THE ROLE OF LIGHT LIMITATION IN THE ONTOGENY OF EXTREMELY LONG-LIVED, TEMPERATE-WATER BIVALVES FROM THE EOCENE OF ANTARCTICA

Non-symbiotic mollusks exhibiting extreme longevity and slow growth are often found at either great depth or high latitude, a life-history strategy that has been most commonly interpreted as an adaptation to cold temperatures. Growth banding and stable isotopic analyses of high latitude bivalves from the relatively warm, shallow waters of the Eocene, however, reveal age estimates of 50-80+ years. These data suggest that prolonged shell growth may largely reflect the importance of light (and hence food) limitation.

Geochemical studies focused on two well-preserved specimens of the bivalve Cucullaea raea from the Eocene La Meseta Formation of Seymour Island, Antarctic Peninsula (64° S, 56° W). Each exhibits many distinct, closely spaced growth bands in cross section. Both d¹⁸O and d¹³C from microsampled shell carbonate yield strong seasonal variation over a significant period of the clam’s life, confirming that bands form annually. Counts of growth bands in these and additional specimens yield ages of 80 or more years. Plots of d¹⁸O versus distance along the shell show sharply peaked negative extremes reflecting growth cessation during the summer and broad positive extremes representing more continuous growth during the winter. Inferred paleotemperatures, assuming an ice-free world, reveal a mean annual temperature comparable to present SSTs off the New Jersey shore (~13-14°C), but less seasonal (~10°C range). Carbon isotope profiles are perfectly negatively correlated with oxygen, reflecting higher primary productivity during the summer months. The association of growth bands with negative d¹⁸O and positive d¹³C suggests that Cucullaea stopped growing during periods of warmest temperatures and highest food and light availability. Such a counterintuitive pattern hints at reproductive strategy, with the organism devoting metabolic energy to spawning rather than growth during optimal conditions such that offspring have the highest chance for survival. Cucullaea’s age, slow growth, and temperate but light-limited environment suggest that extreme longevity in modern bivalves may be more influenced by light availability than previously thought.