Reconstructing intra-annual growth of freshwater mussels using oxygen isotopes

Abstract

Intra-annual growth rates from the bivalve mollusk Lampsilis cardium (Unionidae) were reconstructed using measured oxygen isotope (δ18O) profiles together with high-resolution environmental records. Mussels from a single cohort (2007) were grown in two different settings at the Columbus Zoo & Aquarium Freshwater Mussel Conservation & Research Center (MRC), located in central Ohio, USA. “Outside” specimens were collected from sediment laden cages deployed in the O’Shaughnessy Reservoir. A single “inside” specimen was collected from the raceways in the MRC. Measured δ18Ocarb profiles from these specimens were calibrated with a predicted oxygen isotope envelope (i.e., the daily range of potential δ18Ocarb values) calculated from hourly water temperatures and weekly δ18Owater samples collected in 2010. This exercise suggests the “outside” specimens commenced 2010 shell deposition in late April and ceased in latest October (six month growing season). In contrast, the “inside” growing season lasted only three months (early July and early September). Calculated daily growth rates from both sites were faster early in the year, but highly episodic throughout. Maximum daily growth rates for the “outside” and “inside” specimens were 300 and 436 μm/day, respectively. Analysis of annual growth rates from the entire cohort suggests the “inside” specimens grew slower following transplantation. In subsequent years, however, their growth rates were nearly identical to the “outside” population. Our results suggest that, despite the fact that both populations have similar annual growth rates, they have different intra-annual growth patterns. Thus, caution should be exercised when extrapolating intra-annual growth patterns from cultured specimens to natural populations.