LIFESPANS OF ARCTICA ISLANDICA FROM THE PLIOCENE TJÖRNES BEDS OF ICELAND

Arctica islandica is a species of living marine bivalve that is native to the North Atlantic Ocean. It is also the longest-living non-colonial animal known to science, with the oldest recorded specimen living to be 507 years old. Extreme longevity in polar regions is not exclusive to Arctica and has in fact been documented in other polar species, including the Greenland shark (~400). Bivalves overall are documented to live longer and grow slower at high latitudes in comparison to low. This begs the question - why is it that organisms tend to live longer at high latitudes? There are two environmental hypotheses - cold temperatures and light/food limitation, both of which work through bringing about a reduction in metabolic rate.

To help answer this question, we examined fossil specimens of Arctica islandica from the Tjörnes Beds in northeast Iceland, which date to the mid-Pliocene Epoch. This same species is found in the same area today and routinely lives upwards of 200 years or more. Although the Pliocene was much warmer than today, the latitude and hence seasonal light availability of Iceland has remained the same; thus, the lifespans of these fossil specimens could shed light on the relative importance of temperature and sunlight availability in affecting lifespan.

Arctica, like all bivalves, grows via accretion, and their shells contain growth bands that can mark yearly intervals. We sectioned and polished 11 specimens and counted the number of growth bands in order to constrain lifespans. Two shells were micromilled for stable oxygen isotope variation, and data confirms that growth bands are annual and form in the late summer/fall; growth is fastest in the spring and summer. Seasonally resolved, isotope-based paleo-sea surface temperatures, assuming a latitude-corrected Pliocene water composition, range from 3℃-10℃ in one specimen and 5℃-13℃ in another. Winters are comparable to Iceland SSTs today but warmer than pre-industrial. Summer temperatures are warmer, and seasonally greater, than present.

Growth bands thus far reveal lifespans of about 100 years, on par with Arctica populations off the coasts of continental Europe and the United States. These locations are lower latitude and warmer than Iceland today, supporting the hypothesis that temperature plays a role in enhancing longevity.