Ontogenetic shifts in habitat use and activity in a stream-dwelling isopod

We examined ontogenetic changes in diel patterns of habitat use and activity of the lotic isopod Lirceus fontinalis in relation to differences in predation risk from green sunfish, Lepomis cyanellus. Juveniles are ≤5 mm total length (TL), individuals 6-9 mm TL are mature females (c. 90%) and immature males, and mature males are ≥10 mm TL. Isopods of all sizes were far more dense in Cladophora , a filamentous green algae, than in open silt/sand substrates in fish pools. Densities of each size did not increase in open areas at night in fish pools when risk of predation presumably decreases. Few individuals of any size class occurred in open silt/sand areas in fish pools. However, habitat use did not depend on fish presence. Densities and relative abundances of all sizes were similar between algal and open habitats in fishless pools both day and night, except for mature males which were more abundant in open areas during daylight. All sizes of isopods were significantly more dense in fishless versus fish pools, and density of a size class was influenced by fish presence. Densities of size classes were typically juveniles > individuals 6-9 mm TL > mature males. Green sunfish are size selective feeders on isopods, thus demography of isopods within pools may be partially influenced by the size structure of the fish population.
Isopods reduced their activity, as measured by captures in benthic traps, in the presence of caged green sunfish only at night. This behavior coupled with relatively high activity levels during daylight regardless of fish presence suggests other predators or factors may influence diel activity. Mature males were overrepresented in traps relative to their occurrence in fish runs, indicating high activity levels. Mature males were also more active than gravid and nongravid females and juveniles in the presence of fish in laboratory aquaria. Number of male encounters with nongravid females depended on activity level.