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The impact of invasive species on the recipient ecosystem can be strongly influenced by the presence of associated symbionts. It is therefore important to evaluate the likelihood of co-introduced symbiont establishment, and this requires an understanding of their life history traits. Here, we investigate survival, reproduction and competition in two non-native branchiobdellidan ectosymbionts (Xironogiton victoriensis and Cambarincola aff. okadai) on invasive signal crayfish (Pacifastacus leniusculus). In vivo, X. victoriensis established viable infrapopulations within 10 weeks, whereas C. aff. okadai went extinct within 2 weeks. Both X. victoriensis and C. aff. okadai deposited cocoons in vivo that hatched in 10–27 and 10–11 days, respectively. In vitro, X. victoriensis and C. aff. okadai survived for over 13 and 15 weeks respectively, although both were negatively affected by increased temperature and nitrate, and were absent from kick samples taken in the field. Only C. aff. okadai deposited cocoons in vitro, and this larger species readily predated on X. victoriensis but not vice versa. Both branchiobdellidans possess traits associated with colonisation success, including a relatively fast reproductive rate and extended off-host survival. Given its survival in vivo and known detrimental effect on signal crayfish X. victoriensis is perhaps more likely to influence host invasion dynamics, although its persistence may be affected by the presence of co-occurring symbionts.

Freshwater ecosystems are particularly vulnerable to the effects of alien species and decapod crustaceans, notably crayfish, are a principal threat. Although symbiotic fauna may influence the impact and dispersal of introduced species, this is often overlooked. Here we provide the first record of non-native ecto-symbiotic branchiobdellidan worms on invasive signal crayfish (Pacifastacus leniusculus Dana, 1852) in the British Isles. Using morphological and molecular techniques we identified and re-described two branchiobdellidan species new to the UK, Xironogiton victoriensis Gelder and Hall, 1990 and Cambarincola aff. okadai Yamaguchi, 1933, both of which were found at a single location in the Gavenny River, South Wales. The prevalence of X. victoriensis and C. aff. okadai was 75.34% and 71.23% respectively. Although the level of X. victoriensis and C. aff. okadai co-infection was high at 75.41% of all infected animals, the two species exhibited different micro-habitat preferences on the host with the former being found predominantly on the chelae and walking legs and the latter on the carapace and abdomen. For both branchiobdellidan species, worm burdens were positively correlated with crayfish size. The lack of branchiobdellidan records from signal crayfish in nearby water bodies, and the reports of native white clawed crayfish (Austropotamobius pallipes) in the Gavenny as recently as 2000, indicates that introduction of this worm infested population occurred relatively recently, despite stringent legislation banning the import and transportation of non-native crayfish into the UK. [ABSTRACT FROM AUTHOR]