SUMMARY 1. Exotic zebra mussels, Dreissena polymorpha, occur in southern U.S. waterways in high densities, but little is known about the interaction between native fish predators and zebra mussels. Previous studies have suggested that exotic zebra mussels are low profitability prey items and native vertebrate predators are unlikely to reduce zebra mussel densities. We tested these hypotheses by observing prey use of fishes, determining energy content of primary prey species of fishes, and conducting predator exclusion experiments in Lake Dardanelle, Arkansas. 2. Zebra mussels were the primary prey eaten by 52.9% of blue catfish, Ictalurus furcatus; 48.2% of freshwater drum, Aplodinotus grunniens; and 100% of adult redear sunfish, Lepomis microlophus. Blue catfish showed distinct seasonal prey shifts, feeding on zebra mussels in summer and shad, Dorosoma spp., during winter. Energy content (joules g−1) of blue catfish prey (threadfin shad, Dorosoma petenense; gizzard shad, D. cepedianum; zebra mussels; and asiatic clams, Corbicula fluminea) showed a significant species by season interaction, but shad were always significantly greater in energy content than bivalves examined as either ash‐free dry mass or whole organism dry mass. Fish predators significantly reduced densities of large zebra mussels (>5 mm length) colonising clay tiles in the summers of 1997 and 1998, but predation effects on small zebra mussels (≤5 mm length) were less clear. 3. Freshwater drum and redear sunfish process bivalve prey by crushing shells and obtain low amounts of higher‐energy food (only the flesh), whereas blue catfish lack a shell‐crushing apparatus and ingest large amounts of low‐energy food per unit time (bivalves with their shells). Blue catfish appeared to select the abundant zebra mussel over the more energetically rich shad during summer, then shifted to shad during winter when shad experienced temperature‐dependent stress and mortality. Native fish predators can suppress adult zebra mussel colonisation, but are ultimately unlikely to limit population density because of zebra mussel reproductive potential.

Alligator gar (Atractosteus spatula) populations are declining throughout their range in the United States, even where considered stable. Similar declines in other riverine species are occurring worldwide due to alteration of habitat in river-floodplain systems. Most rivers in the world are highly regulated, resulting in departures from historic hydrology that provides connection to the floodplain habitats used by riverine fishes. Knowledge of the seasonal habitat requirements of alligator gar and similar riverine fishes at a watershed scale is limited, hindering management practices. We used radiotelemetry to monitor movements and habitat use of adult alligator gar (N = 32; 25.0-84.5 kg; 149-224 cm) at macro-and microhabitat scales during 2 years of varying hydrology. Fish showed seasonal differences in their use of main channel and floodplain macrohabitats. Floodplain tributaries and their upper reaches were especially important to reproductive ecology. Minimum distance travelled per day varied seasonally. While using the main channel Fourche LaFave River, gar selected low velocity, shallow depth and complex structure along channel margins. The Fourche LaFave River is unique in its connection to floodplain habitats on a relatively predictable basis, with frequent flooding during the spring and summer reproductive period. Our study emphasises the importance of connectivity between the main river channel and floodplain habitats to a floodplain-obligate riverine species. An intact, heterogeneous riparian zone creates essential microhabitat for the species. Understanding habitat requirements of alligator gar at multiple spatial scales in a river-floodplain system is crucial to management of many other species and river systems.

The proteasome constitutes the main non-lysosomal cellular protease activity, and plays a crucial role not only in the disposal of unwanted material, but also in the regulation of numerous cellular processes. Previously, we have reported that during the replicative senescence of WI-38 fibroblasts there is a significant impairment in proteasome activity, which probably has important implications in the control of MAPK signaling and cellular proliferation. In this study, we report the potential role of the proteasome in the generation of the senescent phenotype in WI-38 fibroblasts. Our results indicate that inhibition of proteasome activity leads to an impairment in cell proliferation, and a shortening of the life span. The results also indicate that inhibition of the proteasome in young cells induces a premature senescent-like phenotype, as indicated by the increase in senescence-associated beta-galactosidase (SA beta-gal) activity and the abundance of both p21 and collagenase mRNAs, as well as a decreased level of EPC-1 mRNA known markers of cellular senescence, not previously shown to depend on proteasome activity. Together, our results suggest a molecular mechanism for the lack of responsiveness of human cells to growth factors, and point towards a role for the proteasome in the control of the life span of both cells and organisms.

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