Oxygen and carbon isotope profiles from Nassa mutabilis shells (Gastropoda): accretion rates and biological behaviour

Six shells of the Gastropoda species Nassa mutabilis L. were collected in the Adriatic Sea at a depth of a few metres off the central Italian coast during 1996 and 1997. The oxygen and carbon isotope composition of spot samples collected from the shell apex to the top was measured according to well-established techniques. The results obtained allow the following conclusions: (1) the CaCO₃ of the N. mutabilis shells is essentially formed by aragonite as proved by X-ray diffraction measurements; (2) the CaCO₃ is precipitated under isotope equilibrium conditions or very close to equilibrium with the environmental water; (3) shell accretion takes place all year round, although the warm period is preferred since the accretion rates are higher during the warm half of the year and considerably reduced in winter; (4) the carbon isotope composition changes according to the age of the specimen, probably in relation to changes in its nutritional regimen; and (5) the alternation of warm and cold periods recorded by the shells allows the evaluation of their age and of the season of their birth and death. Received: 3 December 1998 / Accepted: 7 September 1999