coral reefs, no studies om their rate of
growth have been reported. The rate of
growth of T. gigas over a period of
several vears can now be estimated
by reading the annulations in the shell
and relating them to marks introduced
by radioactivity from nuclear detonations.
Although the ability ef 7. gigas to
Growth Rate of Giant Clam
Tridacna gigas at Bikini Atoll
as Revealed by Radioautography
Abstract. At Bikini Atoll, radicactivity from strontium-90 deposited in the
growing Shell of a viant clam, presmim-
ably during the testing of nuclear
weapons in 1956 and 19588, produced
unmistakable lines on radioautographs
made from transverse sections of the
shell. The reeular banding seen in the
sections
is
interpreted
as
annular
in
nature, One annulus precedes the [956
laver of radioactivity, tvo intervene in
P9858. and six
follow to the time ot
collection. so that this clam Geneth, S>
centuneters} was farts Oth vear of Hfe.
Written
records of the giant clam
Tridacna sivas
Linné have existed
for
centuries. Considered remarkable at
first simply because of its large size
(2 min greatest length and several hundred kilograms in weight). the chint was
later (7) found to contain symbiotic
algae within its tissues. Yonge proposed
that the clam attained such size by
“farming” the zooxanthellae within the
greatly expanded tissues of the siphons,
and by utilizing the photosynthetic
products in nutrition (2-4). However.
in spite of an almost universal curiosity
about the age of these giants of the
concentrate Col’ in its soft parts has
been emphasized (5). dittlhe ts Known
concerning the uptake of radionuclides
by the shell. which ws shown here to contain Sr". Te elucidate the pattern of
deposition of nuclides in the shell after
nuclear detonations, one valve of a
specimen 52 cm in length (6) fem
Bikini Atoll was transversely sectioned
with a Si-cm circular dramond saw.
Figure 1 shows a section, 6 mm in
thickness. from the region immediately
anterior to the umbo. Figure 2 shows
a radioautograph resulting from exposing the section to “Ne Sereen” x-rav
film for a period of 3 months. Two lines
each about 2 mm wide. representing
lavers of radioactive matertal. appeared
on the film. Other sections farther fram
the umbo alse showed these marks. Records (7) reveal that tests of nuclear
devices were conducted at Bikini Atoll
only in 1946. 1954, 1956, and 1958. It
is reasonable to attribute the lavers of
radioactivity tO the twa most recent
test series, The 1956 Redwing series
at Bikini extended fram 2700 Mav
through 20 Judy, and the 1958 Hardtack
series from 11 May through 22 July.
The positions of the layers containing radioactivity. were determined by
superimposing the radioautograph on
the shell section, and are shown us
stippled lines in Fig. |. top. This view
by transmitted Hight) accentuates the
conspicuous alternating dark. relatively
opague. fayers, as contrasted with the
lighter, more translucent bands. clearly
indicating apparent vears of age. Up
to the 1986 line the clam was in its
first vear of lite. Two years intervene
between the twe stippled dimes. to TOSS,
and then six more years to the inner
surface of the shell representing 1964.
so that the clam was in its 9h year The
1956 line corresponds to a shell length
of about 10 cm, and the !9S8 fine, to
about 274 cm.
It is of special interest that a tropical
organism living in Water with a mean
monthly temperature varving fess than
38°C (8) throughout the vear should
display distinct annulations, Seasonally
varving environmental factors other
than temperature, such as winds
cur-
rents. weather, Hight. and the abundance
of
planktonic
food,
could
influence
growth, At Bikini Atoll the relatively
constant winter trade winds from the
east are frequently interrupted in summer by other winds. particularly from
the south (9). and surface currents
would be similarly influenced.
Spawning is probably of a seasonal
nature and thus may influence shell
growth. Yonge (2) cites the spawning
of the closely related genus Aippopus
in January of the Australian summer
and gives 30°C as the minimum temperature for spawning of the giant clam
(3). Wada (70) reported that Tridacna
collected in the Palau Islands in April.
Mas. and June of $938, 1940. and 1941
frequently discharged sperm and cggs
when brought into the laboratory. although he satd nothing of those colJected in other seasons.
During growth. aew shell material is
added exclusively on the inside. Altheugh the mantle is attached only at
the pallial sinus, it: contacts and deposits new material (aragonite) upon
the entire inner surface of the shell.
The extrapallial pertion of the shell,
distal to the pallial sinus and comprising about half of the total inner
surface. is prismatic. while the central
basal part of the shell is nacreous. In
Macroscopic views af sections (Fig. 1,
hettom) the distal. prismatic part ts
relatively Opaque and shows only faint
favering: the central. nacreous part is
more translucent and distinctly layered.
The two areas are clearly demarcated
hy a boundary laver leading from the
basal edge of the existing pallial sinus
obliguely through the shell toward the
umbo at the base (Fig. 1, P).
Figure 3 shows a low-power phatomicrograph obtained by using crossed
polaroid discs of a thin (PS to 20 p)
shell section at the position indicated by
the dashed lines of Fry. 1 (tops. Although Fig. 3 shows the outer border
of the shell at the upper left, it does not
extend to the inner border. The prisMatic outer layer occupies the first and
most of the second column of photographs down to the sloping light area
which is the pallial layer marked P.
while the rest of column 2 and all of
>
columns 3 und 4 consist of nacre The
prismatic layer is camposed of vertical
columns about 45 py in thickness disposed normally to the outer surface
of the shell The nacreous layers below
are more tirregalar. with only slight
or
localized
indications
of vertical
striae, but with both primary, coarse
layering and fine striations orientedsapprosmmately parallel to the tnagr shell