Amphipod ( Gammarus minus ) responses to predators and predator impact on amphipod density

Abstract

Recent theoretical work suggests that predator impact on local prey density will be the result of interactions between prey emigration responses to predators and predator consumption of prey. Whether prey increase or decrease their movement rates in response to predators will greatly influence the impact that predators have on prey density. In stream systems the type of predator, benthic versus water-column, is expected to influence whether prey increase or decrease their movement rates. Experiments were conducted to examine the response of amphipods (Gammarus minus) to benthic and water-column predators and to examine the interplay between amphipod response to predators and predator consumption of prey in determining prey density. Amphipods did not respond to nor were they consumed by the benthic predator. Thus, this predator had no impact on amphipod density. In contrast, amphipods did respond to two species of water-column predators (the predatory fish bluegills, Lepomis macrochirus, and striped shiners, Luxilus chrysocephalus) by decreasing their activity rates. This response led to similar positive effects on amphipod density at night by both species of predatory fish. However, striped shiners did not consume many amphipods, suggesting their impact on the whole amphipod "population" was zero. In contrast, bluegills consumed a significant number of amphipods, and thus had a negative impact on the amphipod "population". These results lend support to theoretical work which suggests that prey behavioral responses to predators can mask the true impact that predators have on prey populations when experiments are conducted at small scales.

Activity level and aggregation behavior in the crustacean gammarid Gammarus insensibilis parasitized by the manipulative trematode Microphallus papillorobustus

“…Several invertebrate species, particularly amphipods, have evolved the ability to reduce activity levels and increase their use of refuges once they have detected a predator's presence by chemical means (Andersson et al, 1986;Wooster, 1998;Dezfuli et al, 2003;Wellnitz et al, 2003;Bollache et al, 2006;Perrot-Minnot et al, 2007;Thünken et al, 2010). In agreement with these findings, we found that uninfected individuals were significantly less active in the presence of bird and fish cues than in control treatment.…”

“…Knowing that aggregation behavior and reduced activity levels are strategies that limit predation in other species of amphipods (Andersson et al, 1986;Krause and Godin, 1994;Wooster, 1998;Dezfuli et al, 2003;Wellnitz et al, 2003), we explored in this study whether M. papillorobustus affects these behavioral responses in G. insensibilis when exposed to either host or nonhost predator olfactory cues in the water (i.e., bird feces and fish mucus, respectively). In lagoon ecosystems of southern France, hundreds to thousands aquatic birds frequently aggregate in rest areas (Cramp and Simmons, 1983 and personal observations).…”

Selective predation by fish: a further reason for the decline of native gammarids in the presence of invasives?

“…Prey either flee or seek refuge when they face a predator. Gammarids seek to hide in the bottom sediments to avoid encounters with fish predators (Williams & Moore 1982, Hoyle & Holomuzki 1990Starry et al 1998;Wooster 1998).…”

Ovigerous Female Amphipods (Gammarus pseudolimnaeus) Face Increased Risks from Vertebrate and Invertebrate Predators

“…It is possible that amphipods are more prone to predation during these behavioral bouts because they are not swimming but are generating motion and vibrations with their pleopods. Several authors have found that stationary amphipods have a lower risk of predation than mobile amphipods for both fish (e.g., Caine 1989, Croy and Hughes 1990, Wooster 1998) and dragonfly larvae (e.g., Cothran 2004). However, the stationary, curled-body response that characterizes amphipod anti-predator behavior (Kaiser et al 1992) is distinct from the more active (but stationary) pleopod beating posture.…”