Small-Scale Habitat Relationships of Corydalus cornutus Hellgrammites in Central Ohio Riffles

Corydalus cornutus hellgrammites are known to inhabit riffles, but information is scarce regarding their habitat relationships at the plot scale and in the northern part of their range in the United States. We investigated the relationship of C. cornutus hellgrammite occurrence, density, and body size with environmental variables at the 1 m2 plot scale within central Ohio riffles. We collected hellgrammites and measured hydrological, substrate, large instream wood, and canopy cover variables in nine riffles in 2023 and ten riffles in 2024. Occurrence and density were best predicted by water velocity and grain size score, while none of the measured variables were a good predictor of head capsule width. Occurrence and density increased with increasing water velocity, grain size score, and substrate richness. Density also increased with increases in edge-interior scores. Head capsule width increased with increasing distances to the nearest plot with hellgrammites. Our results suggest that C. cornutus hellgrammites can serve as an indicator species because their occurrence and density reflect the water velocity and substrate conditions within riffles in the Midwestern United States.