MICROGEOGRAPHICAL VARIATION IN SHELL STRENGTH IN THE FLAT PERIWINKLESLITTORINA-OBTUSATA AND LITTORINA-MARIAE

The strength of molluscan shells has been shown to vary in adaptive wa ys in a number of species and one of the main factors thought to be in volved is shell-crushing by predators. A recent study found that the s ibling species of flat periwinkle Littorina obtusata and Littorina mar iae showed significant differences in the rates at which shell strengt h increased with shell length in specimens which had been collected fr om the same location, where the species were sympatric. This paper des cribes differences between the shells of the two species from a number of localities around Milford Haven in Dyfed, Wales, and local geograp hical variation in the shells. Littorina mariae, which is normally fou nd at lower tidal levels than L. obtusata, matures at a smaller shell length. Both species reinforce the shell as they grow since shell stre ngth, determined as the maximum force applied by a hydraulic tensile t esting machine before the shell cracked, is strongly positively allome tric; it increases at a rate close to the cube of shell length whilst isometric growth would result in strength increasing in proportion to the square of shell length. Because L. mariae matures earlier and rein forces the shell at a smaller size, the mature shell of L. mariae is s ubstantially stronger on average than that of a similar sized but imma ture L. obtusata. At maturity the shell strengths of the two species a re not very different despite the substantial difference in mean shell length. Strength varies significantly from shore to shore, and with t he level of the shore from which the animals were collected. Strength increases down the shore in both species. Shell strength decreases wit h exposure to wave action in L. mariae but increases with exposure in L. obtusata; there is also substantial shore-to-shore variation which is not explained by exposure. Path analysis was used to explore the re lationship between shell strength and other measured shell parameters (mass, length, height, thickness). The best predictor of shell strengt h in both species is a parameter which is heavily positively loaded on LN (shell mass) and strongly offset by negative loadings on LN (shell length) and LN (shell height). This is logical because for a given sh ell length a heavier shell will be thicker and stronger, whilst for a given shell mass a bigger shell will be thinner and therefore weaker. Such differential variation of shell mass and shell length explains mo st of the geographical variation observed in shell strength; shells ar e stronger in snails collected from one place than from another becaus e, for the same shell length they are heavier or, to put it the other way, because at the same shell mass, they are smaller.