Postnatal exposure to methyl mercury from fish consumption: a review and new data from the Seychelles Child Development Study

BACKGROUND

Fish is an important source of nutrition worldwide. Fish contain both the neurotoxin methyl mercury (MeHg) and nutrients important for brain development. The developing brain appears to be most sensitive to MeHg toxicity and mothers who consume fish during pregnancy expose their fetus prenatally. Although brain development is most dramatic during fetal life, it continues for years postnatally and additional exposure can occur when a mother breast feeds or the child consumes fish. This raises the possibility that MeHg might influence brain development after birth and thus adversely affect children's developmental outcomes. We reviewed postnatal MeHg exposure and the associations that have been published to determine the issues associated with it and then carried out a series of analyses involving alternative metrics of postnatal MeHg exposure in the Seychelles Child Development Study (SCDS) Main Cohort.

METHODS

The SCDS is a prospective longitudinal evaluation of prenatal MeHg exposure from fish consumption. The Main Cohort includes 779 subjects on whom recent postnatal exposure data were collected at the 6-, 19-, 29-, 66-, and 107-month evaluations. We examined the association of recent postnatal MeHg exposure with multiple 66- and 107-month outcomes and then used three types of alternative postnatal exposure metrics to examine their association with the children's intelligence quotient (IQ) at 107 months of age.

RESULTS

Recent postnatal exposure at 107 months of age was adversely associated with four endpoints, three in females only. One alternative postnatal metric was beneficially associated with 9-year IQ in males only.

CONCLUSIONS

We found several associations between postnatal MeHg biomarkers and children's developmental endpoints. However, as has been the case with prenatal MeHg exposure in the SCDS Main Cohort study, no consistent pattern of associations emerged to support a causal relationship.

Associations of maternal long-chain polyunsaturated fatty acids, methyl mercury, and infant development in the Seychelles Child Development Nutrition Study

Fish consumption during gestation can provide the fetus with long-chain polyunsaturated fatty acids (LCPUFA) and other nutrients essential for growth and development of the brain. However, fish consumption also exposes the fetus to the neurotoxicant, methyl mercury (MeHg). We studied the association between these fetal exposures and early child development in the Seychelles Child Development Nutrition Study (SCDNS). Specifically, we examined a priori models of Omega-3 and Omega-6 LCPUFA measures in maternal serum to test the hypothesis that these LCPUFA families before or after adjusting for prenatal MeHg exposure would reveal associations with child development assessed by the BSID-II at ages 9 and 30 months. There were 229 children with complete outcome and covariate data available for analysis. At 9 months, the PDI was positively associated with total Omega-3 LCPUFA and negatively associated with the ratio of Omega-6/Omega-3 LCPUFA. These associations were stronger in models adjusted for prenatal MeHg exposure. Secondary models suggested that the MeHg effect at 9 months varied by the ratio of Omega-6/Omega-3 LCPUFA. There were no significant associations between LCPUFA measures and the PDI at 30 months. There were significant adverse associations, however, between prenatal MeHg and the 30-month PDI when the LCPUFA measures were included in the regression analysis. The BSID-II mental developmental index (MDI) was not associated with any exposure variable. These data support the potential importance to child development of prenatal availability of Omega-3 LCPUFA present in fish and of LCPUFA in the overall diet. Furthermore, they indicate that the beneficial effects of LCPUFA can obscure the determination of adverse effects of prenatal MeHg exposure in longitudinal observational studies.

Associations of maternal long-chain polyunsaturated fatty acids, methyl mercury, and infant development in the Seychelles Child Development Nutrition Study

Fish consumption during gestation can provide the fetus with long-chain polyunsaturated fatty acids (LCPUFA) and other nutrients essential for growth and development of the brain. However, fish consumption also exposes the fetus to the neurotoxicant, methyl mercury (MeHg). We studied the association between these fetal exposures and early child development in the Seychelles Child Development Nutrition Study (SCDNS). Specifically, we examined a priori models of Omega-3 and Omega-6 LCPUFA measures in maternal serum to test the hypothesis that these LCPUFA families before or after adjusting for prenatal MeHg exposure would reveal associations with child development assessed by the BSID-II at ages 9 and 30 months. There were 229 children with complete outcome and covariate data available for analysis. At 9 months, the PDI was positively associated with total Omega-3 LCPUFA and negatively associated with the ratio of Omega-6/Omega-3 LCPUFA. These associations were stronger in models adjusted for prenatal MeHg exposure. Secondary models suggested that the MeHg effect at 9 months varied by the ratio of Omega-6/Omega-3 LCPUFA. There were no significant associations between LCPUFA measures and the PDI at 30 months. There were significant adverse associations, however, between prenatal MeHg and the 30-month PDI when the LCPUFA measures were included in the regression analysis. The BSID-II mental developmental index (MDI) was not associated with any exposure variable. These data support the potential importance to child development of prenatal availability of Omega-3 LCPUFA present in fish and of LCPUFA in the overall diet. Furthermore, they indicate that the beneficial effects of LCPUFA can obscure the determination of adverse effects of prenatal MeHg exposure in longitudinal observational studies.

Neurodevelopmental effects of maternal nutritional status and exposure to methylmercury from eating fish during pregnancy

Fish contain nutrients that promote optimal brain growth and development but also contain methylmercury (MeHg) that can have toxic effects. The present study tested the hypothesis that the intake of selected nutrients in fish or measures of maternal nutritional status may represent important confounders when estimating the effects of prenatal methylmercury exposure on child development. The study took place in the Republic of Seychelles, an Indian Ocean archipelago where fish consumption is high. A longitudinal cohort study design was used. A total of 300 mothers were enrolled early in pregnancy. Nutrients considered to be important for brain development were measured during pregnancy along with prenatal MeHg exposure. The children were evaluated periodically to age 30 months. There were 229 children with complete outcome and covariate data for analysis. The primary endpoint was the Bayley Scales of Infant Development-II (BSID-II), administered at 9 and 30 months of age. Combinations of four secondary measures of infant cognition and memory were also given at 5, 9 and 25 months. Cohort mothers consumed an average of 537 g of fish (nine meals containing fish) per week. The average prenatal MeHg exposure was 5.9 ppm in maternal hair. The primary analysis examined the associations between MeHg, maternal nutritional measures and children's scores on the BSID-II and showed an adverse association between MeHg and the mean Psychomotor Developmental Index (PDI) score at 30 months. Secondary analyses of the association between the PDI and only MeHg alone or nutritional factors alone showed only a borderline significant association between MeHg and the PDI at 30 months and no associations with nutritional factors. One experimental measure at 5 months of age was positively associated with iodine status, but not prenatal MeHg exposure. These findings suggest a possible confounding role of maternal nutrition in studies examining associations between prenatal MeHg exposures and developmental outcomes in children.

Mercury levels in newborns and infants after receipt of thimerosal-containing vaccines

OBJECTIVES

Thimerosal is a mercurial preservative that was widely used in multidose vaccine vials in the United States and Europe until 2001 and continues to be used in many countries throughout the world. We conducted a pharmacokinetic study to assess blood levels and elimination of ethyl mercury after vaccination of infants with thimerosal-containing vaccines.

METHODS

Blood, stool, and urine samples were obtained before vaccination and 12 hours to 30 days after vaccination from 216 healthy children: 72 newborns (group 1), 72 infants aged 2 months (group 2), and 72 infants aged 6 months (group 3). Total mercury levels were measured by atomic absorption. Blood mercury pharmacokinetics were calculated by pooling the data on the group and were based on a 1-compartment first-order pharmacokinetics model.

RESULTS

For groups 1, 2, and 3, respectively, (1) mean +/- SD weights were 3.4 +/- 0.4, 5.1 +/- 0.6, and 7.7 +/- 1.1 kg; (2) maximal mean +/- SD blood mercury levels were 5.0 +/- 1.3, 3.6 +/- 1.5, and 2.8 +/- 0.9 ng/mL occurring at 0.5 to 1 day after vaccination; (3) maximal mean +/- SD stool mercury levels were 19.1 +/- 11.8, 37.0 +/- 27.4, and 44.3 +/- 23.9 ng/g occurring on day 5 after vaccination for all groups; and (4) urine mercury levels were mostly nondetectable. The blood mercury half-life was calculated to be 3.7 days and returned to prevaccination levels by day 30.

CONCLUSIONS

The blood half-life of intramuscular ethyl mercury from thimerosal in vaccines in infants is substantially shorter than that of oral methyl mercury in adults. Increased mercury levels were detected in stools after vaccination, suggesting that the gastrointestinal tract is involved in ethyl mercury elimination. Because of the differing pharmacokinetics of ethyl and methyl mercury, exposure guidelines based on oral methyl mercury in adults may not be accurate for risk assessments in children who receive thimerosal-containing vaccines.

Thimerosal distribution and metabolism in neonatal mice: comparison with methyl mercury

Thimerosal, which releases the ethyl mercury radical as the active species, has been used as a preservative in many currently marketed vaccines throughout the world. Because of concerns that its toxicity could be similar to that of methyl mercury, it is no longer incorporated in many vaccines in the United States. There are reasons to believe, however, that the disposition and toxicity of ethyl mercury compounds, including thimerosal, may differ substantially from those of the methyl form. The current study sought to compare, in neonatal mice, the tissue concentrations, disposition and metabolism of thimerosal with that of methyl mercury. ICR mice were given single intramuscular injections of thimerosal or methyl mercury (1.4 mg Hg kg(-1)) on postnatal day 10 (PND 10). Tissue samples were collected daily on PND 11-14. Most analysed tissues demonstrated different patterns of tissue distribution and a different rate of mercury decomposition. The mean organic mercury in the brain and kidneys was significantly lower in mice treated with thimerosal than in the methyl mercury-treated group. In the brain, thimerosal-exposed mice showed a steady decrease of organic mercury levels following the initial peak, whereas in the methyl mercury-exposed mice, concentrations peaked on day 2 after exposure. In the kidneys, thimerosal-exposed mice retained significantly higher inorganic mercury levels than methyl mercury-treated mice. In the liver both organic and inorganic mercury concentrations were significantly higher in thimerosal-exposed mice than in the methyl mercury group. Ethyl mercury was incorporated into growing hair in a similar manner to methyl mercury. The data showing significant kinetic differences in tissue distribution and metabolism of mercury species challenge the assumption that ethyl mercury is toxicologically identical to methyl mercury.

Is susceptibility to prenatal methylmercury exposure from fish consumption non-homogeneous? Tree-structured analysis for the Seychelles Child Development Study

Studies of the association between prenatal methylmercury exposure from maternal fish consumption during pregnancy and neurodevelopmental test scores in the Seychelles Child Development Study have found no consistent pattern of associations through age 9 years. The analyses for the most recent 9-year data examined the population effects of prenatal exposure, but did not address the possibility of non-homogeneous susceptibility. This paper presents a regression tree approach: covariate effects are treated non-linearly and non-additively and non-homogeneous effects of prenatal methylmercury exposure are permitted among the covariate clusters identified by the regression tree. The approach allows us to address whether children in the lower or higher ends of the developmental spectrum differ in susceptibility to subtle exposure effects. Of 21 endpoints available at age 9 years, we chose the Weschler Full Scale IQ and its associated covariates to construct the regression tree. The prenatal mercury effect in each of the nine resulting clusters was assessed linearly and non-homogeneously. In addition we reanalyzed five other 9-year endpoints that in the linear analysis had a two-tailed p-value <0.2 for the effect of prenatal exposure. In this analysis, motor proficiency and activity level improved significantly with increasing MeHg for 53% of the children who had an average home environment. Motor proficiency significantly decreased with increasing prenatal MeHg exposure in 7% of the children whose home environment was below average. The regression tree results support previous analyses of outcomes in this cohort. However, this analysis raises the intriguing possibility that an effect may be non-homogeneous among children with different backgrounds and IQ levels.

The biological monitoring of prenatal exposure to methylmercury

Several biological media have been used as indicators of the fetal body burden of methylmercury and the levels in the primary target tissue, the developing brain. These media include maternal hair and blood. The relative merits of these media will be considered both with regard to current knowledge of the physiology of mercury disposition in the body and also the practicality of field application with respect to sample, collection, transport, storage and processing.

Benchmark concentrations for methyl mercury obtained from the 9-year follow-up of the Seychelles Child Development Study

Methyl mercury (MeHg) is highly toxic to the developing nervous system. Human exposure is mainly from fish consumption since small amounts are present in all fish. Findings of developmental neurotoxicity following high-level prenatal exposure to MeHg raised the question of whether children whose mothers consumed fish contaminated with background levels during pregnancy are at an increased risk of impaired neurological function. Benchmark doses determined from studies in New Zealand, and the Faroese and Seychelles Islands indicate that a level of 4-25 parts per million (ppm) measured in maternal hair may carry a risk to the infant. However, there are numerous sources of uncertainty that could affect the derivation of benchmark doses, and it is crucial to continue to investigate the most appropriate derivation of safe consumption levels. Earlier, we published the findings from benchmark analyses applied to the data collected on the Seychelles main cohort at the 66-month follow-up period. Here, we expand on the main cohort analyses by determining the benchmark doses (BMD) of MeHg level in maternal hair based on 643 Seychellois children for whom 26 different neurobehavioral endpoints were measured at 9 years of age. Dose-response models applied to these continuous endpoints incorporated a variety of covariates and included the k-power model, the Weibull model, and the logistic model. The average 95% lower confidence limit of the BMD (BMDL) across all 26 endpoints varied from 20.1 ppm (range=17.2-22.5) for the logistic model to 20.4 ppm (range=17.9-23.0) for the k-power model. These estimates are somewhat lower than those obtained after 66 months of follow-up. The Seychelles Child Development Study continues to provide a firm scientific basis for the derivation of safe levels of MeHg consumption.

Neuropsychological and renal effects of dental amalgam in children: a randomized clinical trial

CONTEXT

No randomized trials have been published that address the concern that inhalation of mercury vapor released by amalgam dental restorations causes adverse health effects.

OBJECTIVE

To compare the neuropsychological and renal function of children whose dental caries were restored using amalgam or mercury-free materials.

DESIGN AND SETTING

The New England Children's Amalgam Trial was a 2-group randomized safety trial involving 5 community health dental clinics in Boston, Mass, and 1 in Farmington, Me, between September 1997 and March 2005.

PARTICIPANTS AND INTERVENTION

A total of 534 children aged 6 to 10 years at baseline with no prior amalgam restorations and 2 or more posterior teeth with caries were randomly assigned to receive dental restoration of baseline and incident caries during a 5-year follow-up period using either amalgam (n=267) or resin composite (n =267) materials.

MAIN OUTCOME MEASURES

The primary neuropsychological outcome was 5-year change in full-scale IQ scores. Secondary outcomes included tests of memory and visuomotor ability. Renal glomerular function was measured by creatinine-adjusted albumin in urine.

RESULTS

Children had a mean of 15 tooth surfaces (median, 14) restored during the 5-year period (range, 0-55). Assignment to the amalgam group was associated with a significantly higher mean urinary mercury level (0.9 vs 0.6 microg/g of creatinine at year 5, P<.001). After adjusting for randomization stratum and other covariates, no statistically significant differences were found between children in the amalgam and composite groups in 5-year change in full-scale IQ score (3.1 vs 2.1, P = .21). The difference in treatment group change scores was 1.0 (95% confidence interval, -0.6 to 2.5) full-scale IQ score point. No statistically significant differences were found for 4-year change in the general memory index (8.1 vs 7.2, P = .34), 4-year change in visuomotor composite (3.8 vs 3.7, P = .93), or year 5 urinary albumin (median, 7.5 vs 7.4 mg/g of creatinine, P = .61).

CONCLUSIONS

In this study, there were no statistically significant differences in adverse neuropsychological or renal effects observed over the 5-year period in children whose caries were restored using dental amalgam or composite materials. Although it is possible that very small IQ effects cannot be ruled out, these findings suggest that the health effects of amalgam restorations in children need not be the basis of treatment decisions when choosing restorative dental materials.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00065988.

Methylmercury contamination of laboratory animal diets

In the midst of research focusing on the neurodevelopmental effects of mercury vapor in rats, we detected significant levels of mercury (30-60 ng/g) in the blood of nonexposed control subjects. We determined that the dominant form of the mercury was organic and that the standard laboratory chow we used in our vivarium was the source of the contamination. The dietary levels were deemed of potential biologic significance, even though they might have fallen below the limits of measurement specified by the supplier. All investigators employing animals in research must assess such potential contamination because dietary agents may alter a) conclusions based on intentionally administered doses, b) outcomes by interacting with other agents that are the primary focus of the research, and c) outcomes of research unrelated to the toxic effects of experimentally administered agents.

Comparison of blood and brain mercury levels in infant monkeys exposed to methylmercury or vaccines containing thimerosal

Thimerosal is a preservative that has been used in manufacturing vaccines since the 1930s. Reports have indicated that infants can receive ethylmercury (in the form of thimerosal) at or above the U.S. Environmental Protection Agency guidelines for methylmercury exposure, depending on the exact vaccinations, schedule, and size of the infant. In this study we compared the systemic disposition and brain distribution of total and inorganic mercury in infant monkeys after thimerosal exposure with those exposed to MeHg. Monkeys were exposed to MeHg (via oral gavage) or vaccines containing thimerosal (via intramuscular injection) at birth and 1, 2, and 3 weeks of age. Total blood Hg levels were determined 2, 4, and 7 days after each exposure. Total and inorganic brain Hg levels were assessed 2, 4, 7, or 28 days after the last exposure. The initial and terminal half-life of Hg in blood after thimerosal exposure was 2.1 and 8.6 days, respectively, which are significantly shorter than the elimination half-life of Hg after MeHg exposure at 21.5 days. Brain concentrations of total Hg were significantly lower by approximately 3-fold for the thimerosal-exposed monkeys when compared with the MeHg infants, whereas the average brain-to-blood concentration ratio was slightly higher for the thimerosal-exposed monkeys (3.5 +/- 0.5 vs. 2.5 +/- 0.3). A higher percentage of the total Hg in the brain was in the form of inorganic Hg for the thimerosal-exposed monkeys (34% vs. 7%). The results indicate that MeHg is not a suitable reference for risk assessment from exposure to thimerosal-derived Hg. Knowledge of the toxicokinetics and developmental toxicity of thimerosal is needed to afford a meaningful assessment of the developmental effects of thimerosal-containing vaccines.

Exploring nonlinear association between prenatal methylmercury exposure from fish consumption and child development: evaluation of the Seychelles Child Development Study nine-year data using semiparametric additive models

Studies of the association between prenatal methylmercury exposure from maternal fish consumption and neurodevelopmental test scores in the Seychelles Child Development Study have not found adverse effects through age 9 years. The analysis for the most recent 9-year data (Lancet 361 (2003) 1686) employed conventional linear regression models. In this study we reanalyzed the same Seychelles 9-year data using semiparametric additive models with different degrees of smoothing to explore whether nonlinear effects of prenatal exposure were present. Of 21 endpoints in the linear analysis, we chose only those with a two-tailed P value less than 0.2 for the effect of prenatal exposure. Six endpoints met the criterion. A nonlinear effect was identified with the more smooth model for only one endpoint. The test for an overall effect of prenatal exposure was also significant, with a P value of 0.04, while the corresponding P value in the linear regression analysis was 0.08. The nonlinear curve appeared to be nearly flat when the level was below approximately 12 ppm in maternal hair, with a linear trend above that level, suggesting a possible adverse effect in the uppermost range of prenatal exposure included in this cohort. Because of the descriptive nature of semiparametric additive models, the P values are not precise, and certainly there are fewer data above 12 ppm. We conclude that this reanalysis supports the primary linear analysis, showing little evidence for a prenatal adverse effect.

Using measurement error models to assess effects of prenatal and postnatal methylmercury exposure in the Seychelles Child Development Study

Studies of the effects of environmental exposures on human health typically require estimation of both exposure and outcome. Standard methods for the assessment of the association between exposure and outcome include multiple linear regression analysis, which assumes that the outcome variable is observed with error, while the levels of exposure and other explanatory variables are measured with complete accuracy, so that there is no deviation of the measured from the actual value. The term measurement error in this discussion refers to the difference between the actual or true level and the value that is actually observed. In the investigations of the effects of prenatal methylmercury (MeHg) exposure from fish consumption on child development, the only way to obtain a true exposure level (producing the toxic effect) is to ascertain the concentration in fetal brain, which is not possible. As is often the case in studies of environmental exposures, the measured exposure level is a biomarker, such as the average maternal hair level during gestation. Measurement of hair mercury is widely used as a biological indicator for exposure to MeHg and is the only indicator that has been calibrated against the target tissue, the developing brain. Variability between the measured and the true values in explanatory variables in a multiple regression analysis can produce bias, leading to either over or underestimation of regression parameters (slopes). Fortunately, statistical methods known as measurement error models (MEM) are available to account for measurement errors in explanatory variables in multiple regression analysis, and these methods can provide an (either "unbiased" or "bias-corrected") estimate of the unknown outcome/exposure relationship. In this paper, we illustrate MEM analysis by reanalyzing data from the 5.5-year test battery in the Seychelles Child Development Study, a longitudinal study of prenatal exposure to MeHg from maternal consumption of a diet high in fish. The use of the MEM approach was made possible by the existence of independent, calibration data on the magnitude of the variability of the measurement error deviations for the biomarker of prenatal exposure used in this study, the maternal hair level. Our reanalysis indicated that adjustment for measurement errors in explanatory variables had no appreciable effect on the original results.

Mercury concentrations and metabolism in infants receiving vaccines containing thiomersal: a descriptive study

BACKGROUND

Thiomersal is a preservative containing small amounts of ethylmercury that is used in routine vaccines for infants and children. The effect of vaccines containing thiomersal on concentrations of mercury in infants' blood has not been extensively assessed, and the metabolism of ethylmercury in infants is unknown. We aimed to measure concentrations of mercury in blood, urine, and stools of infants who received such vaccines.

METHODS

40 full-term infants aged 6 months and younger were given vaccines that contained thiomersal (diptheria-tetanus-acellular pertussis vaccine, hepatitis B vaccine, and in some children Haemophilus influenzae type b vaccine). 21 control infants received thiomersal-free vaccines. We obtained samples of blood, urine, and stools 3-28 days after vaccination. Total mercury (organic and inorganic) in the samples was measured by cold vapour atomic absorption.

FINDINGS

Mean mercury doses in infants exposed to thiomersal were 45.6 microg (range 37.5-62.5) for 2-month-olds and 111.3 microg (range 87.5-175.0) for 6-month-olds. Blood mercury in thiomersal-exposed 2-month-olds ranged from less than 3.75 to 20.55 nmol/L (parts per billion); in 6-month-olds all values were lower than 7.50 nmol/L. Only one of 15 blood samples from controls contained quantifiable mercury. Concentrations of mercury were low in urine after vaccination but were high in stools of thiomersal-exposed 2-month-olds (mean 82 ng/g dry weight) and in 6-month-olds (mean 58 ng/g dry weight). Estimated blood half-life of ethylmercury was 7 days (95% CI 4-10 days).

INTERPRETATION

Administration of vaccines containing thiomersal does not seem to raise blood concentrations of mercury above safe values in infants. Ethylmercury seems to be eliminated from blood rapidly via the stools after parenteral administration of thiomersal in vaccines.

Perinatal and lifetime exposure to methylmercury in the mouse: blood and brain concentrations of mercury to 26 months of age

Chronic, low-level exposures to environmental toxicants, because they often begin prenatally and then persist throughout the individual's lifetime, pose challenging issues to risk assessment. Exposure to low levels of methylmercury through the diet, based largely on consumption of fish and sea mammals, follows this pattern. Early development is considered to be a period of heightened vulnerability during which even low-level exposures may produce undetected, "silent", damage that is revealed only under conditions that challenge the functional capacities of the individual. Aging, with its diminished functional capacities and compensatory reserves provides such a challenge, but, to explore this possibility, requires basic information about blood and brain levels under conditions of chronic lifetime exposure. The current research was undertaken to provide such information. One hundred female B6C3F1/HSD mice were assigned to one of three dose groups, 0, 1, or 3 ppm methylmercury chloride administered in a 5 nM sodium carbonate drinking solution. They were bred with male CBA/J HSD mice to produce the trihybrid offspring B6C3F1/ HSD x CBA/J HSD. Dosing of the females began 4 weeks prior to breeding and continued for the two methylmercury-exposed groups throughout breeding and gestation. The methylmercury-treated litters were split into two subgroups, one exposed throughout its lifetime (set at 26 months) to the original dose, the other exposed through postnatal day 13 (PND 13). Brain and blood concentrations were assayed by cold-vapor atomic absorption. Samples were obtained on PND 4 and 21, and then at the end of months 14 and 26. On PND 4, brain and blood levels closely reflected maternal dosing. In all groups, concentrations fell sharply from PND 4 to 21, but to a greater extent in the perinatal groups. Blood levels in the 1 ppm lifetime group remained unchanged between months 14 and 26, but brain levels rose modestly. In the 3 ppm lifetime group, both brain and blood levels rose significantly between months 14 and 26, suggesting an interaction between dose and age.

Association between methylmercury exposure from fish consumption and child development at five and a half years of age in the Seychelles Child Development Study: an evaluation of nonlinear relationships

Studies to date of the developmental effects of pre- and postnatal methylmercury exposure from fish consumption in the Seychelles Islands, using linear regression models for analysis, have not shown adverse effects on neurodevelopmental test scores. In this study we evaluated whether nonlinear effects of methylmercury exposure were present, using scores on six tests administered to cohort children in the Seychelles Child Development Study at 66 months of age. Prenatal exposure was determined by measuring mercury in a segment of maternal scalp hair representing growth during pregnancy. Postnatal exposure was measured in a segment of the child's hair taken at 66-months of age. Generalized additive models (GAMs), which make no assumptions about the functional form of the relationship between exposure and test score, were used in the analysis. GAMs similar to the original linear regression models were used to reanalyze the six primary developmental endpoints from the 66-month test battery. Small nonlinearities were identified in the relationships between prenatal exposure and the Preschool Language Scale (PLS) Total score and Child Behavior Check List (CBCL) and between postnatal exposure and the McCarthy General Cognitive Index (GCI) test scores. The effects are best described graphically but can be summarized by computing the change in the predicted test score from 0 to either 10 or 15 ppm and then above this point. For the PLS the trend involved a decline of 0.8 points between 0 and 10 ppm followed by an increase (representing improvement) of 1.3 points above 10 ppm. For the CBCL there was an increase of 1 point from 0 to 15 ppm, and then a decline (improvement) of 4 points above 15 ppm. The GCI increased by 1.8 points through 10 ppm and then declined 3.2 points (representing worse performance) above 10 ppm. These results are not entirely consistent. Two of the trends involve what appear to be beneficial effects of prenatal exposure. The one possibly adverse trend involves postnatal exposure. In every case the trend changes direction, so that an effect in one direction is followed by an effect in the opposite direction. Because of the descriptive nature of GAMs it is difficult to provide a precise level of statistical significance for the estimated trends. Certainly above 10 ppm there is less data and trends above this level are estimated less precisely. Overall there was no clear evidence for consistent (across the entire range of exposure levels) adverse effects of exposure on the six developmental outcomes. Further nonlinear modeling of these data may be appropriate, but there is also the risk of fitting complex models without a clear biological rationale.

Association between prenatal exposure to methylmercury and cognitive functioning in Seychellois children: a reanalysis of the McCarthy Scales of Children's Ability from the main cohort study

Methylmercury (MeHg) is a neurotoxicant whose high-dose effects first became known following a number of poisoning outbreaks that occurred worldwide. The primary human exposure is low dosage from fish consumption. Studies of fish-eating populations have not found a consistent pattern of association between exposures and outcomes. Therefore, examining specific areas of cognitive functioning has been suggested as an important approach to determine whether more subtle effects of MeHg exposure are present. In the Seychelles longitudinal study of prenatal and postnatal MeHg exposure from fish consumption and development, the McCarthy Scales of Children's Abilities (MSCA) were administered to children at age 66 months. No association between MeHg exposure and performance on the MSCA General Cognitive Index was identified. We analyzed these data further to determine whether associations were present on specific subscales of the MSCA. The standard MSCA subscales were analyzed. Then, more specific subscales of the MSCA were defined and analyzed utilizing a neuropsychological approach. The subscales were recombined to approximate the domains of cognitive functioning evaluated in the Faroes and New Zealand studies. Analyses of both the standard and the recombined MSCA subscales showed no adverse associations with MeHg exposure and neuropsychological endpoints. A positive association between postnatal MeHg exposure and performance on the MSCA Memory subscale was found. These findings are consistent with previous reports from the Seychelles study in that no adverse effects of MeHg exposure from fish consumption can be detected in this cohort.

Secondary analysis from the Seychelles Child Development Study: the child behavior checklist

Human exposure to methylmercury (MeHg), a known neurotoxin, is primarily from fish consumption. As part of a large study examining the association between MeHg exposure and child development in a population with high fish consumption we examined school-age behavior using the Achenbach Child Behavior Checklist (CBCL). The CBCL Total T score was a primary endpoint and was reported earlier to show no adverse association with prenatal or postnatal MeHg exposure. In this study we analyzed the T scores of the CBCL subscales to determine if more discrete aspects of measured behavior were associated with exposure. The Seychelles Child Development Study (SCDS) is a prospective, double blind, longitudinal evaluation of over 700 children. The index of prenatal exposure was maternal hair total mercury (T-Hg) in a segment growing during gestation. Postnatal exposure was T-Hg in the child's hair taken at 66 months of age. The child's primary caregiver completed the CBCL during the 66-month evaluation. No association between prenatal or postnatal MeHg exposure and the CBCL subscales was found. In Seychellois children exposed to MeHg from consumption of ocean fish we found no association between either prenatal or postnatal MeHg exposure and behavior as measured by the CBCL subscales.

Neurodevelopmental outcomes of Seychellois children from the pilot cohort at 108 months following prenatal exposure to methylmercury from a maternal fish diet

The Seychelles Child Development Study has been unable to confirm any relationship between maternal exposure to MeHg during pregnancy and adverse developmental outcomes. In this report, 87 children from a pilot cohort were evaluated at 9 years of age. Each child was given a battery testing specific cognitive, visual motor, and motor skills using standardized psychometric and neuro-psychological tests. The results indicated no adverse association between maternal MeHg exposure and any developmental outcome measure. For three endpoints (Boston Naming Test and two tests of visual motor coordination), enhanced performance in males was associated with increasing prenatal MeHg exposure. A secondary analysis including both prenatal MeHg and postnatal MeHg exposure was done even though we lacked postnatal hair for about 35% of the cohort. The results of the secondary analysis mirrored the outcomes of the primary analysis regarding prenatal exposure but were less robust. The results of this study are consistent with earlier findings from the 66-month evaluations of the SCDS Main cohort. Since MeHg is neurotoxic, this effect is likely due to other factors associated with consumption of fish.

Twenty-seven years studying the human neurotoxicity of methylmercury exposure

Research at the University of Rochester (U of R) has been focused on mercury for nearly half a century. Initially studies focused on dosimetry, especially the accuracy of measuring exposure, and experimental work with animal models. Clinical studies in human populations started when the U of R mercury group was asked to assist with dosimetry in the Iraq epidemic of 1971-1972. Initial clinical studies described the effects of methylmercury (MeHg) poisoning on adults and children. A dose-response curve for prenatal exposure was determined and it suggested that relatively low exposures might be harmful to the fetus. Since most human exposure to MeHg is dietary from fish consumption, these theoretical dangers had far-reaching implications. After Iraq, the Rochester team pursued exposure from fish consumption in both adults and children. Populations with high fish consumption were identified in Samoa and Peru for studying adults and in Peru and the Seychelles islands for studying children. The possible health threat to the fetus from maternal fish consumption quickly became the focus of research efforts. This paper reviews the Rochester experience in studying human exposure to MeHg from fish consumption.

Determination of methyl mercury in whole blood by ethylation-GC-CVAFS after alkaline digestion-solvent extraction

A method for the determination of methyl mercury in whole blood samples based on ethylation-gas chromatography-cold vapor atomic fluorescence spectrometry after alkaline digestion-solvent extraction is described. The extraction procedure and conditions were optimized, and the matrix interference after extraction was critically investigated. The storage stability of MeHg in blood samples and a series of extracts was determined. The method detection limit was found to be approximately 0.02 ng/g for a 0.5-g blood sample with relative standard deviations of less than 10%. The accuracy and precision were evaluated by summarizing the quality-control (QC) data generated over a one and one half year period. Appropriate procedures for sample collection, transportation, and storage were adapted to the method. Using this method accompanied by explicit QC protocols and procedures, background levels of MeHg and total mercury in blood for 150 8-10-year-old Portuguese children with nonoccupational and nonamalgamal exposure were determined and reported with summarized QC data.

Segmental hair mercury evaluation of a single family along the upper Madeira basin, Brazilian Amazon

Mercury pollution (MeHg) up the aquatic food chains in the Amazonian ecosystems has been a major concern in environmental health. Riverside people (ribeirinhos) along the Upper Madeira river are heavy fish eaters. Hair is the best biomarker for MeHg exposure. By assuming a constant hair growth rate, it is possible to evaluate a temporal profile of Hg exposure over the recent defined past. In this paper we present the segmental total hair Hg concentrations from a single family from which some of the 10 persons investigated had high hair Hg concentrations (peak of 339 ppm). We also presented the hair MeHg content from 4 out of the 10 family members investigated. There was a wide variation in total hair Hg concentrations (8 to 339 ppm) among these individuals, who were mostly sharing their meals; there was also a wide variation in total Hg concentrations in the same individual over time (136 to 274 ppm). Hg speciation showed a mean and standard deviation in the MeHg content of 62% and 6%, respectively. The wide variation in total hair Hg concentration strongly indicated that it is possible to mitigate critical Hg exposure levels by conducting a fish advisory.

A cluster of pediatric metallic mercury exposure cases treated with meso-2,3-dimercaptosuccinic acid (DMSA)

Nine children and their mother were exposed to vapors of metallic mercury. The source of the exposure appears to have been a 6-oz vial of mercury taken from a neighbor's home. The neighbor reportedly operated a business preparing mercury-filled amulets for practitioners of the Afro-Caribbean religion Santeria. At diagnosis, urinary mercury levels in the children ranged from 61 to 1,213 microg/g creatinine, with a geometric mean of 214.3 microg/m creatinine. All of the children were asymptomatic. To prevent development of neurotoxicity, we treated the children with oral meso-2,3-dimercaptosuccinic acid (DMSA). During chelation, the geometric mean urine level rose initially by 268% to 573.2 microg mercury/g creatinine (p<0.0005). At the 6-week follow-up examination after treatment, the geometric mean urine mercury level had fallen to 102.1 microg/g creatinine, which was 17.8% of the geometric mean level observed during treatment (p<0.0005) and 47.6% of the original baseline level (p<0.001). Thus, oral chelation with DMSA produced a significant mercury diuresis in these children. We observed no adverse side effects of treatment. DMSA appears to be an effective and safe chelating agent for treatment of pediatric overexposure to metallic mercury.

Mercury contamination of heavy metal collection containers

We investigated discordant urinary mercury testing results from 2 patients with potential mercury exposures. Two patients had mercury levels of 634 and > 1,000 micrograms/L respectively. Although repeat 24 h urine mercury levels were elevated, spot urines were negative. Investigation revealed that technical HCl with high mercury content had been added to the 24 h urine collection containers. Subsequently, 20 hospitals were contacted to determine their heavy metals testing procedure and to analyze the acid used for mercury. Most hospitals contacted used acid in the preparation of their urine heavy metal collection containers. Of 13 HCl samples tested, 5 had low levels of mercury and 1 had heavy mercury content. Acid added to heavy metal collection containers should be of high purity grade to avoid mercury contamination of samples.

Association between prenatal exposure to methylmercury and developmental outcomes in Seychellois children: effect modification by social and environmental factors

The Seychelles Child Development Study (SCDS) is testing the hypothesis that prenatal exposure to low doses of MeHg from maternal consumption of fish is associated with the child's developmental outcomes. No deleterious relationships between exposure to MeHg and cognitive functions have been identified in the primary analysis of the main cohort through 66 months of age. We performed secondary analyses to determine if effect modification (EM) from social and environmental factors was affecting associations between MeHg and outcomes.

METHODS

MeHg exposure was determined by analysis of maternal hair growing during pregnancy. Children in our Main Study cohort were evaluated at 6.5 months (N = 740) for visual recognition memory and visual attention using the Fagan Infantest, at 19 months (N = 738) and 29 months (N = 736) with the Bayley Scales of Infant Development (BSID). Interactions between MeHg and Caregiver Intelligence, Family Income and Home Environment were examined by multiple regression analysis.

RESULTS

The median prenatal MeHg exposure was 5.9 ppm (Range 0.5-26.7 ppm). No EM occurred for preferential looking or visual attention at 6.5 months, for the BSID Psychomotor Scale at either 19 or 29 months, or for activity level at 29 months as measured by the BSID Infant Behavior Record. Interactions between MeHg level and both caregiver intelligence and family income were statistically significant for the BSID Mental Scale at 19 months but not at 29 months. These showed enhancement of MDI scores with increasing maternal MeHg in higher caregiver IQ groups at several levels of family income.

CONCLUSIONS

In Seychellois children, consistent major EM by social or environmental factors were not identified. The small EM by caregiver intelligence and social factors at 19 months is consistent with the enhanced performance we reported when this cohort was examined at 66 months.

Semiparametric modeling of age at achieving developmental milestones after prenatal exposure to methylmercury in the Seychelles child development study

Controversy exists concerning the fetal risk associated with exposure to low-dose methylmercury from maternal fish consumption. Previous studies of the effects of acute prenatal mercury exposure identified delays in achieving developmental milestones among exposed children. This led to public health concern that prenatal low-dose exposure from fish consumption could adversely affect the fetus. We evaluated the effects of prenatal methylmercury exposure (through maternal fish consumption) on the age that children walked and first said words in the main study cohort of the Seychelles Child Development Study. We used semiparametric generalized additive models to identify nonlinearities in the relationships between prenatal exposure and developmental outcomes, after adjusting for covariates, and to evaluate their importance. Very slight delays (<1 day) in walking were seen as mercury levels increased from 0 to 7 ppm, but this effect did not persist at the higher exposure levels represented by the cohort, making it difficult to conclude that a cause and effect relationship existed at the exposure levels seen in this cohort. There was no evidence for any association between prenatal exposure and age at talking.

Effects of prenatal and postnatal methylmercury exposure from fish consumption on neurodevelopment: outcomes at 66 months of age in the Seychelles Child Development Study

CONTEXT

Human neurodevelopmental consequences of exposure to methyl-mercury (MeHg) from eating fish remain a question of public health concern.

OBJECTIVE

To study the association between MeHg exposure and the developmental outcomes of children in the Republic of Seychelles at 66 months of age.

DESIGN

A prospective longitudinal cohort study.

PARTICIPANTS

A total of 711 of 779 cohort mother-child pairs initially enrolled in the Seychelles Child Development Study in 1989.

SETTING

The Republic of Seychelles, an archipelago in the Indian Ocean where 85% of the population consumes ocean fish daily.

MAIN OUTCOME MEASURES

Prenatal and postnatal MeHg exposure and 6 age-appropriate neurodevelopmental tests: the McCarthy Scales of Children's Abilities, the Preschool Language Scale, the Woodcock-Johnson Applied Problems and Letter and Word Recognition Tests of Achievement, the Bender Gestalt test, and the Child Behavior Checklist.

RESULTS

The mean maternal hair total mercury level was 6.8 ppm and the mean child hair total mercury level at age 66 months was 6.5 ppm. No adverse outcomes at 66 months were associated with either prenatal or postnatal MeHg exposure.

CONCLUSION

In the population studied, consumption of a diet high in ocean fish appears to pose no threat to developmental outcomes through 66 months of age.

Longitudinal hair mercury concentration in riverside mothers along the Upper Madeira river (Brazil)

Mercury releases from gold mining occurred during the 1980s in the Upper Madeira river, a Southern tributary of the Amazon. Downstream from these areas, riverside residents rely on fish consumption for subsistence. In July of 1993, hair samples were collected for mercury analysis from a group of mothers and their infants and one pregnant woman. By assuming a constant rate of hair growth (1.1 cm per month), a temporal profile of the methylmercury exposure was determined for the previous 2 to 3 years. The length of hair segments corresponded to hair growth during pregnancy and the subsequent breastfeeding periods. During all periods, hair mercury concentrations in six mothers were in the range of 4.5-26.8 ppm, slightly lower than those of their infants (8.2-28.4 ppm). Further segmental analyses of hair mercury from another six mothers showed concentrations in the range of 12.2-41.0 ppm during the three trimesters of pregnancy and 4.0-33.5 ppm during breast feeding-slightly lower than their infants (11.6-50.4 ppm). Another four mothers showed hair mercury concentrations in the range of 21. 3-84.4 ppm.

Effects of prenatal methylmercury exposure from a high fish diet on developmental milestones in the Seychelles Child Development Study

Mercury is widespread in the environment and exists in several physical and chemical forms. Prenatal exposure to methylmercury disrupts brain development. The most common mode of prenatal methylmercury exposure is maternal fish consumption. Studies of human prenatal exposure in Iraq following maternal ingestion of methylmercury treated grain suggested that maternal hair mercury concentrations above 10 ppm may be related to delayed developmental milestones and neurological abnormalities. This level of exposure can be achieved by frequent consumption of fish. The Seychelles Child Development Study analyzed developmental milestones similar to those determined in Iraq in a large controlled, prospective study of children exposed prenatally to methylmercury when their mothers ate fish. As part of this ongoing study, cohort children were evaluated at 6.5, 19, 29, and 66 months of age. At 19 months care-givers were asked at what age the child walked (n=720 out of 738) and talked (n=680). Prenatal mercury exposure was determined by atomic absorption analysis of maternal hair segments corresponding to hair growth during the pregnancy. The median mercury level in maternal hair was 5.8 ppm with a range of 0.5-26.7 ppm. The mean age (in months) at walking was 10.7 (SD = 1.9) for females and 10.6 (SD = 2.0) for males. The mean age at talking (in months) was 10.5 (SD = 2.6) for females, and 11.0 (SD = 2.9) for males. After adjusting for covariates and statistical outliers, no association was found between the age at which Seychellois children walked or talked and prenatal exposure to mercury. Normal ages at achievement of the developmental milestones walking and talking were found in Seychellois toddlers following prenatal exposure to methylmercury from a maternal fish diet. These results do not support the lowest effect levels in young children following prenatal methylmercury exposure predicted by the dose response analysis of the Iraq data. More detailed studies in older children are needed to determine if there are adverse effects in fish eating populations.

Biological monitoring of iodine, a water disinfectant for long-term space missions

In order to establish guidelines for exposure of astronauts to iodine, used as a water disinfectant in space, we studied the usefulness of hair, saliva, and urine for biological monitoring in humans and in the human hair/nude mouse model. The monitoring of iodine in patients that received 150 mCi of Na131I (carrier-free) showed similar patterns of elimination for blood, saliva, and urine. The mean correlation coefficient (r) between iodine elimination for blood/saliva was 0.99, for blood/urine, 0.95, and for saliva/urine, 0.97. The absolute value of iodine concentrations in urine revealed marked variability, which was corrected by adjusting for creatinine levels. The autoradiographic studies of human hair demonstrated that iodine is rapidly incorporated into external layers of the hair root and can be removed easily during washing. These data were confirmed after iodine exposure using the human hair/nude mouse model. Hair does not provide satisfactory information about exposure due to unstable incorporation of iodine. The most useful medium for biological monitoring of astronauts exposed to high doses of iodine in drinking water is urine, when adjusted for creatinine, and saliva, if quantitative evaluation of flow rate is provided.

Methylmercury poisoning: long-term clinical, radiological, toxicological, and pathological studies of an affected family

For 3 months in 1969 a family in the United States that included a pregnant mother consumed pork containing methylmercury. Children, aged 20, 13, and 8 years and a neonate, developed severe neurological signs. Twenty-two years later, the 2 oldest had cortical blindness or constricted visual fields, diminished hand proprioception, choreoathetosis, and attentional deficits. Magnetic resonance images showed tissue loss in the calcarine and parietal cortices and cerebellar folia. The youngest had quadriplegia, blindness, and severe mental retardation until their deaths. The brain of the 8-year-old who died at age 30 showed cortical atrophy, neuronal loss, and gliosis, most pronounced in the paracentral and parietooccipital regions. The total mercury level in formalin-fixed, left occipital cortex was 1,974 ng/gm as measured by atomic absorption. Regional brain mercury levels correlated with extent of brain damage. A control patient had 38.5 ng of mercury/gm in the occipital cortex. Systemic organs in the patient and a control subject had comparable mercury levels. In mercury-intoxicated rats, we found that only 5 to 10% of total brain mercury was lost by formalin fixation. Brain inorganic mercury in the patient ranged from 82 to 100%. Since inorganic mercury crosses the blood-brain barrier poorly, biotransformation of methyl to inorganic mercury may have occurred after methylmercury crossed the blood-brain barrier, accounting for its persistence in brain and causing part of the brain damage.

DMPS as a rescue agent for the nephropathy induced by mercuric chloride

The effectiveness of 2,3-dimercaptopropane-1-sulfonate (DMPS) as a rescue agent for the acute nephropathy induced by HgCl2 was studied in uninephrectomized (NPX) and sham-operated (SO) rats. NPX and SO rats that were given a toxic 2.5-mumol/kg dose of HgCl2 developed severe renal damage within 24 hr after the HgCl2 was administered. Renal injury was assessed by measuring plasma creatinine, creatinine clearance, fractional excretion of several biological markers, the rate of excretion of cellular enzymes and plasma solutes and severity of morphologically demonstrable necrosis in the pars recta of proximal tubules. When a 10-mg/kg dose of DMPS was given to the NPX and SO rats 1 hr after treatment with the 2.5-mumol/kg dose of HgCl2, the nephropathy induced by the dose of HgCl2 was less severe. Moreover, the NPX rats had significantly less severe renal damage than did the SO rats. Renal damage was completely absent from both NPX and SO rats that were given a 100-mg/kg dose of DMPS 1 hr after treatment with the 2.5-mumol/kg dose of HgCl2. These data indicate that at the 10-mg/kg dose of DMPS, NPX rats are protected against the nephrotoxic effects of a 2.5-mumol/kg dose of HgCl2 to a greater extent than are SO rats. Moreover, the data show that complete rescue from the 2.5-mumol/kg dose of HgCl2 is afforded to NPX and SO rats given 100 mg/kg of DMPS within 1 hr after treatment with HgCl2. This complete rescue is afforded for at least 24 hr. Based on the present findings DMPS may prove to be a useful rescue agent against the acute nephropathy induced by HgCl2.

Early biochemical effects of an organic mercury fungicide on infants: "dose makes the poison"

Phenylmercury absorbed through the skin from contaminated diapers affected urinary excretion in infants in Buenos Aires. The effects were reversible and quantitatively related to the concentration of urinary mercury. Excretion of gamma-glutamyl transpeptidase, an enzyme in the brush borders of renal tubular cells, increased in a dose-dependent manner when mercury excretion exceeded a "threshold" value. Urine volume also increased but at a higher threshold with respect to mercury. The results support the threshold concept of the systemic toxicity of metals. gamma-Glutamyl transpeptidase is a useful and sensitive marker for preclinical effects of toxic metals.

Rare-event analysis methods for detection of fetal red blood cells in maternal blood

Methods for analyzing very small subpopulations of cells ("rare-event analysis methods") are of interest in many areas of biology. In this paper we demonstrate the ability to detect one Rh-positive erythrocyte per 100,000 Rh-negative erythrocytes. This is the level needed to detect human fetal red blood cells in the maternal circulation in the case of fetal-maternal Rh-incompatibility. Erythrocytes stained with fluorescent immunobeads are more than 100 times as bright as the same cells stained by indirect immunofluorescence (or almost 40 times as bright as cells stained by the biotin-avidin technique). More importantly there is greater than a sevenfold increase in signal-to-noise (S/N) ratio, which proves critical in detecting very low frequency ("rare event") cell subpopulations. Although equal mixtures of positive and negative cells can easily be detected, positives cannot be easily distinguished from negatives when they are present in relative frequencies of 1:1000 or less as measured by either indirect immunofluorescence or biotin-fluorescent avidin labeling of D antigens. However, use of fluorescent immunobeads and high-speed flow cytometry allows detection of subpopulations of the same cells at relative frequencies of less than 1:100,000. Based on calibrations using free-fluorescent immunobeads, the average Rh-positive cell (when present in a relative frequency of 1:100,000) labeled with 36 +/- 19 fluorescent immunobeads. Rh-positive cells of relative frequencies 1:10,000 and 1:100,000 can be detected above background (Rh-negative red blood cells treated with the same amount of fluorescent immunobeads). Using defined mixtures of Rh-positive and negative cells, the expected numbers of positive cells were observed over a range of 1:100 to 1:100,000 relative Rh-positive cell frequencies. These methods should prove of general use in applications where original frequency information about rare cell subpopulations is important and would be lost if enrichment procedures were to be used.

ASSIMILATION OF PHOTOSYNTHETIC PRODUCTS OF ZOOXANTHELLAE BY A REEF CORAL

1. The hermatypic coral, Pocillopora damicornis was incubated in the laboratory and in its reef habitat with Na₂¹⁴CO₃ for 1-24 hours. Controls were incubated in darkness. ¹⁴C fixation in light exceeded that in darkness. 2. Fractionation of corals labeled on the reef for 24 hours revealed that 35-50% of the total ¹⁴C fixed appeared in the animal tissue lipid (as ¹⁴C-glycerol) and protein. From a comparison with dark controls it is concluded that photosynthetic products of zooxanthellae are translocated to host coral tissue. The skeletal organic matrix also acquires ¹⁴C. 3. Zooxanthehellae isolated from corals and incubated in a homogenate of host coral tissue selectively release glycerol and traces of other organic material including glucose, alanine, and glycolic acid confirming previous observations.