Adaptive morphology of Capulus subcompressus Pelseneer, 1903 (Gastropoda: Capulidae) from Terra Nova Bay, Ross Sea (Antarctica)

Capulus subcompressus Pelseneer, 1903 (Gastropoda: Capulidae) is a small epibiont gastropod living at Terra Nova Bay (Ross Sea) down to 540 m on the calcareous tubes of its unique host, the serpulid Serpula narconensis Baird, 1865. This polychaete forms bush-like aggregates which host a rich microfauna of crustaceans, hydroids and molluscs. In contrast to all other capulids, C. subcompressus shows an evident oval shell aperture, which is due to an allometric growth that can be imputed to the Serpula tube morphology. Since the allometric growth is detectable in all size classes, it could be deduced that the compressed shape of the C. subcompressus shell is the stable result of a signi®cant evolutionary history which binds tightly these two spe- cies in Antarctic waters.