First report and molecular characterization of Epistylis sp. infestation in the narrow-clawed crayfish (Pontastacus leptodactylus) from Xinjiang, China

Introduction The narrow-clawed crayfish ( Pontastacus leptodactylus ) is an emerging aquaculture species in Xinjiang, China. Here, we report the first recorded outbreak of a severe epibiotic infestation caused by the sessile ciliate Epistylis sp. in a commercial crayfish farm. Materials and methods Combined morphological analysis and molecular identification (employing 18S rDNA and ITS sequencing) confirmed the epibiont as Epistylis sp., sharing 100% sequence similarity with Epistylis chrysemydis . Metagenomic sequencing and MALDI-TOF mass spectrometry were further employed to characterize the bacterial community associated with this ciliate. Results A striking age-specific susceptibility was observed, with 100% prevalence in adult crayfish and near-zero incidence in juveniles, a difference strongly associated with the higher molting frequency of juveniles, which facilitates regular epibiont shedding. Environmental DNA detection verified the water column as a persistent reservoir for the ciliate. The outbreak coincided with elevated water temperatures and high organic load, conditions that reduced dissolved oxygen. While a 0.5% saline immersion proved effective in controlled laboratory settings, integrated farm-scale management strategies showed limited efficacy against adult crayfish infestation. Conclusions These findings identify Epistylis sp. as a significant emerging threat to P . leptodactylus aquaculture, providing critical insights for developing targeted control measures. Moreover, the results also underscores potential transboundary biosecurity risks, given the location of the farm within international watershed of the Irtysh River.