Agric. Biol. Chem., 48 (5), 1117-1121, 1984 1117 Body Burdens and Distribution of Mercury and Selenium in Striped Dolphins Kazuomi Itano, Shin'ichiro Kawai, Nobuyuki Miyazaki,* Ryo Tatsukawa** and Toraya Fujiyama*** Osaka City Institute of Public Health and Environmental Sciences, 8-34, Tohjo-cho, Tennoji-ku, Osaka 543, Japan *Department of Zoology, National Science Museum, 3-23-1, Hyakunin-cho, Shinjuku-ku, Tokyo 160, Japan **Department of Environment Conservation, Ehime University, 3-5-7, Tarumi, Matsuyama 790, Japan ***Department of Marine Sciences, University of the Ryukyus, Nishihara-cho, Okinawa 903-01, Japan Received May 16, 1983 Weexamined variations in the burden of total mercury and selenium with age in striped dolphins (Stenella coeruleoalba). The amount of total mercury accumulated in the whole body increased with age and reached a constant level at about 16 years of age. The constant total mercury burden level in males was slightly higher than that of females. A similar tendency was observed for the body burdens of methylmercury in males and females. The body burden of selenium remained constant for the dolphins of ages between 5 to 12 years, and after that it sharply increased with growth until about 17 years of age. The sum of total mercury in muscle, liver and blubber was about 90% of that in the whole body. Approximately 90% of the methylmercury in the whole body was in the muscle. The total selenium content in muscle, liver and blubber comprised 67 ~ 80%of that in the whole body of immature individuals and mature males. The accumulation of selenium in various tissues varied among mature males, pregnant females and lactating females. The con? centration ratio of total mercury to selenium was lower in females at the later gestational stage than in mature males, and the ratio wasthe highest in females at the early gestational stage and lactating females. The change in mercury level with age, body weight and size of marine mammalshas been extensively examined in recent years.1 ~3) But there is a great lack of information on the amount of mercury accumulated in marine mammals. For estimating the bioconcentration and the fate of mercury in marine mammalsfrom their food, the mercury concentration in mammalson a whole-body basis is a necessary piece of information. However, it is difficult in practice to determine the concentration or burden of mercury in the case of large animals such as marine mammals. Therefore, the fate of mercury accumulated in the tissues of marine mammals in natural environments has been little studied. In our preceding paper,4) we reported the levels of mercury and selenium in various tissues of striped dolphins (Stenella coeruleoalba). In this paper, we present the changes of mercury and selenium contents in whole bodies of dolphins with age, and the distribution of both elements in dolphins of different ages. MATERIALS AND METHODS The striped dolphins used in this study were collected in December 1978 and 1979 at Taiji, Kii Peninsula, on the Pacific coast of Japan. Each tissue removed from individuals was precisely weighed. The amounts of mercury and selenium accumulated in the tissues were determined from the tissue weight and concentrations of the two elements.4) The concentrations of total mercury and selenium were uncorrected for recovery. The total burdens of mercury and selenium were estimated as the sum of accumulated amounts in all tissues except skin, eyes and teeth.
1118 K. Itano et al. RESULTS Changes with age in mercury and selenium contents of the whole body Figure 1 shows the relationship between the age and mercury content of the whole body. The total mercury burden reached a constant level at about 16 years of age. This is because both body weight and total mercury concentration leveled off above 17 years of age.5) The constant total mercury burden level in males was slightly higher than that of females. The body burden of methylmercury also increased with age, and the amounts of methylmercury in males and females reached a plateau after about 16 years of age. The relationship between the age and selenium content in the whole body is shown in Fig. 2. The body burden of selenium remained 200 /. F3100 /,,-: " J^' 10 15 20 25 30 35 40 Age(years) Fig. 1. Variation of Mercury Contents of Striped Dolphins with Age. Total mercury: å, immature; #, mature males; æf, mature females. Methylmercury: D, immature; O, mature males; O, mature females. 5 10 15 20 25 30 35 40 Age(years) Fig. 2. Variation of Selenium Contents of Striped Dolphins with Age. å, immature; #, mature males; æf, mature females; Fx, lactating female ; F2, pregnant female (fetus, 0.23 kg) ; F3, pregnant female (fetus, 4.63kg); F4, pregnant female (fetus, 10.0kg). Table I. Distribution and Concentration of Total Mercury in Striped Dolphins T-Hg, total mercury.
Mercury and Selenium in Striped Dolphin 1119 constant for the animals of ages between 5 to 12 years. The selenium burden in males over 12 years of age increased with the increase of the mercury amount until about 17 years of age, and after that it attained a constant level. The selenium burden in pregnant females increased with growth of the fetus. Distribution of mercury and selenium in tissues Table I shows the distribution of total mer- Table II. Distribution and Concentration of Methylmercury in Striped Dolphins The ages of samples are shown in Table I. M-Hg, methylmercury; T-Hg, total mercury. Table III. Distribution and Concentration of Selenium in Striped Dolphins The ages of samples are shownin Table I. Se, selenium; T-Hg, total mercury; c.r., concentration ratio; Fl9 F2, F3 and F4, see the footnote to Fig. 2.
1120 K. Itano et al. cury in the muscle, liver and blubber of striped dolphins of different ages. The amount of total mercury was the highest in muscle, followed by liver and blubber in both sexes. The ratio of tissue weight to body weight of dolphins is 50-57% for muscle, 14-19% for blubber and 1.2-1.7% for liver. The tissue distribution of total mercury was found to be different between the developmental stage and the saturated stage in body weight for immature individuals and mature males. The sum of total mercury in the three tissues comprised about 90% of the total amount in the whole body. No remarkable difference was observed between the concentrations of total mercury in the whole bodies of mature males and females. As shown in Table II, approximately 90% of methylmercury in the whole body was distributed in the muscle. The methylmercury concentration in mature females on a whole body basis showed a slight fluctuation. Table III presents the selenium distribution in muscle, liver and blubber of striped dolphins of different ages. The selenium distribution in the above tissues for immature individuals and mature males was found to be classified into three groups; 57 to 96.3, 106 to 145 and more than 160kg in body weight. For mature individuals, the order of selenium amounts in muscle, liver and blubber also varied among mature males, pregnant females and a lactating female. The concentration of selenium in the whole body showed relatively small differences regardless of the age and sexual condition. The concentration ratio of total mercury to selenium was lower in females at the later gestational stage than in mature males. The highest ratio was observed in the female at the early gestational stage and the lactating female. DISCUSSION Little work has been done on the uptake of mercury and selenium by marine mammals from their food, because of the difficulties of examining the total burdens of these elements in the whole bodies of mammals.the average concentration ratio of total mercury to selenium in mature striped dolphins was 6.9. The concentration ratio for mature male dolphins was about 75 times higher than that for squids (Table IV), one of main foods of the dolphins. This suggests that the bio-concentration factor of mercury is muchgreater than that of selenium. The selenium concentrations in striped dolphins were at approximately the same level as those in the main foods of the dolphins. The selenium concentration on a whole body basis may not be very different among marine animals. A methylmercury poisoning incident in Iraq6) allowed clinical investigation of the threshold body burden of methylmercury in man. The threshold burdens of methylmercury for the onset of paresthesia and death were calculated to be approximately 25 and 200 mg, respectively, for an average body weight of 51 kg. Similar threshold values for paresthesia have been estimated by the Swedish Expert Committee from data for Japanese incidents. The concentration of total mercury in the mature striped dolphin was 2.5 times as high as Table IV. Total Mercury, Methylmercury and Selenium Levels in Whole Bodies of Squid and Myctophid Collected off the Pacific Coast of Japan A principal food of striped dolphins is the fish, Diaphus coelucens, ll to 13cm in total length. The sample was pooled from ten fish.
Mercury and Selenium in Striped Dolphin 1121 the fatal concentration reported for man. The methylmercury level of striped dolphin also reached the fatal level for humans. In spite of the fact that the concentration of methylmercury in the dolphin was at a level expected to be fatal, symptoms of mercury poisoning were not evident. These pieces of information suggest that selenium may offer protection against mercury poisoning. The concentration ratio of total mercury to selenium was lower in females at the later gestational stage than in females at the early gestational stage and the lactating stage, although their total mercury burdens were not appreciably different. The selenium burdens in pregnant females increased with the growth of the fetus. From these results, selenium seems to be accumulated in various tissues during the gestational stage and transferred from the mother to the suckling through lactation. Our observation of selenium accumulation by the gestational stage was inconsistent with the result for humans.7) Differences in mercury amount amongtissues were observed in striped dolphins. Muscle was the main tissue for mercury accumulation. The tissue distribution of mercury in dolphins varied between the developmental stage and the saturated stage in body weight. Particularly, the mercury amountin the muscle increased at the saturated stage. Almost all of the extra mercury was present in the inorganic form. The biological half-lives of inorganic mercury in rats, mice and freshwater fishes were reported to be shorter than those of methylmercury.8~10) However, Parizek et al.n) showed that selenium in rats injected with equimolar doses together with mercuric chloride could change the tissue distribution of mercury and increased the body retention of mercury. From these results, the inorganic mercury that is present together with selenium in the muscle of older striped dolphins is considered to be in a steady state. Tillander et al.12) found that in a ringed seal dosed orally with methylmercury proteinate, the organic mercury had a biological half-life of about 500 days. Gaskin et al.3) supposed that the half-life of mercury was up to 700+ days from a study of wild marine mammals. The coaccumulation of mercury and selenium in muscle was not observed in marine mammalsother than the striped dolphin.1'3'40 Consequently, the excretion rate of methylmercury taken with the food in older striped dolphins seems to be slow in comparison with that of other marine mammals. Acknowledgment. This study was supported in part by a Grant-in-Aid for Scientific Research (project No. 343056) from the Ministry of Education, Science and Culture of Japan. REFERENCES 1) J. H. Koeman,W. S. M. vandeven,j. J. M. de Goeij, P. S. Tojioe and Environ., 3, 279 (1975). J. L. van Haaften, Sci. Total 2) P. J. H. Reijnders, Netherlands J. Sea Res., 14, 30 (1980). 3) D. E. Gaskin, K. I. Stone field, P. Suda and R. Frank, Arch. Environ. Contam. ToxicoL, 8, 733 (1979). 4) K. Itano, S. Kawai, N. Miyazaki, R. Tatsukawa and T. Fujiyama, Agric. Biol. Chem., 48, 1109 (1984). 5) N. Miyazaki, Y. Fujise and T. Fujiyama,' Sci. Rep. Whales Res. Inst., 33, 27 (1981). 6) F. Bakir, S. F. Damlujio, L. Amin-Zaki, M. Murtadha, A. Khalid, N. Y. Al-Raw, S. Tikriti, H. I. Dhahir, T. W. Clarkson, J. C. Smith and R. A. Doherty, Science, 181, 230 (1973). 7) C. Casey, B. Guthrie, G. M. Friend and M. F. Robinson, Arch. Environ. Health, 37, 133 (1982). 8) Y. Kido, G. Urakubo and A. Hasegawa, /. Hyg. Chem., 14, 76 (1968). 9) S. Yamanaka, K. Ueda and T. Yoshida, Jpn. J. Hyg., 28, 582 (1974). 10) S. Yamanaka and M. Nishimura, Jpn. J. Hyg., 37, 871 (1983). ll) J. Parizek, I. Ostadalova, J. Kalouskova, A. Babicky and J. Benes, "Newer Trace Elements in Nutrition," ed. by W. Mertz and W. E. Cornatzer, Marcel Dekker Inc., New York, 1971, pp. 95-96. 12) M. Tillander, J. K. Miettinen and I. Koivisto, "Marine Pollution and Sea Life," ed. by M. Ruivo, Fishing News (Books) Ltd., England, 1972, pp. 303-305.