Styloniscus is a small, slater-like isopod with a mottled grey-brown colouring and longer antennae than most aquatic isopods. This isopod normally lives above the water amongst damp vegetation, but it appears to deliberately enter the water on occasion.
While they are not normally aquatic, many Styloniscus were observed moving around under water on the walls of an Auckland swimming pool, and we often find them in samples collected from bush-covered streams.
They probably feed on dead plant matter and the associated decomposer organisms.
Being only an occasional visitor to aquatic habitats, Styloniscus may indicate little about stream water quality. There are no tolerance values assigned to this genus but the Order Isopoda has been assigned tolerance values of 5 (hard bottom sites) and 4.5 (soft bottom sites).