Microdistribution of the polymorphic snailLittorina saxatilis(Olivi) in a patchy rocky shore habitat

Abstract

Galician populations of the marine snail Littorina saxatilis have two main morphs on exposed rocky shores. The ridged and banded morph inhabits the barnacle (Chthamalus stellatus) dominated upper shore, while the smooth and unbanded morph is mainly found in the lower-shore zone of blue mussels (Mytilus galloprovincialis). The distribution of the two morphs overlaps in the mid shore where hybrids are also present. The mid-shore habitat is a mixture of barnacle and mussel patches. We hypothesised that the different architectural complexity of barnacles and mussels would affect the distribution of the two morphs. Fractal dimensions were used to describe the substratum complexity of 96 small patches from three sites with different proportions of barnacles and mussels. Increased proportions of mussels in a patch increased the fractal dimension, and thus surface complexity. Snail abundance decreased with increased substratum complexity in the smooth and unbanded morph and in the hybrids but not in the ridged and banded morph. Furthermore, snail size increased with surface complexity. Barnacles seemed a more suitable habitat than mussels as the interstitial volumes between barnacles available as refuges for snails were of a similar size range to that of the snails, while the mussel refuges were considerably larger.