Heather Hawk’s question about radiometric verification of the age of elderly clams reminded me of an old but fine study by my late UW colleague Kelly Bonham. He transversely sectioned the shell of a 52-cm-long Tridacna gigas he collected at Bikini Atoll in 1964. He placed the sectioned surface against an x-ray film in a dark closet on the campus here for three months. At the end of this period two lines, each about 2mm wide, paralleling the then inner surface of the shell, appeared where radioactive decay had exposed the film. He traced these bands onto the sawn surface of the shell and reproduced this image as Fig. 1 in his paper (K. Bonham, Growth rate of giant clam Tridacna gigas at Bikini Atoll as revealed by radiography. Science 149:300-302, 1965). The two radioactive bands, determined to be from Sr90 incorporated in the shell in place of Ca, must have represented the last two Bikini
H-bomb tests, in 1956 (Operation Redwing) and 1958 (Operation Hardtack). (Incidentally, somewhere I still have my Operation Redwing film badge, as I was doing research on Conus at Enewetak at that time) The positions of the radioactive bands paralleled the growth bands in the shell. The 1956 band occurred in the clam’s first year (when it attained a length of 10 cm). The 1958 band occurred in its third year, when it was 24 cm long. Two dark and one light growth bands occur between the radioactive bands, and 6 more dark growth bands occur between the 1958 radioactive line and the inner edge of the shell in 1964, when the clam was sacrificed, in its ninth year at a length of 52 cm. The relationship between the radioactive markers and the growth bands thus substantiates the latter as annual, even in the tropical Pacific.

The T. gigas growth rate resulting from Bonham’s observations is consistent with David Campbell’s estimate of a life span “in the low decades.” Bonham gives the maximum size of T. gigas as 2m; David, as 4 feet. I measured a shell that for years was mounted in concrete at the Kwajalein Airport; it was 5 feet long. And David: do I recognize your epigram’s author correctly as Ivar Haglund?

Alan Kohn



On Sun, 4 Nov 2007, Heather Hawk wrote:

> Is there, indeed, any evidence that longevity in these clams is related to
> depth or shell size?  Has there been any radiometric verification of the
> ages of these "Killer" and other large/old species?  When including
> bomb-radiocarbon- and lead-dating, there are sometimes great discrepancies
> among aging techniques of fish and molluscs; esp. compared with
> ring-counting; and esp.  in deep-sea species.  As any tropical forest
> ecologist will tell you, growth regions (in trees as well as calcareous
> mollusks) don't always equal years when there are no annual growth seasons
> to which the organism responds.
>
> Heather L. Hawk
>
> Graduate Student
> Moss Landing Marine Laboratories
> 8272 Moss landing Road
> Moss Landing, California 95039 USA
> 678-570-8990
> [log in to unmask]
>

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