Hermit Crabs Prefer Winkle Shells

Pagurus bernhardus, the Common Hermit Crab

Pagurus bernhardus, the Common Hermit Crab, is a scavenger of sheltered shores, and is the most common hermit crab in Irish waters. It can often be spotted in rock pools, seen as a seashell with legs scurrying into corners. It has, along with all the hermit crabs, the last thoracic plate on the ventral side free of the carapace (1) and because of this, P. bernhardus lives inside the empty shells of molluscs, exchanging the shells for larger ones as the animal grows. The fifth pair of walking legs are also reduced to allow it to fit into the shell which also allows the animal to grip the inside of the shell (2) and the right handed pincer is larger than the left to block the shell entrance when the crab retreats inside (3). It has been suggested that hermit crabs originated from crevice dwlling ancestors, which had progressively lost their abdominal calcification, and took to using empty mollusc shells due to their mobility (4).

Pagurus bernhardus emerging from Edible Periwinkle (Littorina littorea) shell

Young P. bernhardus individuals have a marked preference for winkle shells, both Littorina littorea (Edible Periwinkle) (5) and L. obtusata (Flat Perieinkle) (6) over Gibbula spp. (Whelk) shells. This preference may be due to a number of factors, such as greater ease of locomotion and ease of manipulation during mating, and does have an effect on the overall performance of an individual. A 1995 survey of female P. bernhardus individuals showed that those living in the preferred shells produced eggs earlier in the season, produced more eggs in the first brood, and produced a second brood more often than did females in the less preferred shells (6). However it was unclear whether this reflected a reduced capacity for mating or if competition for the best shells resulted in low quality crabs occupying the less preferred ones.

Pagurus bernhardus, the Common Hermit Crab. Note the difference in size of the pincers

References

1. Lancaster, 1988 Field Studies 7 pp. 189-238
2. Challinor et al., 1999 A Beginner's Guide to Ireland's Seashore p. 142
3. Sterry, 2004 Collins Complete Guide to Irish Wildilfe p. 168
4. McLaughlin, 1983 Journal of Crustacean Biology 3 608-621
5. Elwood et al., 1979 Animal Behaviour 27 pp. 940-946
6. Elwood et al., 1995 Marine Biology 123 pp. 431-434