Growth Rate of Giant Clam Tridacna Gigas at Bikini Atoll as Revealed by Radioautogrphy (Reprinted from Science, July 16, 1965, Vol. 149, No. 3681, Pages 300-302)

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Growth Rate of Giant Clam Tridacna gigas at Bikini Atoll as Revealed by Radioautography l{eprintecl from Science, July 16, 1965, Vol. 119, So.3681, pages 300-302

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Growth Rate of Giant Clam Tridacna gigas at Bikini ,itoll as Rwcakd by Radioaotograph} coral reefs, no studies on their rate of growth have been reported. The rate of several years can now be estimated by reading the annulations in the shell and relatln: them to marks introduced hccn emphasized (5). little is knotln conccrntng the uplak of radio nr.lclide~ dcpmition 01 nuclidm in the shell :iftcr Figure 1 sho~~s a waion. 6 [mm in thick ncs~. from Ihc region immcdiaielv each about 2 mm uidc. reprcseni in: on Ihc filnl. Other sections farther from the umho also showccl Ike mar~s. Redevices were conducted at Bikini .A[olt lighter. more translucent fmrxis. clcarlv 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 [hc south (9). and surface currents perat Llrc for ~p;iv.nin: of the giant clam c{)l[cctcd In rhc Palau Islands in April. Ila>. and June of 193S, 1940. and 1941 though he said nothing (>f those collected tn olhcr seasons. During growth. ncw shell material is added exclusively on the inside. Although lhc mantk is :ittached only at the cnlirc inner \ur Jacc <~1 the shell. I;)! cring: lhc central, nacrcous part is umbo at the haw ( Fiy. 1. f’). microgrzph ohtoinul by using crossed pol;lroid Jiscs Of a thin ( 15 to 20 p ) of the shell at [he (rppcr left. it Lioc\ not extend ro the inner h(~rdcr. The pris-

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surface, The degree of separation of the fine striations seems to affec.i the differentiation of the dark and light annular bands revealing age: the striations are finer and more closely packed (about 15 ~,. between centers) in the dark, relatively opaque areas of Fig. 1 than in the lighter, more translucent regions (25 14). Probably the light areas are deposited during seasons of warmest 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. 1 even though it was collected on 22 August. Triducna :~igm grows fast compared to other molluscs. Wilbur and Jodrey (11 ) estimated from uptake of Ca43 that the shell of the oyster. a relatively fast-growing mollusc. increased in weight about 1 g per 70 cm2 of surface per month, The annLlal increment in thickness of the giant ciam shell under consideration was approximately I cm. With J shell density of 2.75 this would grams per 70 cm2 per month, or 16 times the growth rate of the oyster. Annular bands on the only other speciin this laboratory indicaie growth approximately l.5 times m 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 50 cm or more in from 6 to 9 years, probably gives the giant clam the distinction of being the fastest growing of bivalves. In order to identifv the racfion Llclides responsible for the lines on the rxlioautographs, a strip about 7 mm wide along the [958 Iinc of radi(>;lctivity was bandsawed and chipped from the shell section shown in Fig. 1. top. The broken pieces of this exciwd \trip, weighing 14 g. yiekkxf 8 net counts per min Llte in a low-level. anticoin - cidence gross beta counter of ().4 geometry with a background of 0.8 count per minute. There was no detectable peak above backgroLlnd even after 3900 minutes of counting on a 256-channel dium iodide crystal detector. The material was finally analyzed for Sr’’’’-Y””. considered the most likely radionuclides K:IIC 2.5 times the Iowcr figure. \howing by \tipplcci Iincs the

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because of the absence of gamma activity and because of the association of Sr with ca. Beta-counting of the integrations per minute per gram of shell material, and the decay rate was appropriate for Y{)’). Thus, although there were undoubtedly other radionuclides in the shell shortly after the detonations, physical decay left deLaboratory of Radiation Biclogy, University 0/ Washincqton, Seattle 5 References and Notes English translation by W, S. Dallas, Ann. seum (Natural History). London. 19361. vol. 3. —, Proc. Zool. Sot. London 123, 551 6. Soecimen 302 was the smallest of six ranging to 73 cm in length collected between 2 and 4 p.m. on 22 August 1964 about 100 m off the north point of Airukiiji Island in water 1 to 3 m deep. On the following day the soft tissues were removed for radioassay. The dry half-shell, brought to Seattle, was sectioned about 11 a.m. on 12 August 1964 in water less than I m deep within 20 m of the north shore of Engebi Island, Eniwetok Atoll. Work done under contract AT(45-I)138S with the AEC. . thank also Mr. Egil Oas for preparing the thick and thin sections of the sheil, Mks Lorna Matson for the radioautographs, and Dr. Grant Gross for identification by x-my diffraction of aragonite in