Antarctic sea ice is vital for global climate regulation and local ecosystems. After decades of increase, Antarctic sea-ice extent and persistence has declined precipitously since 2014; declining sea ice has implications for global climate and threatens to upend light, phytoplankton, and disturbance regimes that structure the environments of unique coastal Antarctic benthos. Understanding sea-ice dynamics before satellite images requires paleoenvironmental proxies. The Antarctic scallop Adamussium colbecki is an attractive candidate to proxy sea-ice persistence because it is an ecosystem engineer, relatively long-lived, widely geographically distributed, and has a genus history in Antarctica to the Oligocene. This dissertation compares growth and chemistry of A. colbecki valves from two sites on western McMurdo Sound that differ by sea-ice duration: Explorers Cove has multiannual sea ice and Bay of Sails has sea ice that breaks out annually. Valve growth is slower for scallops living under multiannual sea ice. Yearly growth is also more variable under multiannual sea ice, suggesting locally heterogenous food distribution. Subannual growth lines (striae) are not good candidates for ontogenetic ageing of A. colbecki, but comparison of concurrently growing juveniles suggests that interstrial growth increments reflect environmental conditions, rather than individual biology. Striae are therefore suitable candidates for further environmental analysis. Stable isotopes of oxygen in A. colbecki valves are indistinguishable between the sites. In contrast, stable isotopes of carbon in narrow and wide striae groups reflect greater seasonality of nutrient conditions at the annual sea ice site and help differentiate valves living under annual and multiannual sea-ice conditions. Trace element concentrations form clear cycles over ontogeny in valves from the annual sea ice site, and cycles are absent in valves at the multiannual sea ice site. Covariation (wavelet coherence) between trace elements and interstrial growth increments and among individual trace elements is stronger for longer sections of ontogeny under annual sea ice than multiannual sea ice. Moreover, Mn/Ca and Pb/Ca may proxy sea ice. Growth and valve chemistry differ under annual and multiannual sea ice in A. colbecki in Antarctica, thus A. colbecki is a promising proxy for past sea-ice duration in Antarctica.