Breaking down the mussel (Mytilus edulis) shell: Which layers affect Oystercatchers' (Haematopus ostralegus) prey selection?

Predators are able to identify fine characteristic features of prey and use them to maximise the profitability of foraging. Oystercatchers Haematopus ostralegus select thin-shelled mussels Mytilus edulis to hammer through because they are easier to crack than thick-shelled mussels. But mussel shells are composite structures, so we need to ask what it is about these thin-shelled mussels that make them vulnerable. Here we show that the mussels damaged by Oystercatchers were mainly distinguished by having a significantly thinner prismatic layer than undamaged mussels. Regression analysis indicated that the Oystercatchers' shell selection was independently influenced by the thickness of the prismatic and nacreous layers, but the coefficient for the thickness of the prismatic layer was almost one and a half times that for the nacreous layer. Thus the thickness of the prismatic layer largely determines the vulnerability of the mussel shells. Oystercatchers were more likely to attack mussels by the right valve than the left, and this tendency was accentuated in larger mussels and those with a thicker nacreous layer.

Research Highlights
► Oystercatchers are known to select thin-shelled mussels to feed on.
► Earlier studies on mussel selection ignored the multilayer structure of the shells.
► Shells damaged by Oystercatchers had relatively thin prismatic and nacreous layers.
► The thickness of the pyramidal layer had the larger effect on attack tendency.
► A thick nacreous layer raises the birds' excess tendency to attack the right valve.