ORIGINAL PAPER S. Schiaparelli á R. Cattaneo-Vietti á M. Chiantore Adaptive morphology of Capulus subcompressus Pelseneer, 1903 (Gastropoda: Capulidae) from Terra Nova Bay, Ross Sea (Antarctica) Accepted: 17 July 1999 Abstract 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. Introduction In Antarctica, only a few data regarding the speci®c interactions occurring among benthic organisms living on hard bottoms are available, and most of these rela- tionships are unknown. In some cases these living as- semblages may produce structured communities with a high degree of biodiversity. Among these, the bush-like aggregations of the calcareous tubes of the polychaete Serpula narconensis Baird, 1865 are frequent at Terra Nova Bay (Ross Sea) on rock outcrops and gorgon- aceans between 200 and 500 m depth (Di Geronimo et al. 1990; Cantone and San®lippo 1992; Cattaneo- Vietti et al., in press). This microcosm shelters a varied fauna of small crustaceans (scalpellids, ostracods, amphipods and isopods), hydroids, sponges and mol- luscs. During the last PNRA expeditions, a study on the structure of these micro-reefs was carried out, with special emphasis on the morphological adaptation of the sessile epibiont gastropod Capulus subcompressus Pelse- neer, 1903 (Caenogastropoda), the unique capulid known from all the Antarctic continental margins, ex- cept the Antarctic Peninsula where it is absent (Dell 1990; Numanami 1996). The genus has a widespread distribution, with about 25 species living in all oceans, mainly on bivalves. They exploit the strong feeding currents of the host bivalve (Yonge 1938), or steal their accumulated food and/or pseudofaeces (Graham 1988) using a pseudoproboscis. Their shell morphology is quite similar, being the result of an adaptation to grow on ¯at surfaces, and follows a morphological empirical rule, which states that their aperture is generally circular in outline (Morita 1991) and without distinct overlap whorls (Linsey 1977; McNair et al. 1981). Capulus subcompressus showing an evident oval ap- erture, as its name suggests, represents a singular ex- ception. Several authors (Pelseneer 1903; Powell 1958; Arnaud 1972) have suggested that this form could be due to cidarid spines or an anterior siphonal gastropod canal to which C. subcompressus adheres with its foot, and Numanami (1996) reported a high occurrence of cidarid echinoids at the same stations in which C. sub- compressus was collected. However, Hove (1994) re- ported a personal communication (from H. Zibrowius and P.M. Arnaud) in which C. subcompressus was re- corded on tubes, near the mouth, of Serpula narconensis in the Weddell Sea. Within the framework of the ecological studies car- ried out in Terra Nova Bay (Ross Sea), a signi®cant number of specimens of this quite rare species was col- lected, allowing the description of its gross anatomy and a partial reconstruction of larval ontogeny. A biometric evaluation of the numerous specimens permitted the identi®cation of substrate constraints (S. narconensis) as the driving force in shell compression. Polar Biol (2000) 23: 11±16 Ó Springer-Verlag 2000 S. Schiaparelli (&) á R. Cattaneo-Vietti á M. Chiantore Dipartimento per lo Studio del Territorio e delle sue Risorse ± DIP.TE.RIS, Viale Benedetto XV, 5, I-16123 Genoa, Italy e-mail: isamge1@unige.it, Fax: +39-10-3538102