Assessing the invasion potential of non-native branchiobdellidans: experimental studies of survival, reproduction and competition

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 knowndetrimental 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.