Signal crayfish as a threat for European ectosymbionts: overlooked biodiversity losses

The decline of native crayfish biodiversity caused by introductions of invasive crayfish has been well documented; however, the fate of their associated biota remains overlooked. Native European branchiobdellids (Annelida: Clitellata)-crayfish ectosymbionts-were monitored in Czech Republic in three localities with past or present sympatric occurrence of the native noble crayfish Astacus astacus (AA) and invasive signal crayfish Pacifastacus leniusculus (PL). No branchiobdellids were detected in two localities with abundant populations of PL but with absences of AA, although they were recently recorded there. Two species of branchiobdellids (Branchiobdella parasita and B. pentadonta) were detected on both crayfish species in the locality where AA still coexists with PL. However, PL harboured a significantly lower abundance of both branchiobdellids than AA (80.7% and 90.8% decrease, respectively) and a significantly lower number of their attached cocoons (95.7% and 95.0% decrease, respectively). Nevertheless, the abundance of branchiobdellids was positively correlated with the carapace length in both crayfish species. Other potential predictors related to crayfish biological parameters (e.g. sex or missing appendages) were not significantly related to the abundances of both branchiobdellids and their cocoons. We also performed laboratory experiments with both branchiobdellids distributed on size-matched individuals of AA and PL to verify the results of field observations. The probability for branchiobdellid survival was significantly higher on AA in both branchiodellids for all the time of the experiments. The absence or decreased abundances of the native branchiobdellids in the invasive crayfish suggest their limited capability to deal with the replacement of host species.

Automated summary

The study found that the decline of native crayfish biodiversity caused by invasive crayfish introductions has been well documented, but the fate of their associated biota has been overlooked. The presence of native European branchiobdellids (annelid worms that are ectosymbionts of crayfish) was monitored in three localities in Czech Republic. The results showed that branchiobdellids were not detected in areas with invasive signal crayfish but without the native noble crayfish, and the number of branchiobdellids on signal crayfish was significantly lower compared to the native noble crayfish.