THE OXYGEN AND CARBON STABLE ISOTOPE RECORD OF MODERN TURRITELLINE GASTROPODS: IMPLICATIONS FOR BIOLOGY, SCLEROCHRONOLOGY AND (PALEO)ENVIRONMENTAL INTERPRETATIONS

Turritelline gastropods are high-spired, often sedentary, ciliary suspension feeding marine gastropods that occur worldwide from the intertidal to at least 2000 m water depth, in tropical to arctic waters. They have an abundant, Cretaceous-Recent fossil record, but data on their modern ecology are scarce. New size-frequency data from exhaustive surveys of three assemblages of turritelline gastropods on modern tidal flats (Thailand, Sea of Cortez) reveal that no individuals smaller than 5 cm are present. This is consistent with narrow size distributions in many other fossil and modern assemblages. Nothing is known about the environment or biology of juvenile individuals for these taxa, but their absence on these tidal flats suggests they may favour a deeper marine habitat. Stable oxygen isotope ontogenetic profiles indicate that shells of 5 cm in length correspond to an age of ~1 year and that individuals live to reach 2.5 years of age. Growth rate analyses indicate slowing in later ontogenetic stages at least in some species. Stable carbon isotope profiles in all populations consistently show a reduction in δ¹³C values in the later stages of life (> 1-1.5 yrs). The trend to reduced shell carbon values thus appears to coincide with a change in the biology of the animals: in the specimens from the Sea of Cortez, incorporation of isotopically light carbon coincides with the appearance of the animals on the tidal flat at sizes above 5 cm. The ratio of the carbon isotopes being incorporated into the shell depends on several factors. Primarily, the carbon isotopic composition of the shell reflects the isotopic composition of dissolved inorganic carbon in the ambient seawater. However, some percentage of isotopically negative respired carbon can also be incorporated into the shell. Our observations suggest that incorporation of more metabolic carbon in the later stages of life, possibly in connection with reproductive activity and reduced shell growth, is the primary reason for the observed trend in these taxa. Therefore, δ¹⁸O profiles of Recent and fossil turritellines can be used for age estimates and in ideal cases as paleotemperature indicators. When interpreting δ¹³C records, however, ontogenetic trends must be taken into account.