New paper out in Polar Biology!

Lifespan and growth of Astarte borealis (Bivalvia) from Kandalaksha Gulf, White Sea, Russia
David K. Moss, Donna Surge, and Vadim Khaitov

What we did:
We were interested in understanding lifespans and growth rates of the polar bivalve Astarte borealis from a small population in the White Sea, Russia (67° N, 32° E). Bivalves record growth lines in their shells that are similar to the rings of trees. However, bivalves complicate things a bit, because they can record growth lines at several different periodicities: daily, tidal, fortnightly, monthly, and annual. Therefore, you cannot cut a shell in half a count everything that is there. Oxygen isotopes though can be used to distinguish annual from non-annual growth lines. This is because the ratio of 18O/16O is partially dependent on temperature. A. borealis like many other clams makes its shell out of aragonite (CaCO3) and faithfully records the ratio of 18O/16O. Essentially, clam shells act like thermometers that keep a record of temperature while the organism was growing. Using this information, we can go back and understand at what time of the year different growth lines formed. When we did this with A. borealis we found that it records an annual growth line (dark line in Fig1 below) in the late summer.

In bivalves, annual growth lines represent a time of significantly slowed growth. They can result from a number of things but most often, they have to do with temperature stress or reproduction. In the White Sea, summer temperatures do not reach the known thermal maximum for A. borealis so we rule that out as a cause. Instead, my co-authors and I suggest that the summer shutdown might be a time of reproduction in this species. More work is need to confirm our hypothesis.
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Knowing which lines were annual allowed us to determine the lifespan and growth rate of A. borealis from the studied population. Though we had a small sample (n=18), we found that A. borealis is both slow growing and long-lived (oldest individual was 48 years). 

Fig 1. Top image: individual of A. borealis. Dashed line shows axis of maximum growth along which specimen was cut. Bottom image: polished cross-section of individual above captured using a light microscope; red arrows indicate annual growth lines. 

​Why is it important?
Before our study, only one studied had used oxygen isotopes to determine the lifespan of A. borealis. In that study, which looked at individuals from extreme northern Greenland (Torres et al. 2011), the authors found individuals up to 150 years old! Most other studies looking at lifespans of this species relied on ridges on the external surface of the shell, which can be misleading. Because A. borealis is so long-lived, it might be a potentially important archive of past climate conditions at high latitudes. In other words, they can act as thermometers before human records.  
Future questions
Our study in the White Sea is part of a larger one that looks at how the growth strategy (i.e., when do they form annual lines) vary on the planet today and what happens to that strategy when Earth’s climate changes. This information can inform us about what might happen to this, and other species, in the near future. We are currently examining this species from the Baltic Sea and closely related species from the Atlantic Coast of the United States.
 
Link to paper here: https://link.springer.com/article/10.1007/s00300-018-2290-9
Please email me ([email protected]) if you would like access to the pdf version.