Experimental spillover of an exotic ectosymbiont on an European native crayfish: the importance of having a chance.

Symbiont spillover involves the emergence of new interactions that can influence both the symbiont and the host invasion processes. Here we aimed at experimentally testing the transmission of an exotic ectosymbiont, Ankylocythere sinuosa (Ostracoda, Entocytheridae), from a crayfish invader (Procambarus clarkii) to a native European crayfish (Austropotamobius italicus). Crayfish transmitter and receiver were placed in the same container during 4 days. Three experimental treatments were designed: (i) crayfish transmitter alone (i.e. control group); (ii) cleaned P. clarkii as crayfish receiver; (iii) A. italicus as crayfish receiver. The proportions of colonised crayfish were 92% for P. clarkii and 100% for A. italicus. Mean symbiont emission rate of the control group was 2.75%, with 2.5 times odds increase under the presence of crayfish receiver. Mean transmission rate in treatments with P. clarkii receivers was 38.48%, with higher odds to bigger crayfish receivers (5% increase per gram) and to A. italicus (2.1 times). All entocytherid growth stages were transmitted. We demonstrated that entocytherids harbour transmission features that allow them to achieve successful spillover in nature. However, the symbiont was not found on native crayfish in nature hitherto, probably due to host competitive exclusion and crayfish plague acting on native hosts.