Temperature does not influence functional response of amphipods consuming different trematode prey

Abstract

Direct consumption on free-living cercariae stages of trematodes by non-host organisms interferes with trematode transmission and leads to reduced infections in the next suitable hosts. Consumer functional responses provide a useful tool to examine relationships between consumption rates and ecologically relevant prey densities, whilst also accounting for abiotic factors that likely influence consumption rates. We investigated how temperature influences the consumer functional response of the amphipod <i>Gammarus lacustris</i> towards the cercariae of three freshwater trematodes (<i>Diplostomum, Apatemon</i> and <i>Trichobilharzia</i>). Amphipods displayed different functional responses towards the parasites, with Type II responses for <i>Diplostomum</i> and Type I responses for <i>Apatemon</i> prey. Temperature did not alter the consumption rate of the amphipod predator. <i>Trichobilharzia</i> was likely consumed at similar proportions as <i>Diplostomum</i>; however, this could not be fully evaluated due to low replication. Whilst Type II responses of invertebrate predators are common to various invertebrate prey types, this is the first time a non-filter feeding predator has been shown to exhibit Type I response towards cercarial prey. The prey-specific consumption patterns of amphipods were related to cercarial distribution in the water column rather than to the size of cercariae or temperature influence. The substantial energy flow into food webs by non-host consumer organisms highlights the importance of understanding the mechanisms that modulate functional responses and direct predation in the context of parasitic organisms.