More about old clams:

In March 1989 Dr. Douglas S. Jones, now director of the Florida Museum
of Natural History contributed an article to American Conchologist,
"Growth Rings and Longevity in Bivalves." In it he confirms most of what
you say, Rick and tells us more about how science knows this. I quote
liberally from Dr. Jones' article below:

"In almost every instance, bivalve ages that are based upon annual,
internal growth increments are far more accurate than other estimates.
In addition the number of annual increments suggests lifespans
substantially greater than had previously been imagined. For example,
consider the case of Spisula solidissima, one of the largest and most
common bivalves from Canada to Cape Hatteras. On the basis of external
shell rings this clam was once thought to live about seventeen years.
Counts of documented yearly growth increments from shell cross-sections,
however, reveal that individuals may actually live over 30 years.

"Equally famous among seafood lovers is the New England Softshell or
Steamer Clam, Mya arenaria. Once they were thought to live for only a
few years, but growth increment analysis has extended the known life
span of this clam to at least 28 years. The story is much the same for
Mercenaria mercenaria, the Hard Clam or Northern Quahog (perhaps more
familiar as cherrystone, little neck or chowder clam -- depending on its
size). Scientists recently reported finding two live specimens that had
been tagged 33 and 36 years ago and that possessed 33 and 36 growth
increments, respectively. Large specimens from Rhode Island that lived
over 50 years, occasionally approaching ages of 75 years, have been
documented through growth increment analysis.

"Soviet marine biologists have employed these age determination
techniques with similar results. A study of lifespans of common bivalves
from the far eastern seas of the USSR reported that over half the
species had lifespans in excess of 20 years. Many were found to live
beyond fifty years.
Although the idea of clams living for 20, 50 or 75 years seems a bit
difficult to accept, the most surprising result of this entire research
effort was the discovery of several bivalve species with over 100 annual
shell growth increments. Naturally, substantial independent
corroboration was sought to verify these age assessments, and the
results largely have been upheld. A decade ago, Margaritana
margaritifera, the European Freshwater Mussel, was considered to be the
longest-living invertebrate, with an estimated lifespan of 100 years.
Today it is but one of many bivalve centenarians, and definitely not the
longest lived.

"For example, specimens of Panope generosa [now known as Panope abrupta
(Conrad, 1849)] the geoduck (pronounced gooey duck) clam, harvested
commercially from the West Coast of the United States, have been found
to possess 120 annual increments. On the other side of the Pacific,
Crenomytilus grayanus from Peter the Great Bay is reported to have the
greatest lifespan encountered among mollusks from the USSR, 150 years.
Perhaps the slowest-growing centenarian bivalve, however, comes from the
deep sea. The diminutive Tindaria callistiformis grows extremely slowly
in the cold dark world it inhabits, attaining a size of only 8.4mm in
100 years.

"The Ocean Quahog, Arctica islandica, currently holds the longevity
record for bivalves as well as for all non-colonial invertebrates, and
may, in fact, be the longest-lived animal. Individuals dredged from the
middle Atlantic continental shelf often show over 150 annual growth
increments. One specimen had 220! Because of these unusually high age
estimates, mark-and-recovery experiments were supplemented by
radiometric dating techniques to test the yearly periodicity of the
internal growth rings. The results verified the annual nature of the
rings and confirmed the conclusions regarding age. The longest-lived
animal on earth may well be a bivalve!"

American Conchologist 17(1): 12,13

The remainder of the article (the first half, actually, and it is
available if anyone is interested) deals with the measurement of
incremental growth rings in clams as a method of measuring bivalve age,
analogous to counting growth rings in trees.

Lynn Scheu
Louisville, KY