Born to kill: Predatory functional responses of the littoral amphipod Echinogammarus marinus Leach throughout its life history

Studies of predator–prey interactions, and in particular the derivation of functional responses, have typically focussed on adults or single life-history stages, with subsequent food web analyses often simply using average body sizes or assuming that all conspecifics are equal. Such limited consideration of ontogeny in the context of predatory functional responses may have important implications for our understanding of community structure and function. Further, large predators may be physically restricted in their ability to feed on very small prey individuals and/or avoid such predation due to its limited energetic return. Here, we elucidate the predatory nature and impact of the marine intertidal amphipod, Echinogammarus marinus, throughout its ontogeny, towards a prey species, the isopod Jaera nordmanni. Firstly, we found J. nordmanni in the guts of field caught E. marinus, from juveniles through to adults of the latter, indicating that E. marinus may be predatory throughout ontogeny. Secondly, juvenile, sub-adult and adult E. marinus exhibited Type II functional responses towards size-matched J. nordmanni prey. In addition, the largest adult E. marinus fed on the smallest J. nordmanni, again with Type II functional responses, in both homo- and heterogeneous habitat. Thus, the prey do not benefit from any ontogenetic or size refuge from the predator. These findings demonstrate the significant predatory ability and potential population level impacts of E. marinus throughout its ontogeny, which may determine local prey species exclusion and persistence. Determining the functional responses of predators throughout their ontogeny may thus improve our understanding and prediction of their community impacts.