surface. The degree of separation of
the fine striations seems to affeci the
differentiation of the dark and light
annular bands revealing age: the stria+ tions are finer and more closely packed
(about 15 , between centers) in the
dark, relatively opaque areas of Fig. !
than in the lighter, more translucent
regions (25 ,). Probablythe light areas
are deposited during seasons of warmest
water temperature. 29° to 30°C, which
occur at Bikini Atoll from August to
October, inclusive (8). Possibly a cool
summer accounts for the lack of a
distinct light area on the inside of the
shell in Fig. | even though it was collected on 22 August.
Tridacna gigas grows fast compared
to other molluscs. Wilbur and Jodrey
(//) estimated from uptake of Cat?
that the shell of the oyster, a relatively
fast-growing
mollusc,
increased
in
weight about 1 g per 70 cm* of surface
per month. The annual increment in
thickness of the giant clam shell under
consideration was approximately | cm.
With a shell density of 2.75 this would
yield
(1.0) (70) (2.75)/12 ='16
grams per 70 cm’ per month, or
16 times the growth rate of the oyster.
Annular bands on the onty other specimen (/2) of T. gigas sectioned to date
in this laboratory indicate growth approximately 1.5 times as fast, to a
length of 55 cm in only 6 years. Such
rapid growth, where length increased
by 5 to 8 cm per year, or by 5O cm or
more in from 6 to 9 years, probably
Fig. | (above). Transverse section, 6 mm in thickness. of 7ridacna gigas shell near the umbo. (Top) Basal portion to a
scale 2.5 times the lower figure. showing by stippled lines the
positions of the layers of radioactivity traced from the radioautograph (P. indicates the pallial mark). (Bottom) Entire
section.
Fig. 2 (above right). Radioauutograph of basal portion of section
of shell of Tridacna gigas shown in Fig. 1. Retouching was
limited to the dotted lines indicating the outlines of shell and
eroded areas. The two tight undulating Iines originating near
the umbo resulted from the beta activity of Sr” presumed to
have been deposited in 1956 and [9S8. respectively. Radioactivity ts abso evident tn the umbonat cleft and elewhere on
the outer surface as well as tn the eroded spaces. The white
Spot left of center resulted from Ca’ mixed
to label the section Sf (see Fig.
with
4
big. 3 right). Composite photomicrograph of
section ground to a thickness of 1% to 20 ua
he position indicated by dashes on the tight
(top). Length and width of section, 44 by 2
Hintious strip as divided for convenience into
farting
swith
the outside of the
dicates the pallial marke.
the
ink
used
1).
shell
at
the
thin) transverse
Removed trom
side of Fig 1,
mm The confour columns
upper
deft
€P
ine
gives the giant clam the distinction of
being the fastest growing of bivalves.
In order to identify the radionuclides
responsibile for the lines on the radioautographs, a strip about 7 mm wide
along the 1958 line of radioactivity was
bandsawed and chipped from the shell
section shown
in
Fig.
1. top.
The
broken pieces of this
excised strip.
weighing 14 gs. yielded 8 net counts
per minute in a low-level, anticoincidence gross beta counter of 0.4 geometry with a background of 0.8 count
per minute. There was no detectable
peak above background even after 3900
minutes of counting on a 256-channel
gamma spectrometer with a 7.6-cm sodium iodide crystal detector. The material was finally analyzed for Sro-Y°",
considered the most likely radionuclides