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The Unsuspecting Thermotolerance of an Antarctic Bivalve

Paper Reviewed
Waller, C.L., Overall, A., Fitzcharles, E.M. and Griffiths, H. 2017. First report of Laternula elliptica in the Antarctic intertidal zone. Polar Biology 40: 227-230.

One of the most feared concerns about CO2-induced global warming is that temperatures will rise so fast and furiously that many stenothermal organisms (those capable of living only within a limited range of temperatures) will be driven to extinction. As a result, many laboratory studies have been conducted on these types of species to determine their vulnerability to future temperature rise.

One such example is the soft-shelled burrowing bivalve Laternula elliptica, which inhabits marine waters of the Southern Ocean surrounding Antarctica. According to Waller et al. (2017), approximately "75 percent of the recorded specimens [of L. elliptica] are from localities shallower than 100 m," where the populations are exposed to "low and stable water temperatures in the range of -1.9 to +1.8 °C" (the remaining 25 percent inhabit cooler waters of the continental slope down to ~700 m). Experimental analyses conducted in the laboratory have revealed that this saltwater clam suffers "50 percent failure in essential biological activities at 2-3 °C and complete loss of function at 5 °C," rendering L. elliptica a fine example of a stenothermal organism -- or so it was thought!

During the austral summer of 2007, Waller et al. sampled the intertidal or littoral zone (region of the coast that is submerged at high tide, but above water and in the air at low tide) at locations along James Ross Island, East Antarctic Peninsula, writing that "prior to this study, there have been no reports of [L. elliptica] animals surviving the more variable environmental conditions of the littoral zone south of the Antarctic Circumpolar Current." To their great surprise, however, they report finding specimens of this supposedly stenothermal clam at densities "similar to many subtidal locations," ranging in age from one to eight years.

Commenting on their findings, the five United Kingdom researchers state that "the presence of this species in intertidal sediments raises questions about their physiological tolerances and capacity to cope with warming sea temperatures." Indeed, for at the time of their collection by Waller et al., temperatures within the sediment were measured at 7.5 °C while air temperatures were even greater at 10 °C -- both values far above laboratory-defined tolerance limits! This discrepancy between laboratory and field temperature tolerances, in the words of the authors, "has major implications for our understanding and interpretation of the physiological tolerances of Antarctic shallow water marine organisms. If one of the best-studied model species can be found surviving far beyond its predicted environmental envelope, then our use [of] laboratory-based experimental results may underestimate the ability of polar organisms to cope with environmental change."

And so it is that laboratory-based analyses showing negative impacts of rising temperatures on stenothermal organisms should be taken with a large grain of salt; and we can be much more optimistic of their future survival.

Posted 7 April 2017