Habitat Use and Fish Avoidance Behaviors by the Stream-Dwelling Isopod Lirceus fontinalis

We examined patterns of habitat use of an isopod, Lirceus fontinalis, that coexists with predatory fish in second and third order permanent streams in central Kentucky, USA. Isopods were significantly more dense in Cladophora, a filamentous green alga, than on bare substrates of silt, sand, and gravel, and in submerged leaf litter when predatory fish were present. The dense, entangled filaments of the algae precluded most fish from entering Cladophora mats and eating isopods. Nonetheless, fish predation significantly reduced total isopod densities in these habitats. In laboratory manipulations, emergences from and entrances into Cladophora along with overall movements by isopods in open sandy areas were significantly reduced when green sunfish (Lepomis cyanellus) were present. Lirceus responded similarly to only water conditioned with green sunfish, indicating isopods use chemical cues, in part, to detect predatory fish. Isopods predominantly ate decaying leaf tissue (i.e., coarse particulate organic matter [CPOM] > 0.5 mm in diameter), but amounts of CPOM were significantly lower in Cladophora than in sediments of bare substrates. Lirceus did not eat the algae. Over 80% of the organic particulates in algal mats were < 0.5 mm (fine POM). Isopods reared on FPOM grew significantly slower than those on CPOM. Long-term use of algal mats poor in forage by isopods may adversely affect relative fitness since egg production by females is dependent on body size.