Riparian soil conditions associated with the timing of millipede mass entry into a stream, western <scp>Japan</scp>

Abstract Millipedes play a crucial role in forest ecosystems as decomposers of organic litter and as food resources. A few studies have reported that millipedes are preyed upon by fish and gastropods and that assemblages of millipede carcasses can be found in streams. However, the mass entry of millipedes into aquatic environments and the effects of environmental conditions on this phenomenon remain unclear. In this study, we characterized the distribution of millipedes, Parafontaria tonominea (Attems, 1899), in a mountain stream in western Japan and examined riparian environmental conditions at the time of their entry into this stream. From June 7 to November 30, 2022, we surveyed a 650 m stream reach two to three times per week. We discovered a high density of millipede carcasses at a single site (661.3 carcasses/m 2 ). Millipede entry occurred between June 17 and 20. After that period, no millipedes were observed in the stream. Meteorological and hydrological observations in the riparian zone indicated that millipedes entered the stream when there was no rainfall, when the soil was relatively wet, and when the daily minimum soil temperature exceeded 15°C for the first time that year. All individuals observed in this study were juveniles, suggesting that their entry was unlikely to be related to their breeding behavior. Soil moisture and temperature conditions may have been favorable for increased millipede activity. Further research is needed to elucidate the reasons for this mass entry and its ecological significance, based on millipede behavioral traits and soil environmental conditions.