Mate choice in divergent morphs of the gastropod mollusc Littorina saxatilis (Olivi): speciation in action?

We investigated mate choice in the gastropod Littorina saxatilis (Olivi) in the laboratory, using snails taken from two shores, 20 km apart. The snail s occur in two distinct shell morphs in the high (H) and mid (M) shore zone s, known to show signs of a postzygotic reproductive barrier between them. We found that mating was assortative not only between the forms H and M on a single shore, but also between snails from the two different shores. Mati ng preference appeared to be based on morph type over this distance. The sn ails may be detecting and responding to an extrinsic attribute reflecting a common microhabitat (but across a distance of 20 km), and/or an intrinsic attribute to do with the organisms themselves. These findings are further e vidence of behaviour associated with the evolution of reproductive isolatio n in this gastropod species. (C) 1999 The Association for the Study of Anim al Behaviour.