The branchiobdellidans are small aquatic worms that live symbiotically on the surface of crayfish bodies. They belong to the phylum Annelida, which includes earthworms and leeches. Other branchiobdellidans that parasitize crayfish gills are rather rare in Europe. The majority of species in the Czech Republic often constitute even mutually beneficial relationships with crayfish, as they scrape overgrown microscopic organisms from their exoskeletons.
During our observations of the invasive North American signal crayfish in the Czech Republic, we noticed that these crayfish were rarely infested by native European branchiobdellidans. To investigate this further, we conducted a field experiment and discovered that signal crayfish hosted 81 to 91% fewer branchiobdellidans compared to native noble crayfish captured at the same locality. The reduction in the number of egg cocoons was even more pronounced. In other locations where noble crayfish had been completely exterminated, we found no occurrence of branchiobdellidans on signal crayfish individuals.
Our laboratory experiment yielded similar results, showing that branchiobdellidans had significantly lower survival rates on signal crayfish compared to noble crayfish. Based on our findings, we concluded that the increased grooming behavior of signal crayfish was responsible for this trend. However, we do not yet understand why this invasive crayfish is specifically intolerant to these European species of branchiobdellidans, while it can tolerate at least some North American branchiobdellidans.
Nevertheless, it is clear that the signal crayfish poses a threat to the native European fauna, including the branchiobdellidans.
Detailed information can be found in the original article: Let, M., Ložek, F., Kouba, A., Buřič, M., Bláha, M., 2023. Signal crayfish as a threat for European ectosymbionts: overlooked biodiversity losses. Aquatic Sciences 85: 30. https://doi.org/10.1007/s00027-022-00932-w
Written by: Marek Let and Filip Ložek
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