Morphometric variation in two intertidal littorinid gastropods

Here we investigate spatial variation in shell shape of Littorina saxatilis, an ovoviviparous species, and Melarhaphe neritoides, a species with planktonic eggs and larvae. Populations of both species were sampled in 6 sites located along the west coast of the Iberian Peninsula. Shell shape was studied using landmarkbased morphometric methods. Landmark data was used to estimate individual size and to describe shell shape. Prior to statistical analysis specimens were aligned using Generalised Procrustes Analysis (GPA). Ordinations based on GPA of landmark data and using Principal Components Analysis (PCA), revealed a clear spatial segregation of sites for both species, although this was more evident for L. saxatilis than M. neritoides. Statistical analysis revealed significant multivariate variation in shape among sites and a significant effect of allometry on shape for both species. In contrast to this last result, there was no significant linear relationship between any of the first three PC axes and size in L. saxatilis but highly significant associations between the first and third PC axes and size for M. neritoides. Spatial variation in the shape of both species was primarily related to variation in the height of the apical whorls and in the width of the aperture for L. saxatilis. Further variation in shape was related to the shape of the last whorl in L. saxatilis and shell elongation and a change in aperture shape in M. neritoides.