Life history of the Pliocene scallop Fortipecten, based on oxygen and carbon isotope profiles

Oxygen and carbon isotope profiles from the outer calcite layer of two well-preserved specimens of the Pliocene scallop Fortipecten takahashii exhibit distinct attenuation in their annual ranges throughout the complete growth history of approximately 10 years. The segments of oxygen isotope profiles corresponding to winter or summer extremes were compressed, suggesting growth retardation during extreme water temperatures. These profile compressions correspond well with the positions of growth rings on the outer shell surface. One scallop shell, which evidently grew in warmer environments, based on mean oxygen isotope values, developed growth rings in summer while a second shell with colder mean-temperature values exhibits winter growth rings. A drastic reduction in isotope fluctuation occurred at a shell age of approximately 3 years, and possibly is related to the onset of the reproductive phase, which may have suppressed shell growth in favor of building up reproductive tissue. Sexual maturity brought a change in life habit, from a swimming to a reclining mode, which is clearly shown by the change in shell architecture. Although scallop shells during the juvenile stage may record a complete seasonal cycle, isotopic profiles in the adult stage contain repeated growth breaks. As a result, using isotope signals as paleoenvironmental proxies must be done carefully, at least with scallop shells.