Optimal foraging theory predicts that animals’ foraging behaviours will maximise energy intake relative to energy expended. It is commonly used to explain animal foraging behaviour across a wide range of phyla. Among marine invertebrates, the invasive European shore crab, Carcinus maenas, is often presented as an optimal forager. In experiments where C. maenas is offered pair-wise choices between bivalves, the crab selects those size-classes where energy return is greatest per unit handling time. It is questionable whether these experiments really provide support for optimal foraging theory because bivalve density, rather than volume, is held constant between size-classes, potentially leading to differences in bivalve encounter rate. To assess the extent to which prey selection is influenced by encounter rate, we offered crabs five size-classes of pygmy mussels, Xenostrobus securis, in pairwise trials that (1) held mussel density constant, but not volume and (2) held mussel volume constant but not density. In addition, we assessed whether prey selection in pair-wise experiments predicted prey selection when mussels were offered at multiple size-classes simultaneously, at their natural size-frequency distribution. Across trials in which mussels were offered at equal density, crabs repeatedly consumed more of the larger prey items that maximised energy return per unit handling time, but that were also encountered at a greater rate, proportional to their volume. By contrast, when mussel volume was held constant, crabs consumed more of the smaller sub-optimal bivalves, in proportion to the rate at which they were encountered. When offered multiple mussel size-classes at their natural size-frequency distribution, crabs consumed prey items in proportion to their abundance. These findings raise doubts as to whether C. maenas really forage optimally.