DISSOLVING THE FUTURE RECORD: A NEGATIVE CORRELATION BETWEEN PRE-MORTEM DECALCIFICATION OF MOLLUSKS AND DEAD-SHELL DENSITIES IN PUGET SOUND

Ocean acidification (OA) threatens shellfish communities worldwide: carbonate undersaturation makes it difficult for animals to secrete and maintain biomineralized hardparts, even leading to elevated local mortality. Declines in shellfish populations or shell thickness should thus decrease shell input into sediments, where skeletal debris creates habitats for epibenthic fauna, buffers porewaters against low pH, and, ultimately, promotes persistence into the permanent fossil record. Puget Sound exemplifies a setting with an inherently strong taphonomic bias against shell preservation even without OA (e.g., strong tides, cold bottom waters). Within the past decade, biomonitoring of mollusks from soft-sediment benthos has revealed increasing evidence of shell dissolution during life, to the point where shells are transparent, flexible, or even absent.

Does the intensity of dissolution damage on living mollusk shells, presumed to arise from OA, vary along environmental gradients and/or with dead-shell availability in Puget Sound? We assessed damage patterns on 7,275 live-collected bivalves and gastropods (68 species) from sediment grabs at 48 subtidal stations in 2019. Each specimen was assigned to a within-species size class (small, medium, or large) and a damage score combining severity and extent on an ordinal scale from 0 to 12. Dead-shell assemblages from sediment grabs at 11 stations were processed for the total number (density) of dead shells. Damage to living shells varied strongly, with the highest values at terminal inlets with high organics. For most species, small individuals had higher damage values. Dead shell density was anticorrelated with damage to living shells, with the highest densities associated with deep-water stations with low damage values. The implication is that OA-related damage to the living assemblage is also leading to the loss of dead shells from the same seabeds, or at least to a new failure of shell input to compensate for ordinary rates of postmortem shell loss.

The damage-state of living shellfish is thus a potentially insightful metric for detecting ongoing consequences of OA. The results presented here suggest that the emergence of OA will likely hinder future replenishment of dead-shell assemblages that serve as important records of historical community structure.