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 signific antly different in shape. Rare intermediates between these two distinc t forms were occasionally found, usually with the low shore animals. T he two forms show different reproductive High shore females (L. saxati lis 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 inte rmediate females (L saxatilis I) contain eggs of both sizes, giving ri se 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 incide nce of abortion (about 60%) and the incidence of abortion in the small eggs was also increased (about 30%). Thus there appears to be a parti al 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 dista nce of about 66 km. The facts are suggestive of a hitherto unrecognise d reproductive barrier with L. saxatilis, the nature and strength of w hich is under investigation.