osman m–1; Arens et al. 2011). The values reported here and Whitlatch 2004). Similarly, a previous study for both Ramsay et al. (2008b) and Arens et al. led some of the same researchers to negate (2011) are based on assessments of spring- species dominance (i.e. superior ability to settle) socked mussel socks in October of the socking as a factor in settlement and instead explain the year; thus on similar socks, colonial tunicates success of a settler based on species richness on add 10-times less weight to the sock than a substrate (Stachowicz et al. 1999). In C. intestinalis. particular, Stachowicz et al. (1999) found that In Cardigan River in eastern PEI, Mytilus more diverse communities prevented settlement edulis is grown throughout the estuary, and C. by invaders, likely by occupying all available intestinalis has become a serious fouler on substrates and quickly filling new space, e.g. mussel leases. Since being discovered in this through expansion of existing colonies, such that estuary in 2005, this tunicate has spread rapidly new invaders had little chance of recruitment. throughout the ecosystem and quickly Determining the competitive interactions established itself as the dominant tunicate among the invasive tunicate species on PEI is species. Ramsay et al. (2008a) also reported this important to better understand recruitment phenomenon in the adjacent Brudenell River. hierarchies of these fouling organisms on However, anecdotal reports from mussel growers aquaculture gear. Mitigation strategies could be and SCUBA divers working on the leases more effective if designed to target a dominant suggest that densities of C. intestinalis were tunicate species or allowing a lesser nuisance lower on socks heavily fouled by the colonial species such as a colonial tunicate to outcompete ascidians Botryllus schlosseri and Botrylloides a more detrimental species like C. intestinalis. violaceus. Therefore, the objective of this study was to Colonial tunicates have 2 main advantages assess the effect of the presence of Botryllus that could lead to their dominance over solitary schlosseri or Botrylloides violaceus on the species on hard substrates. First, colonial larvae recruitment and growth of the solitary tunicate are larger than those of solitary species, giving C. intestinalis. This assessment involved an in them survival and growth advantages (Marshall situ study comparing C. intestinalis % cover, and Keough 2005; Marshall et al. 2006). Among length and abundance on bare plates (controls) the study species, the larvae of Botrylloides and plates that were pre-settled with Botryllus violaceus are approximately 3 times larger than schlosseri or Botrylloides violaceus. those of C. intestinalis and Botryllus schlosseri (Bullard and Whitlatch 2004), and colonial juveniles grow up to 4 to 6 times larger than C. Methods intestinalis within the first week post-settlement Development of colonial tunicate colonies (Bullard and Whitlatch 2004). Second, colonial on experimental surfaces tunicates can reproduce asexually by budding and fragmenting/ regenerating and are therefore The first phase of the project (pre-settlement; 29 not dependent on the time limitations of gamete June to 20 September 2007) was conducted in 2 production. With asexual reproduction comes the bays located on the north shore of PEI (Canada): possibility of continued mat-like growth over a St. Peters Bay (46.432°N, 62.676°W) and Savage 16 Effect of colonial tunicate presence on Ciona intestinalis recruitment Figure 1. The Savage Harbour and St. Peters Bay collection sites and the study site in Cardigan River in Prince Edward Island (PEI), Canada. QC: Quebec, NB: New Brunswick, NS: Nova Scotia, NL: Newfoundland. Harbour (46.416°N, 62.833°W) (Figure 1). These SCUBA divers 2 months after deployment and bays were selected as sites for pre-settling transported to the Cardigan River study site in experimental substrates because both bays ambient seawater, taking care to prevent contained no solitary tunicates but high levels of interspecific mixing and damage to the tunicate the colonial tunicates, Botryllus schlosseri and colonies. Water temperatures at both sites ranged Botrylloides violaceus, respectively (J. Hill and from 14 to 22°C (18 ± 2.5°C, mean ± SD) during J.D.P. Davidson pers. obs.).