Fig.4. Mean heavy metal concentrations (µg/g dry weight) in sediment soft tissue and three sections (one, two and three) of Bufonaria echinata specimen collected from Salakh site.
Percentages of fine fraction (clay and silt) and TOM in the sediments from Salakh site were slightly higher than those in Suza site.
According to the Kruskal Wallis test metals occurrence were significant differences (p<0.05) in sediments and tissues among the sites, in order to: for sediments in the case of Cd: Salakh site > Suza site and for Ni, Pb: Suza site > Salakh site, in soft tissues in the case of all elements: Suza site > Salakh site and in the case of shells just for Cd: Salakh site > Suza site (Figure 3, 4).
The main positive correlation was found between concentration of Pb (p<0. 05, r= 0.834) and Ni in the soft tissues (p<0.05, r= 0.941) and Cd (p<0. 05, r= 0.891) in shells with sediments.
According to the Wilcoxon Signed Ranks test in comparison to metal bioaccumulation in the soft tissues with shells from both sites except for Ni in the Suza site significant differences existed in all elements (P < 0.05), at both sites for Cd: soft tissue> shell, for Pb and also in the case of Ni in the Salakh site: shell> soft tissue (Figure 3, 4).
Possible associations among the elements in the selected soft tissues of the specimens were assessed by Spearman’s correlation coefficient test. Only significant correlation was found between Ni and Pb (P= 0.008, r= 0.928).
Significant correlation coefficients were also found for some size-dependent characteristics of the shells (p< 0.05): positive width-dependent relationship in the case of Ni (r= 0.812, p= 0.050), positive correlation between the maximum shell length and Cd accumulation in the soft tissues from Salakh site (r= 0.870, p= 0.024).
Weight-dependent trend observed for accumulation of the Cd in soft tissues from sampling Salakh site.
The composition of the mollusk’s shell is strongly linked to the chemical mineralogy which includes metal accumulated from the environment, and therefore metal concentrations in the shell follow the metal concentrations in their environments (Amin et al., 2006). Shells also have some feasible advantages over the use of soft tissue as they can expose less variability, metal concentrations consortium over the life of the organisms, able to give an idea on the metal levels in the past and offer signal advantages in easy preservation and storage (Palpandi et al., 2010), therefore Gastropod shells record environmental pollution so that it can be profitable in studying metal accumulation.
According to the achieved results, in relation to all examined elements except for Ni section one with three and Cd in three sections from Souza site, significant difference (p<0.05) between each shell sections were visible (figure 3, 4), Most of the lipophilic shell apparatus located at the growing margin of the shell encounter and may be instrumental in facilitating the delivery of lipophilic defiles from the soft tissue to the shell (Walsh et al., 1995). Therefore, the differential accumulation of heavy metals in the three section of shell of B. echinata may reflect the operation of detoxification mechanisms that involve translocation of contaminants out of the metabolically active tissue into the safe storage matrix of the shell. Major source emissions of these three elements (Pb, Ni and Pb) in the environment were probably due to oil pollution from the oil emissions in these sites so heavy metals gathering up could be due to presumably higher rate accumulation in some of the shell sections of B. echinata in the desired period of its life time which were utilized in our research.
Cd, Ni and Pb accumulated tell us that most prevalent contaminant in B. echinata are from oil related activity in those sites but the significant differences, which were observed between metals concentration in the sediment samples collected from two sites, indicate that, heavy metals do not completely originate from the same sources. For example, the elevated levels of Cd in salakh site could be related to inputs from oil smuggling or might be due to percentage of clay fraction from sampling salakh site were slightly higher than Suza site. On emancipate into the aquatic background, heavy metals may undergo divers destiny-determining chemical and physical processes. Chemical processes are ion exchange, adsorption, precipitation, dissolution, volatilization, and complications (Peijnenburg & Jager, 2003).
Clay minerals are essential obvious ion reversal materials therefore in outward soils; the clay minerals are generally coated with metal oxides (Peijnenburg & Jager, 2003). These coating substances supply the facet typical that are notable in the reversal of mentioned heavy metals (Peijnenburg & Jager, 2003), on the other hand smaller particles have a larger surface area: volume ratio and therefore contain higher concentration of metals (Parizanganeh, 2008).
According to Presley et al., (1980) the elemental concentration of sediments not only depends on anthropogenic and lactogenic causes, but also on the textural characteristic, organic matter content, mineralogical composition and depositional environment of sediments (e.g. Cauwet 1987; Biksham et al., 1991; Martincic et al., 1990; Villaescusa-Celaya et al., 2000). In general, a concentration of dissolved Ni and Pb in sediment of the Suza site was significantly higher than concentration of them in Salakh site (p<0.05), which may be transported by tidal currents and waves. The fact that tidal current is responsible for transporting and spreading of contaminant in the coastal waters have often supported by previous summarizes (Teuchies et al., 2008). Since in contrast to the Salkh site, Suza site is located at less distance to Coastline therefore sources of this local Pb contamination could be connected with leaded petrol from cars and specially boats or ships in Suza pier. The same as shell length and width, the weight of gastropods varied slightly between sites (gastropods in suza site are slightly smaller than which in salakh site) and this had effected on metals accumulation, clearance rate, and heavy metal body burden, It is known that growth performance of marine organisms depends on many environmental factors such as temperature, bottom types, oxygen availability, salinity, etc (Yap & Edward, 2010). It means that the growth performance of the marine organisms in the heavy metals polluted area is limited by some of these environmental factors which put the organisms into a lower metabolic state and therefore, affecting their growth rate thus lowering the values of allometric parameters(Yap & Edward, 2010).
The higher bioaccumulation of Cd in soft tissues of gastropods from sampling Suza site than Salakh site can be explained by this fact that concentration of cadmium decreases with increasing size, because the concentration of metals in the younger animals, that have smaller size, is higher due to higher metabolic activity (Kontopoulos et al., 2003). However, in normal circumstances, the higher allometric parameters could also influence lower accumulation of heavy metals in the snail (Larger/adult individual accumulate lower concentrations of metal than smaller/young individual) (Yap & Edward, 2010).
Cadmium forms only weak complexes with organic matter (Peijnenburg & Jager, 2003), therefore the slightly increased of TOM in Salakh site couldn’t have affected in the cadmium availability.
In the case of Cadmium in shell increases of Cd in the sediment of salakh site can indicate the amount of them in shell of this gastropod, in this site.
The sediment is the habitat for benthic animals and B. echinata is the benthic gastropod (Hosseinzadeh et al., 2001). According to the achieved results (figure 3, 4) Pb, Ni accumulation in the soft tissues, and Cd in the shells were reflected the amount of them from sediments. The soft tissues of marine gastropods are broadly admitted as more methodical accumulators of heavy metals than shells (Amin et al., 2006). as indicated by Cd bioaccumulation in gastropods soft tissues from the Suza site.
However, while low Cd concentration was found in the sediment samples of Suza site, surprisingly high concentrations of it was measured in soft tissues of B. echinata sample from this site. Heavy metals accumulation by gastropods not only depends on the metals concentration in ambient environment but also, many other factors such as salinity, temperature, and pH may affect speciation and bioavailability of metals (Lu et al., 2005). Farther more interaction among heavy metals can outcome both in stimulation and antagonism, as described by Luoma, (1983). Cadmium uptake is stimulated (induces uptake of one metal synthesis of binding sites that affect accumulation of both metals) by lead concentration (Geffard et al., 2003). That might be affected the amount of the cadmium uptake by B. echinata in these sites and its elevation in soft tissues of B. echinata from the suza site. In the other hand for Cd in Salakh site it might has been entranced to this site recently and presumably hasn’t still been accumulated from sediment of this site by the sampling time.
Generally, Ni in Salakh site and Pb levels were higher in the shells compared to the soft tissues. This could be due to the calcareous nature of the shells which allows it to retain the metals as compared to the soft tissues which can excrete (Dambo & Ekweozor, 2000). Heavy metals impact on aquatic ecosystems produce a variety of complex feedback reigned by divers fundamental points: (1) nature of the toxicant; (2) concentration; (3) exposure time; (4) environmental characteristics of the receiving system; (5) age, condition, etc.
, of exposure organisms; and (6) the present of other toxicants (Nebeker et al., 1992).
The differential rapport of the metals to the binding sites might be related with the dissimilar metal accumulations found in the different tissues. The towering level of a metal found in a particular tissue could be due to the metal being tightly bound to the metallothionein as was reported by Viarengo et al., (1985).
There was higher concentration of Pb in shells than in the soft tissues and this was also observed by Fowler, (1990), Dambo & Ekweozor, (2000), Howard et al., (2008) , Yap & Cheng, (2009) lead accumulated preferentially in the shells of marine shellfishes rather than in the soft tissues, could probably be due to the case that the crystalline structures of the shell matrix have a higher roominess for incorporation of these metals than the soft tissues (Amin et al., 2006).
Pollutant analysis of B.echinata revealed that both the shell and soft bio accumulated a diverse range of heavy metals (Figure 3, 4). Heavy metals in the soft tissue and shell of B.echinata may reflect the operation of detoxification mechanisms that involve translocation of contaminants out of metabolically active tissue into the safe storage matrix of the shell. Compounds that cannot be fractured freely by detoxification systems then either accumulate within lipophilic components of the soft tissue or be totally isolated from cellular metabolism by translocation to and storage within the shell (Walsh et al., 1995). The bioavailability of sediment-bound metals to the aquatic organisms might be connected to the speciation of metal in aqueous and solid phases (Forstner, 1990), nutritional mineral need the organisms, binding site of metals in biological boundary and solubility of metals connected to chemical and physical condition. That can clarify the key correlation was established between concentrations of heavy metals in tissues with sediments (Watanabe et al., 1997).
The associations between heavy metal levels in soft tissues of different snail’s species and the biometric characteristics (especially length) regularly have been researched and not only the results varied between the studies but also sometimes even contradictory results have been obtained from different researches. Significant correlation coefficients were found for some size-dependent characteristics of the shells:
As mentioned before, in the present study soft tissues size dependent relationships (positive) were observed only for Cd and Pb in Salakh site. The relatively similar results have been reported by e.g. Davies et al., (2006) and Lacerda et al., (2007). This can be attributed to the feeding way, the longtime of the stay in polluted environment and metabolic activity of adult’s organisms.
Some of our results regarding the relation of heavy metal contents to Cubadda et al., (2001) body weight comparable to those of previous studies, according to the relationship between metal content in the marine snails Monodonta turbinata Born and Monodonta mutabilis Philippi and the body weight highly significant straight line correlation for cadmium.
Unfortunately, investigated gastropod has not been considered in Iran For the purpose of this there aren’t any standard for measuring heavy metals in gastropods and comparison official figure for human’s nutrition. The widest and narrowest range of variations can be observed in the case of Cd and Pb, respectively. These data declare that in compared to other world areas metal contents in the gastropod species from the studied region in most cases except in the case of Ni and Cd in soft tissue of suza site either fell within the range for other areas or are lower. However, it should be noted that some of the values mentioned in the table correspond to the relatively polluted sampling sites. The divergence in accumulation of the metals which can be seen clearly in the table especially in the case of b. Rana inter- genus may be arose from unequal feeding rituals, the variation in the aquatic environments affecting the source and degree of water pollution expanding rates of the species and some other additional (Türkmen & Ciminli, 2007). The highest mean value of Ni in the tissues in this study was higher than that obtained by Blackmore & Rainbow, (2000) from Bufonaria Rana from the southeastern waters of Hong Kong region and Astani et al., (2012) from Thais mutabilis from intertidal zone of Bandar Abbas South Golshahr intertidal zone of Bandar Abbas Posht-e- shahr waterfront regions.
Comparison of the gained data from this study with the global baseline values, different guidelines and results of several other researches on marine sediments shows that in the case of Cd our results in Suza site were considerably higher than the global baseline values and world average shale. The mean concentrations of Pb and Ni in Salakh site from the study area were relatively higher than both values. In the case of Cd in Salakh site and Pb and Ni in Suza site our results were higher than the global baseline values and the world average shale.
The comparison of heavy metals level in the sediment of the present work with the result of the previous studies performed by other researchers in other parts of the Persian Gulf is given in table 4 shows that the level of Pb in Suza site was higher than those reported by ROPME, (1999) (RSA, Northeastern Iranian coast), (ROPME, 1999; Al-Darwish et al., 2005) (Persian Gulf, Bahrain).
According to the mentioned researches, heavy metals contamination in the Iranian coasts of the Persian Gulf had been increasing from year 1999 to 2005 (Pourang et al., 2005) concluded that oil tankers, oil drilling in the region and current transportation were possible sources of metals contamination. The concentrations of Cd, Ni and Pb in the sediment from north part of the Persian Gulf (Pourang et al., 2005) were higher than present study. The highest levels of measured metals in the sediment in present study did not exceed the Effects Range Low (ERM), Probable Effects Level, USEPASV: U.S. Environmental Protection Agency Screening Values for bed sediments (PEL) proposed by NOAA (National Oceanic and Atmospheric Administration), except for Ni concentration in Suza site.
Amount of the Cd, Ni and Pb measured in the ambient sediments, shells and soft tissues of the Bufonaria echinata are declared that Suza and Salakh sites are impressively contaminated by those heavy metals, furthermore have been extracted from the result that the shell of these gastropods can accumulate the amount of heavy metals from the surrounding sediments over a period of time (fig 3, 4). Also from eco toxicological point of view, the accumulative of Cd, Ni and Pb, and consistently high levels of them in the two tissues of B. echinata population collected from two sites, indicated that this species is a potential biomonitor of heavy metal pollution especially in suza site. Soft tissues of B. echinata accumulated Cd several times more than marine sediment, so it is noticeable to declaration that gastropods have a vital position in the ecosystem and they supply essential connection between primary producers and higher consumers in the food chain (Lance et al., 2008). From the results are concluded that Bufonaria echinata shell type is suggested as an appropriate index for biomonitoring of Pb, Ni, Cd, however, complementing research is required for definite judgment beside of that it is suggested that molluscs play a vital role in the removal of these heavy metals from the marine environment via the eventual deposition of pollutants traced in the shell matrix to the sediments and therefore decreasing their availability within the food chain (Gobbeler & Klussmann-Kolb, 2011).
With regard to the receiving various types of pollutions by Suza and Salakh sites, regular monitoring of these sites is highly recommended.