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 t he Adriatic Sea at a depth of a few metres off the central Italian coast du ring 1996 and 1997. The oxygen and carbon isotope composition of spot sampl es collected from the shell apex to the top was measured according to well- established techniques. The results obtained allow the following conclusion s: (1) the CaCO3 of the N. mutabilis shells is essentially formed by aragon ite as proved by X-ray diffraction measurements; (2) the CaCO3 is precipita ted under isotope equilibrium conditions or very close to equilibrium with the environmental water; (3) shell accretion takes place all year round, al though the warm period is preferred since the accretion rates are higher du ring the warm half of the year and considerably reduced in winter; (4) the carbon isotope composition changes according to the age of the specimen, pr obably in relation to changes in its nutritional regimen; and (5) the alter nation of warm and cold periods recorded by the shells allows the evaluatio n of their age and of the season of their birth and death.