Natural selection on a vertical environmental gradient in Littorina saxatilis:: analysis of fecundity

Two different morphs of the intertidal snail Littorina saxatilis are found in the wave-exposed coasts of Galicia, NW Spain. One lives on the upper shore and the other on the lower shore. They meet and produce phenotypically intermediate hybrids in a narrow transition band on the mid-shore. To understand the maintenance in sympatry of such an extreme intraspecific polymorphism, we studied the change in fecundity in each morph along two vertical transects of the coast. We did not find a progressive reduction in fecundity in any morph as it approached the midshore, and conclude therefore that the observed spatial segregation of the morphs cannot be explained by divergent natural selection acting through fecundity. We also measured 16 environmental variables, bur none explained an important proportion of fecundity variability.