Morphological divergence and evidence for reproductive isolation in Littorina saxatilis (Olivi) in northeast England

Current work at Old Peak (Ravenscar) on the northeast coast of England indicates that boulder-dwelling Littorina saxatilis (Olivi) comprise at least two distinct forms, inhabiting high- and mid-shore boulders. Using multivariate morphometrics we show that these forms are significantly different in shape. Rare intermediates between these two distinct forms were occasionally found, usually with the low shore animals. The two forms show different reproductive High shore females (L. saxatilis H) mature at a smaller size and produce a smaller number of large eggs giving rise to large crawlaways. The mid-shore form (L. saxatilis M) matures at a larger size and produces a large number of small eggs which give rise to small crawlaways. A novel observation is that intermediate females (L saxatilis I) contain eggs of both sizes, giving rise to a bimodal distribution of egg sizes within the brood pouch. The usual incidence of abortion (observed in H and M forms) is very low (<1%). However, in the intermediates the large eggs showed a high incidence of abortion (about 60%) and the incidence of abortion in the small eggs was also increased (about 30%). Thus there appears to be a partial reproductive barrier between the two forms, presumably resulting in reduced fitness due in the intermediates to reproductive dysfunction. These findings extend to other shores in the region, over total distance of about 66 km. The facts are suggestive of a hitherto unrecognised reproductive barrier with L. saxatilis, the nature and strength of which is under investigation.