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size limit that can be opened being directly related to the size of the crab (references in Seed 1976). Crabs will almost always choose small‐sized prey when offered a range of sizes. It is handling time rather than the energetic costs of handling, estimated as a mere 2% of corresponding gains, that is the basis on which prey are selected (Rovero et al. 2000). During the handling period, the crab is at risk from other predators, competitors and even claw damage. Small mussels are therefore particularly vulnerable to predation, since they are easily crushed by most size classes of crab. A mussel must attain a shell length of at least 45 mm before it is relatively safe from crab predation. Once again, mussels show several defence mechanisms. In laboratory experiments, M. edulis increased byssus volume in response to waterborne cues from Cancer pagurus and Carcinus maenas (Cote 1995; Leonard et al. 1999). Similar findings were reported when P. viridis were exposed to crab (Thalamita danae) that had recently consumed conspecifics (Chiu et al. 2011). In addition, mussels subject to heavy predation develop thicker and more robust shells in response not just to crabs but also to the broken shells of other mussels (Leonard et al. 1999; see also Freeman & Byers 2006 and comments from Rawson et al. 2007 and Freeman & Byers 2007). Similar effects have also been reported in mussels subject to heavy whelk predation (Smith & Jennings 2000), and increased production of byssal threads has been shown in P. viridis in response to crab and gastropod predation (Cheung et al. 2004a,b, 2006, 2009). A behavioural strategy in response to crab predation has been reported in the Wadden Sea, Germany, where oysters (Crassostrea gigas) have invaded native mussel (M. edulis) beds (Eschweiler & Christensen 2011). Mussels subjected to direct contact with crabs (C. maenas) migrated from the top of the oyster reef to interspaces at the bottom of the reef, where they showed significantly reduced growth rates and conditions to mussels on the top of the reef. Mussels experience a trade‐off between survival and food supply, preferring to take refuge from predation even when this decreases growth and condition. In a previous study, Shin et al. (2008) investigated the effect of cues from damaged conspecifics and heterospecifics on the induction of refuge seeking and enhancement of byssus production as responses to the crab Carcinus maenas in the mussel Brachidontes variabilis, hypothesising that the mussel would seek refuge more readily and prefer a smaller refuge. More byssal threads (thicker and longer ones) should also be produced. They found that B. variabilis was able to differentiate between the sizes of available refuges and to stay in appropriate ones according to the level of risk they perceived. Staying in a smaller refuge would reduce the chance of the mussels being dislodged and consumed by the crabs. This helps explain why B. variabilis tended to stay in smaller refuges when predation risk was high, as simulated by the presence of damaged conspecifics and heterospecifics. In contrast, higher food and oxygen availabilities were found in large refuges, although the predation risk was also higher. Staying in larger refuges would be advantageous to the mussels only when predation risk was low. The preference toward certain sizes of refuge, therefore, should be a trade‐off between physiological requirements and the risk of predation.

Aquatic prey encounter an array of threat cues from multiple predators and killed conspecifics, yet the vast majority of induced defences are investigated using cues from single predator species. In most cases, it is unclear if odours from multiple predators will disrupt defences observed in single‐predator induction experiments (Freeman et al. 2009). Freeman et al. (2009) compared the inducible defences of M. edulis to waterborne odours from pairwise combinations of three predators representing two attack strategies, the sea star A. rubens, which pulls mussel shells open, and the crabs Carcinus maenas and Cancer irroratus, which crush or peel⁷ shells. Mussels increased adductor muscle mass in response to cues from unfed Asterias and increased shell thickness in response to unfed Carcinus. However, they did not express either predator‐specific response when exposed to the combined cues of Asterias and Carcinus, and did not increase shell thickness when exposed to cues from Cancer alone or any pairwise combination of the three predators. Shell closure or ‘clamming up’ did not occur in response to any predator combination. These results suggest that predator‐specific responses to Asterias and Carcinus are poorly integrated and cannot be expressed simultaneously.

Biotic invasions can result in the displacement of native species. This can alter the availability of native prey and the choices made by native predators. Skein et al. (2018) investigated prey selection by two native South African predators, the lobster Jasus lalandii and the starfish Marthasterias africana, in response to the invasive mussels M. galloprovincialis and Semimytilus algosus and the native mussels Aulacomya atra and Choromytilus meridionalis. As the diets of the two predators are broad, the authors hypothesised that they would consume the most abundant prey, regardless of its native or alien status. Laboratory studies presented predators with varying proportions of native and invasive mussels that represented pre‐ and post‐invasion scenarios. Both predators exhibited preference toward the native mussel C. meridionalis, even when it was the least abundant prey. The selection of native species occurred despite mussel parameters (shell strength, adductor muscle size and energy content), suggesting that invasive species would be easier to consume. These findings highlight the potential for facilitation of prey invasions, especially when predators avoid alien prey and select for native comparators that may offer resistance to the invasion through interspecific competition. This study does not present the first observation of native predators failing to select for invasive prey (e.g. López et al. 2010; Veiga et al. 2011). However, in other cases the avoided invasive prey were suggested to possess physical characteristics that might hinder predation from native predators (Skein et al. 2018).

Several bird species are predators of mussels. The main ones in western Europe that feed on mussels at low tide are the oystercatcher, Haematopus ostralegus, the common eider duck, Somateria mollissima, and the herring gull, Larus argentatus, although the latter only feeds on small mussels on newly established beds. Of these, the most significant is the oystercatcher (Figure 3.11). Predation is seasonal, with birds switching in the spring from mussels (and cockles) to deep‐living clams such as Scrobicularia plana and Macoma balthica, and back to surface bivalves in autumn in order to maximise intake rate (Zwarts et al. 1996a). The birds cannot survive if their diet is restricted to one or two prey species; they need to switch between three or four, and have to roam over feeding areas measuring at least some tens of km². Oystercatchers use one of two general techniques when preying on M. edulis: they either ‘stab’ their bills between the gaping valves of the mussel or ‘hammer’ through the shell on the dorsal or ventral side (Zieritz et al. 2012 and references therein). Stabbers and dorsal hammerers open the mussel in situ, but ventral hammerers usually tear the mussel off from the substrate and carry it to a suitably firm patch, where the shell is broken at the ventral region. They invariably select thin‐shelled mussels to hammer through because these are easier to crack than thick‐shelled mussels; it is the thickness of the prismatic layer that largely determines the vulnerability of mussel shells (Le Rossignol et al. 2011). Wintering oystercatchers feed extensively on M. edulis in the estuaries of southern Britain. They show a marked preference for brown‐shelled mussels over the commoner black‐shelled morph, and show that this enables them to maximise their rate of energy gain over a longer period than a single foraging bout (Nagarajan et al. 2002a). The brown and black mussels do not differ in ventral thickness or energy content, which are the main criteria for mussel selection and the most important for short‐term optimisation. The brown mussels contain significantly less moisture, so by selecting them, oystercatchers can pack more mussel flesh into their limited oesophageal storage capacity. This enables them to increase their overall consumption during a feeding bout and increases their long‐run energy gain rate, to an extent that is large enough to be significant for survival, especially during the short exposure of the mussel beds in winter. All birds show size selection within the prey species; this is because flesh content increases more steeply with prey size than handling time (Zwarts et al. 1996b). Oystercatchers are highly selective toward mussels of between 35 and 50 mm shell length,

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