Falling of terrestrial isopods (Isopoda: Oniscidea) into a stream: Possibility of downstream dispersal

This study was conducted in a riparian zone and small stream in western Japan to investigate the potential downstream movement and dispersal of terrestrial isopods (Crustacea, Isopoda, Oniscidea) via a stream. A total of 345 individuals were captured using pitfall traps on the riparian forest floor, while bankside traps along the streambank captured 416 individuals. Most individuals belonged to the families Ligiidae (three species) and Armadillidae (two species). Capture rates per crossable trap perimeter (individuals m −1 day −1 ) were 0.39 ± 0.02 (mean ± SE) for pitfall traps and 0.77 ± 0.14 for bankside traps, with the latter being 1.97 times higher, indicating that terrestrial isopods do not actively avoid streams and frequently fall into them. Generalized Linear Mixed Models (GLMMs) analysis further revealed that the likelihood of females falling into the stream was higher than that of males. Drift‐net surveys conducted once in July and August (24 h each) captured four live individuals, which were converted to represent the number of individuals traversing the channel cross‐section per day, resulting in 13.7 individuals for Ligiidae in July and 5.0 (Ligiidae) and 5.1 (Armadillidae) in August. Additionally, the stomach contents of 152 amago trout ( Oncorhynchus masou ishikawae Jordan & McGregor, 1925) were examined to evaluate predation during the downstream drift, and terrestrial isopods were detected in only six individuals (4%). These results suggest that terrestrial isopods frequently fall into rivers, survive while drifting, experience minimal predation from fish and potentially disperse downstream via the river.