Heavy metals in . the rock oyster Crassostrea iridescens ( Filibranchia : . Ostreidae ) from Mazatlan , Sinaloa , Mexico

Abstraet: Two populations of Crassostrea iridescens were sampled off Mazatlan and in a zone free of anthro­ pogenic activity. The .bivalves were collected from February 1992 to February 1993. Concentrations of Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn were determined by flame atomic absorption spectrophotometry. Annual mean val­ ues in the sampling �bne off Mazatlan were higher in Co, Cu, Fe, Mn, Ni and zn than in the sampling zone free of anthropogenic acti:vity; however, differences were not statistically significant at 95% confidence leve!. Urban discharges in the coastal zone in fron! of Mazatlan city, have litt¡.e or no heavy metal concentratiolls dissolved or particulated.

In the Pacific region of Mexico, the rock oyster Crassostrea iridescens (Hanley 1954), is present along the coast, and it is asso ciated with subtidal rocky suhstrates (Páez Osuna et al. 1995).The species is geographi cally distributed from La Paz, Gulf of California, to northern Peru (Keen 1971).
Recently, its r):p roductive cycle was described by Frías-Estiericueta et al. (1997): there is a resting : pJtase \ trom NQvember through January;/ a matu�ation phase from April to May; and a spawning phase from July to October.

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Bivalves arf! extensively used in moni toring programs �n the marine environment 4qe to their abilit)i to concentrate pollutants to �y�ral orders of magnitude aboye ambient ¡,fey d Js in sea w�ter (AI-Madfa et al. 1998).
. According to Phillips and Rainbow (1993) an Regarding heavy metals, various pracesses influenced by anthropogenic activi ties may contribute to increase concentrations in natural waters (Singh and Steinnes 1994): (1) run-off fram agricultural and urban are as, (2) discharges from mining, factories and municipal sewer systems, (3) leaching fram dumps and former industrial sites, (4) atmos pheric deposition.
The purpose of this study was to evalu ate the concentratíon of eight heavy metals (Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn) in two dif ferent sites in Mazatlan, Sinaloa, Mex.One site with little influence and another without urban discharge influence.

MATERIALS AND METHODS
Individuals of C. iridescens were col lected (9-10 samplings) fram two sites along the coast of Mazatlan, Sínaloa, Mexico from February 1992 through February 1993.One station was located in front of Mazatlan, which receíves Httle urban wastewater (23° 15' 41.5"N and 106° 15' W).The second sta tíon was located north of the city between 23 ° 28' and 23° 29' N and 106° 36' and 106° 37' W, an area characterízed by the absence of significant anthropogenic activity.Oysters were sampled by scuba diving, collecting organisms of similar size to minimize the effect of body weight (Anonymous 1980;Marina and Enza 1983).In the laboratory, after taking biometric measurements, the soft tissue of each composite sample of 15-20 organísms was dried at 70 oc.AH glassware and plastic devices used in the manipulatíon of the samples were completely acíd-washed (Moody and Lindstrom 1977).Pulverization and homogenization were achieved by grind ing the tissue samples in a teflon mortar.Samples and blanks for analysis were pre pared by digesting aliquots of dry mate rial with concentrate quartz-distilled nitric acid using the standard addítion method and the metal concentrations were determined by flame atomic absorption spectrophotometry (Páez-Osuna et al. 1993a, 1995).The heavy metal co n centration confidence intervals were calculated using the method proposed by MiIler and Miller (1987).The performance of the method was evaluated by analyzing a reference material mussel homogenate MA M-2IMT (Anonymous 1985).The concentra tions found for Cd, Co, Cr, Cu, Ni and Zn were in the acceptable range reported, only Fe and Mn were underestimated (Páez-Osuna et al. 1995) (Table 1).

RESULTS
Table 2 and 3 list heavy metal concen trations in the soft tissue of C. iridescens in both stations.
There were no significant correlations between metal concentratíon and body length (T-students, p > 0.05).Boyden (1974) sug gested that for sorne metals the concentra tions in small organisms may be greater than those in large organisms.With regard to the correlation coefficient between metal concen trations, only Cu-Zn revealed a significant coupling with r= 0. 95 (p=0.005) and r= 0.88 (p=0.05) for stations 1 and 2, respectively (Fig. 1).The intervals of concentrations for Cd, Co, Cr, Cu, Fe, Mn, Ni and Zn were: 2.2-3.3,4.4-6.5, 1.6-2.9,35.8-118, 110-271, 11.5-20.3, 3.1-9.8 and 639-822 Ilglg (dry weight), respec tively, for station 1; while for station 2: 1.9-3.9, 4. 5-5.4, 1.7-2.8, 32.3-112, 15-377, 11-19.2, 2.8-8.2 and 604-843Ilg/g (dry we i ght), respectively.In station 1 the highest concentration of Cd, Co, Cr, Cu and Zn was determined in autumn-winter months; while the Ni, Fe and Mn its highest value occurred in the Spring Summer months (Table 2).Regarding station 2 the highest concentration of Cd, Co, Cu, Fe and Zn was found in autumn-winter; and Cr, Mn and Ni in spring-summer months (Table 3).There were no significant differences between the highest values of spring-summer and autumn-winter.Only Ni, in both stations, Mean values of Co, Cu, Fe, Mn, Ni and Zn were higher in station 1 than in station 2, only Cd and Cr mean values were higher in sta tion 2. However, no significant difference was observed (means comparison test, p> 0.05).
In both stations the decreasing order of metals values was the same.Zn>Fe>Cu>Mn>Co>Ni>Cd>Cr

DISCUSSION
Around the world, the coastal zones are subjeet to the direct release of urban and indus trial discharges, sueh inputs are known to eon tain heavy metals, whieh may inerease traee Concerning correlations between heavy metal concentrations in the soft tissue of C. iri descens, only Cu-Zn in both stations were sig nificant.These correlations may be attribulable to the fact Iha! C. iridescens metabolically requires Cu and Zn in the same proportion dur ing its life cycle, and similar physicallchemical properties of the metals involved in the meta bolic processes (Szefer et al. 1994).Paéz-Osuna and Marmolejo-Rivas (l990a) found the same correlations in Crassostrea corteziensis from the Port of Mazatlán, Sinaloa, Mexico.
Trace metal seasona!variations have been reported for severa!species of molluscs (Bryan 1973).The causes of seasonal fluctua tions have been associated with the food sup ply, changes in the run-off of particulate metal to the sea, due to high precipitation (Fowler and Oregioni 1976), variations related to the reproductive cycle (Latouche and Mix 1981;Paéz-Osuna and Marmolejo-Rivas 1990 b).
In stations 1 and 2, the peak of Mn was determined in July and May, respectively.According to Frías-Espericueta et al. (1997) in these months C. iridescens was in the matura tion phase and close to spawning.Marina and Enzo (1983) found in Donax trunculus that Mn plays an important role in those phases prior to spawning, possibly as an enzymatic catalyst (Galtsoff 1964).Watling and Watling (1976) found in Choromytilus meridionalis that the trace metal distribution changes as a function of growth and maturation.
In the present work, essential metals were accumulated in amounts higher than non essential metals.This order suggests that C. iridescens accumulates heavy metal s accord ing to the biological effect of metal s andJor the concentratíon in the surrounding seawater.Páez-Osuna and Marmolejo-Rivas(1990a) found that, when the levels of trace metal s in the oyster Crassostrea corteziensis were corre-lated with those in the particulate fraction of the month irnrnediately preceding, a significant relationship was obtained for several metals.This is the result of specific mechanisms as uptake, transport, storage and excretion of metals (Simkíss andMason 1983, Phillips andRainbow 1989).Zn showed the highest concentrations.Generally, oysters have been considered as strong accumulators of zinc, and these organ isms accumulate high concentrations of zinc in detoxified granules (George et al. 1978).With regard to Cu, Eisler (1981) observed thatsome species could exhibit high Cu concentrations dtie tothe proximity to anthropogenic source or to biotic/abiotic factors capable of modify ing Cu uptake and retention.Rainbow (1988), and Depledge and Rainbow (1990), found that heavy metals are taken up and accumulated by marine invertebrates to tissue and body con centrations usualIy much higher on a wet weight basis than concentrations in the sur rounding seawater.Metal accumulation strate gies fall along a gradient from the strong accu mulation of aH metal taken up, to the regula tion of the body metal c o ncentration to an approximately constant level; by balancing metal excretionluptake (Rainbow 1993).Zn, Fe, Cu, and Mn are biologically essential a nd play an important role as cofactor in enzymat ic processes (S i ngh and Steinnes 1994)" Comparing metal concentrations in oysters from this study with other bivalves colIected in other zones of the world (Table 4), our values are lower than those reported by Goldberg et al. (1978) for a polluted zone (except for Ni), besides metal concentrations of this study are slightly higher than those of Langebahn lagoon (South Africa), an area rel atively unpolluted with respect to trace met als, and according to Watling and Watling (1976), these values represent near-back ground levels and that metal uptake is "nor mal".However, and according to Rainbow (1993), defining a background concentration for a metal is too difficult, given the role of biological variables.
A general conclusion is that urban dis charges in th� coastal zone in front ofMazatlan city, have liule or no heavy metal concentra tions dissolved or particulated .. This work require ;¡ future investigations to analyze urban dischar g es.However, dle analysis of total col iform t e c es (Ortiz-Arellano, in prep.)revealed higher values in station 1 than station 2.

ACKNOWLEDGMENTS
The authors thank H.M. Zazueta-Padilla for his help during the analytical work and Marcos Merino for English corrections.We gratefulIy acknowledge the.anonymous reviewers for t h eir thoughtful comments.RESUMEN Dos poblaciones de Crassostrea iridescens fueron muestreadas en la zona coster¡l de la ciudad de Mazatlan y en una zona libre de actividad antropogénica.Los bivalvos fueron colectados de febrero de 1992 a febrero de 1993.Concentraciones de Cd, Co, Cr, Cu, Fe, Mn, Ni y Zn fueron 4etenninados por espectrofotome!ria de absorción atóIJrica.Los valores medios anuales en la estación de muestreo de la ciudad de Mazatlán fueron mayores en Co, Cu, Fe, Mn, Ni y Zn que los determinados en la zona libre de actividad antropogénica; sin embargo, las diferencias no fueron significativas a! nivel de confianza del 95%.Una conclusión es que las descargas urbanas en la zona costera en frente de Mazatlán no tiene, o tiene, bajas concentra ciones de metales pesados disueltos o particulados.
ideal biomonitor should fulfill several requi sites: should be sessile or sedentary in order to be representative of the study• area; should be abundant in study areas, easy to identify and sample at all times of year, and should have sufficient tissue for analysis of the cont aminant of interest; should be hardy, tolerat ing wide ranges of contaminant.concentra tion, thereby permitting the design of trans plant experiments and laboratory studies of contaminant kinetics; and should be strong accumulators of the relevant trace metal.These characteristics, its wide distribution and commercially valuable seafood species, .makes C. iridescens a very suitable species as an indicator of pollution (biomonitor) for heavy metals.
Fig.l.Re!atinoship between Zn and Cu (¡.tg/g, dry weight) in the soft tissue of the oyster Crassoslrea tridescens alo .ng study periodo A: station 1; B: station 2.
Certified with satisfactory (A) or acceptable (B) degree of confidence; values not certified (C)(Anonyrnous 1985).

TABLE 2
Heavy metals concentrations (/lg/g, dry weight) in soft tissue of Crassostrea iridescens collected in sta/ioni.

TABLE 3
Heavy metals concentrations (/lg/g, dry weight) in 50ft tissue of Crassostrea iridescens collected in station 2.
metal concentrations in the coastal zone, some of which are toxic and can endanger human health (AI-Madfa el al. 1998).In the present study, station 1, which receives httle urban di s charges, apparently showed the highest annual mean values of the majority of metals.