The hidden passengers: On the role of exotic crayfish in the spread of symbiotic and pathogenic organisms in Northern Iberian Peninsula.

The spread of invasive crayfish species poses a growing threat to freshwater ecosystems and public health, not only through direct ecological impacts but also by facilitating the transmission of symbionts and potential zoonotic pathogens. This study characterizes the symbiont diversity associated with the crayfish species that occur in northern Spain. In addition, the first comprehensive and comparative compilation of reported symbiont and parasite species of crayfish species reported in Iberia is provided. A combination of scanning electron and optical microscopy analysis and a review of the literature was employed to identify the symbionts and evaluate their spatial distribution on the host, their taxonomy, and their zoonotic potential. A total of five crayfish species were analyzed, with symbionts recorded across multiple body regions, particularly the chelipeds, gills, and pleopods. The most prevalent taxa belonged to the phyla Ciliophora, Annelida, and Platyhelminthes. Some symbionts, such as Xironogiton victoriensis and Uncinocythere occidentalis, were identified as obligate ectosymbionts that could have a negative impact on protected species such as Austropotamobius fulcisianus. Furthermore, several symbiont taxa (e.g., Fusarium sp., and Paragonimus spp.) are known to be zoonotic and are associated with conditions such as keratitis, onychomycosis, and paragonimiasis. The results highlight the dual ecological and epidemiological risk posed by crayfish invasions. As vectors of both invasive symbionts and zoonotic agents, these crustaceans may facilitate the emergence of new infectious diseases in freshwater environments. This underscores the need for integrated monitoring strategies that consider symbiont-host dynamics in invasive species management and public health surveillance and highlights the importance of implementing targeted management actions to mitigate ecological impacts and reduce associated health risks.