The Thickness of the Flat Periwinkle Shell

Flat Periwinkle, Littorina obtusata

The flat periwinkle Littorina obtusata is a very common grazer of seaweeds on the middle shore around Ireland (1) and is a large constituent of the diet of the common shore crab Carcinus maenas (2). C. maenus feeds on the shellfish by crushing the shell and extracting the periwinkle. The evolution of the shell is though to be driven mainly by the selection pressures associated with such shell crushing predators (3). However Geoffery Trussell proposes that morphological co-evolution between predators and their gastropod prey may be driven by natural selection on reaction norms rather than genetically fixed phenotypes (4). In his study of geographical variation in L. obtusata in the Gulf of Maine, he found that periwinkles in areas with more C. maenas had thicker shells. This result was underlined in a subsequent laboratory experiment when L. obtusata raised in the presence of C. maenas again had much thicker shells. The trade off for better protection was smaller individuals with an accompanied reduction in body growth.

Shore Crab, Carcinus maenas

References:

1. Chinery 1987, Field Guide to the Wildlife of Britain and Europe p. 200
2. Ropes 1968, Fishery Bulletin 67 pp. 183-203
3. Clements et al. 2008, Biology Letters 4 pp. 179–182
4. Trussell 2000, Evolutionary Ecology Research 2 pp. 803-822