Shell Microstructure as an Indicator of Changing Environmental Conditions in Coastal Oceans

Ocean ecosystems are undergoing pronounced changes in temperature and chemistry. Biomineralizing animals, such as mollusks, that produce calcium carbonate shells can be sensitive to these changes, and this may be reflected in the microstructure of their shells. Shell microstructure potentially offers conservation paleobiologists an additional tool for assessing spatiotemporal changes in environmental conditions resulting from human activities. Previous work suggests that environmental factors, such as temperature and pH, can affect how mollusks build their shells. Certain mollusks continuously lay down tablets of nacre on the interior of their shells, and the thickness of these tablets may reflect the temperature of the environment in which the organism lived. We have been investigating spatial and temporal variation in tablet thickness in two groups of marine mollusks. Our first case study focuses on a variety of present-day abalone species along a temperature gradient. Data were collected from individuals raised in aquaculture at controlled temperatures, as well as wild-grown individuals from different latitudes in the Pacific Ocean. Our second case study focuses on the marine bivalve Nucula proxima from the northern Gulf of Mexico. Live and dead Nucula proxima specimens were collected from 20 meters water depth offshore Louisiana and Alabama; radiocarbon analyses indicate that these specimens represent both present-day and pre-Industrial populations. Our preliminary results show: 1) little variation in tablet thickness among abalone grown in aquaculture at different temperatures, suggesting that fine scale variation in temperature has little effect on abalone microstructure; and 2) an increase in tablet thickness during the past 250 years for N. proxima offshore Louisiana, but little change in N. proxima microstructure in coastal Alabama over past centuries.