Preferential feeding by the crab Necora puber on differing sizes of the intertidal limpet Patella vulgata

The crab Necora puber L. is a common predator of limpets, the major grazer on rocky shores in Northern Europe. Information on interactions between crabs and their limpet prey is limited, extending mainly to limpet defensive and predator offensive tactics, while the importance of prey size on the outcome of such interactions remains largely unknown. Here, a laboratory approach was used to test for preference in feeding habits. Predation by N. puber with cheliped height 3 to 27 mm (carapace width [CW]: 16 to 77 mm) was examined on Patella vulgata with shell length 5 to 60 mm. Predator size (10, 11–15, 16–20 and 21–25 mm cheliped height) and prey size (5–10, 15–20, 25–30 and 35–40 mm shell length) were examined, with 2 replicate tests for each predator-prey size combination. Crabs >10 mm in cheliped height (35 mm CW) predominantly crushed the shell of limpets <10 mm, while in the remaining combinations of predator and prey sizes, crabs prised limpets from the substratum. Size of limpet shell (vulnerability to crushing force) and resistance to leverage force were both important factors influencing the outcomes of crab-limpet interactions. For the largest crab tested (27 mm cheliped height; 77 mm CW), there was a size refuge for limpets >41 mm in shell length. Field observations showed that the majority (94%) of limpets present in the intertidal zone are of a size that is vulnerable to predation by N. puber. For all sizes of crab examined, there were clear preferences for limpets smaller than the maximum size that the crabs were actually able to consume. Intriguingly, however, the preference experiment showed that, when given a choice, crabs consistently consumed proportionately more limpets of a larger size-class than when presented only with a single size-class at a time. Although further in situ studies are necessary, the present study indicates that size-selective predation by N. puber and other crabs may have an important influence on limpet population structure