Stable carbon and oxygen isotope fractionation in bivalve (Placopecten magellanicus) larval aragonite

The relationship between stable isotope composition (δ13C and δ18O) in seawater and in larval shell aragonite of the sea scallop, Placopecten magellanicus, was investigated in a controlled experiment to determine whether isotopes in larval shell aragonite can be used as a reliable proxy for environmental conditions. The linear relationship between δ13CDIC and δ13Caragonite (r2 = 0.97, p < 0.0001, RMSE = 0.18) was:δ13CDIC=1.15(±0.05)∗δ13Caragonite-0.85(±0.04)The relationship between δ13CDIC and δ13Caragonite described for P. magellanicus resulted in larval shell aragonite that was depleted on average by 1.82‰ (SD = 0.22‰, range = 1.1-2.1‰) from predicted equilibrium values based on the relationship calibrated for inorganic aragonite. The average contribution of metabolic carbon that resulted in this depletion was 5.4% (SD = 0.57%; range = 3.4-7.8%). Stable oxygen isotopes were deposited into the larval shell in equilibrium for most samples, and the linear relationship described by least squares regression between temperature and δ18Oaragonite-δ18Owater (r2 = 0.90, p < 0.0001, RMSE = 0.63) was:T(°C)=20.0(±0.4)-4.6(±0.3)∗(δ18Oaragonite-δ18Owater)However, larvae reared under “stressful” conditions were depleted from oxygen isotope equilibrium. Further studies are necessary to determine the variable contribution of metabolic carbon to the larval shell in field conditions, the potential effects of growth rate on carbon isotope composition, and the factors influencing oxygen isotope depletion in P. magellanicus larval shell before the isotope composition of larval shells can be used to reconstruct δ13CDIC or temperature of the seawater in which the larvae developed.