Digging Into Clam Biochemistry
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You'll recall the arctic quahog - a species of bivalve clam - has a maximum lifespan somewhere north of 400 years. Other bivalve species fall into the 30 to 100 year range, with a fair degree of uncertainty. While you can age a bivalve by the growth rings in its shell, there are a lot of different species, and not that many people out there performing a rigorous analysis of growth rings. The range of 30 to 400 years is not out of line with the range of life spans in mammals, and one might expect to see similar biochemical reasons for differences in life span between bivalve species. In mammals, it appears to have a lot to do with mitochondria:

In animal cells, mitochondria are semiautonomous organelles [with] their own code and genome (mtDNA). The semiautonomy and restricted resources could result in occasional 'conflicts of interests' with other cellular components, in which mitochondria have greater chances to be 'the weakest link,' thus limiting longevity. ... (1) to what extent the mammalian maximum life span (MLS) is associated with mtDNA base composition? (2) Does mtDNA base composition correlate with another important mitochondria-associated variable - resting metabolic rate (RMR) - and whether they complement each other in determination of MLS? ... Analysis of 140 mammalian species revealed significant correlations ... To the authors' knowledge, it is the highest coefficient of MLS determination that has ever been reported for a comparable sample size. Taking into account substantial errors in estimation of MLS and RMR, it could mean that [this explains] most of the MLS biological variation. [This leads us to] mitochondria as a primary object for longevity-promoting interventions

Back to the bivalves: you can do more with growth rings than count them, as this research paper demonstrates. Growth rings contain a record of some aspects of the biochemistry of the clam over its life:

The ocean quahog Arctica islandica is the longest-lived of all bivalve and molluscan species on earth. Animals close to 400 years are common and reported maximum live span around Iceland is close to 400 years.

High and stable antioxidant capacities are a possible strategy to slow senescence and extend lifespan and this study has investigated several antioxidant parameters and a mitochondrial marker enzyme in a lifetime range spanning from 4-200 years in the Iceland quahog.

In gill and mantle tissues of 4-192 year old A. islandica, catalase, citrate synthase activity and glutathione concentration declined rapidly within the first 25 years, covering the transitional phase of rapid somatic growth and sexual maturation to the outgrown mature stages ( approximately 32 years). Thereafter all three parameters kept rather stable levels for > 150 years. In contrast, superoxide dismutase activities maintained high levels throughout life time.

These findings support the 'Free Radical-Rate of Living theory', antioxidant capacities of A.islandica are extraordinarily high and thus may explain the species long life span.

A little explanation here:

  • Mitochondria churn out damaging free radicals as a side-effect of their job as the cell's power plants. The chain of biochemical events that follows on from this fact is a major component of age-related damage, disease and degeneration. You can look back into the Fight Aging! archives for an introduction to that topic.

  • It has been demonstrated that soaking up the free radicals produced by mitochondria right at the source extends life span. This has been achieved by means of antioxidants like catalase, targeted to the mitochondria by gene therapy or other bioengineering means. This is quite different from taking antioxidants as a supplement, I should add; those don't go anywhere near your mitochondria, and thus don't do much good.

  • So it's reasonable to theorize that if you happen to be a member of a species that naturally generates a lot of antioxidants around the mitochondria, you're going to live longer than members of another, similar species with worse luck in the antioxidant stakes.

It remains to be seen if this is enough to explain all of the disparity in bivalve life spans. Certainly, researchers are aiming in that direction to explain the ninefold difference in life span between naked mole-rats and other rodents of similar size, so it seems at plausible at this point in time. Personally, I hope that researchers demonstrate that mitochondrial biochemistry accounts for a very large chunk of aging and relative life span, given that the medical technology needed to repair and upgrade our own mitochondria is right around the corner.

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