Prey size selection and bottom type influence multiple predator effects in a crab–bivalve system

Abstract

Consumption by multiple conspecific or interspecific predators on shared prey is not always predicted when data from isolated predators are combined. Predator interactions can result in non-independent predator effects on prey, and may be influenced by prey size selection and bottom type. We examined stone crab Menippe mercenaria and blue crab Callinectes sapidus predation of hard clams Mercenaria mercenaria. Interactions between these predators may become common in mid-Atlantic estuaries as stone crab abundance increases with a poleward range shift driven by climate change. Crabs in isolation, in conspecific pairs and in the interspecific pair were offered clams in 5 size classes simultaneously (10-60 mm shell length [SL]) or 1 size class (10-20 mm SL). Trials were on sand and hard bottom. On sand, all predator pairs had independent effects on prey, regardless of the presence or absence of multiple prey sizes. On hard bottom, when multiple prey sizes were present, blue crab conspecific and the interspecific pairs consumed less than predicted from isolated crabs. Strong selection of small clams by blue crabs led to conspecific interactions that reduced foraging compared to isolated crabs. Because stone crabs consumed all clam sizes, behaviours other than prey size selection caused the non-independent effect on prey by the interspecific pair. When multiple prey sizes were absent on hard bottom, most predator pairs had independent effects on prey. Thus, an expected poleward range shift of stone crabs may not increase interactions with blue crabs when foraging on sand. However, on hard bottom, increased predator interactions could reduce overall predation risk for prey, especially when consuming multiple prey sizes. In a climate-changed midAtlantic estuary, stone crabs will consume a wide range of clam sizes, and the size refuge large clams usually have from blue crab predation will be lost.

Exploring the decline of oyster beds in Atlantic Canada shorelines: potential effects of crab predation on American oysters (Crassostrea virginica)

“…Crab-bivalve interactions have been shown to be complex [56,57] particularly those involving habitat-forming species like oysters [20,27,58]. As this study illustrates, to better understand the role of newly arrived predators on a coastal system, it is important to gather information from multiple sites and conduct experimental manipulations [42].…”

Eastern oysters Crassostrea virginica deter crab predators by altering their morphology in response to crab cues

“…Prey size often affects vulnerability to consumers (Wong et al 2010), and crabs prey more heavily on smaller bivalves possibly because they require less energy to open (Nakaoka 2000, Johnson & Smee 2012 and are less likely to cause injury to crab claws (Juanes 1992). We therefore elected to focus on oyster spat for this experiment because they are readily consumed by many organisms, including mud crabs (Newell et al 2000, Johnson & Smee 2012.…”

Size matters for risk assessment and resource allocation in bivalves

“…Surprisingly, few studies have examined how predator size or biomass influences the propagation of nonlethal effects on prey populations (but see Schoeppner & Relyea 2008, Toscano & Griffen 2012. Similarly, it is well established that prey size influences risk of being consumed (Micheli 1995, Wong et al 2010), but few studies have examined whether prey size influences reactions to predation risk (but see Nakaoka 2000).…”