Freshwater Molluscs as Indicators of Bioavailability and Toxicity of Metals in Surface-Water Systems John F. Elder* and Jerilyn 1. Collins t
Contents I. Introduction .................................................... II. Characteristics of Freshwater Molluscs .............. . . . . . . . . . . . . . . . A. Gastropoda (Snails) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Pelecypoda (Clams, Mussels) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Introduced Bivalve Species .................................... III. Freshwater Molluscs as Bioindicators of Metals .................... A. Characteristics that Pertain to Biomonitoring . . . . . . . . . . . . . . . . . . . . B. Special Characteristics of Corbicula Clams. . . . . . . . . . . . . . . . . . . . . . . IV. Bioaccumulation and Toxicity of Metals to Freshwater Molluscs. . . . . . A. Bioconcentration and Depuration .............................. B. Differential Bioaccumulation Among Tissues and Organs. . . . . . . . . . C. Relation to Individual Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . D. Bioaccumulation in Relation to Environmental Concentrations .... E. Effects of Metal Mixtures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Biochemical Effects and Metallothioneins . . . . . . . . . . . . . . . . . . . . . . . . G. Metals as Molluscicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Comparisons with Nonmollusc Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary........................................................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37 39 40 41 41 42 42 45 46 47 58 59 61 64 65 66 66 68 69
I. Introduction During the past several decades, studies from a variety of locations have demonstrated widespread occurrence of metals in surface waters at concentrations significantly higher than background levels. Elevated concentrations are not limited to certain water types or polluted areas; they appear in all types of systems and in all geographic areas. It is clear that metals enter the aquatic systems from diverse sources, both point and nonpoint, and they can be readily transported from one system to another. Transport routes *U.S. Geological Survey, 6417 Normandy Lane, Madison, WI 53719. tU.S. Geological Survey, 227 N. Bronough St., Suite 3015, Tallahassee, FL 32301.
© 1991 by Springer-Verlag New York Inc. Reviews of Environmental Contamination and Toxicology, Vol. 122. 37
38
J.F. Elder and 1.1. Collins
include atmospheric, terrestrial, subterranean, aquatic, and biological pathways (Elder 1988; Salomons and Forstner 1984). Many trace metals have toxic effects on some biological species at concentrations common in water bodies that are contaminated to some degree. Elevated metal concentrations can cause severe reduction or elimination of intolerant species, thereby having a significant effect on the diversity and trophic structure of the biological community (Peterson 1986). Bioaccumulation-the uptake of contaminants from the surrounding media or food and storage of the contaminants in body tissues-can lead to latent toxic responses in the affected organism or toxicity to organisms higher in the food chain. Among the effects of such propagation through the food chain, there is the possibility of human health hazards. Concerns about contaminants in the aquatic environment have led to extensive research on chemical behavior and biological effects of such contaminants. Although many questions about sources, distribution, partitioning, and transport can be addressed by hydrologic monitoring and chemical analyses of environmental samples, other questions having to do with fates of the metals and their effects on the ecosystem can be addressed only with biological research, or more appropriately, research that makes use of combined biological, chemical, and hydrologic data. Biological research of contaminant problems in aquatic systems may take anyone of several forms. One of the most common general approaches is the use of bioindicator organisms. This involves measurement of contaminant effects on aquatic biota that are exposed to the contaminants either in the natural environment or by some simulation of the natural setting. The organisms may be natives or introduced for the purpose of the study. The exposure time may be short-term or long-term, depending on whether the research is primarily focused on acute effect or chronic effects. Three general approaches involving the use of bioindicator organisms are commonly applied to the assessment of contaminant effects in aquatic systmes. These approaches are: 1. TisS1Je analysis. Organisms collected from exposed environments are used for chemical analysis of tissues to detect bioaccumulation. The procedure involves the use of either native organisms or introduced organisms that are contained at the study site for a limited time. The procedure can also be used in the laboratory, where the test organisms in controlled enclosures are exposed to known concentrations of contaminants. 2. Toxicity testing. Organisms are exposed experimentally to contaminants and their toxicological responses to such exposure are monitored. Acute toxic effects are usually measured as the rate of mortality caused by the exposure and chronic effects are manifested by a variety of sublethal responses. Testing usually involves exposure to a gradient of contaminant concentrations, ranging from a concentration that is not expected
Mollusc Indicators of Metals in Surface-Water
39
to produce response to one that is equal to or greater than any concentrations that are likely to be found in nature. If possible, the test systems include controls, in which no contaminants are present. A great variety of testing procedures are employed. 3. Ecological surveys. Existing biological communities in aquatic systems are examined and described where contaminati-on is suspected. Commonly measured ecological variables include community structure, biomass, species diversity, and metabolic functions such as primary productivity or respiration. If there are significant trends over time and (or) space, these may reflect stresses caused by contaminants. Molluscs have been used extensively as bioindicators of pollution in estuarine and marine systems (Bayne et al. 1985), but much less in freshwater systems. Like the marine species, freshwater mussels and snails have the potential to be very useful for biomonitoring studies. This potential has been realized with increasing frequency in recent years, as reflected in the literature on inland surface-water contaminant studies. Although much more limited in terms of species diversity than their marine counterparts, the freshwater molluscs are widely distributed in nearly all types of surface-water systems, and they are frequently very abundant. The purpose of this review is to examine current information on concepts and principles of metal biogeochemistry and toxicity as elaborated by studies with freshwater molluscs. There is a limited but expanding body of literature describing work with freshwater molluscs. Because of the limited literature, there are information gaps. Some processes of metal biogeochemistry that have been thoroughly studied in marine animals, or in other freshwater groups, have been rarely, if ever, demonstrated in studies of freshwater molluscs. In recognition of this limitation, this review is restricted almost entirely to freshwater work to focus on a relatively narrow scope. It is hoped that a synthesis of this work will facilitate evaluation of the current status of research in this area and the most promising and needed directions for future research. Important advantages and disadvantages of the use of freshwater molluscs as indicators of metal contamination are also discussed. As stated in an earlier review (Elder 1988), some results from research on metals in aquatic systems are controversial; comparisons among different studies reveal apparent inconsistencies. Even if the focus is narrowed to studies of freshwater molluscs as bioindicators of metals, some of the same inconsistencies persist. One of the objectives of this review is to examine the inconsistencies and offer possible explanations for them.
II. Characteristics of Freshwater Molluscs The taxonomy of molluscs and other freshwater invertebrates is described by Pennak (1978). Included in that discussion are detailed descriptions of morphologicaL behavioral, developmental, and ecological characteristics of
40
J.F. Elder and 1.1. Collins
each taxonomic group. The following is a synthesis of mollusc characteristics that are particularly important with respect to their use as contaminant bioindicators. Molluscs are found in almost all types of freshwater habitats, from the smallest ponds to large lakes and rivers. Only in extremely cold alpine lakes and streams, acidic waters, or grossly polluted areas, they are likely to be absent. Even in cold climates, they can be successful if there are seasonal warm periods; they can survive the cold periods by burrowing into bottom mud or organic debris and hibernating. Mollusc distribution can be limited by very low concentrations of dissolved salts, especially calcium carbonate, which is important for shell construction. Because of their requirement for calcium carbonate, molluscs are more likely to live in alkaline waters and they are rare in waters more acid than pH 6. Because of rather high dissolved oxygen requirements, they are also excluded from anoxic or near-anoxic environments. The principal predators of freshwater molluscs are fish. However, only a few fish species, such as suckers, perch, and whitefish, depend on molluscs for a significant portion of their diet. Snails are also eaten by ducks, shore birds, and sometimes amphibians. Clams and mussels are commonly eaten by terrestrial mammals, particularly muskrats. Some mollusc species are hermaphroditic (each individual has male and female sex organs and is capable of self-fertilization). More commonly, the sexes are separate. Eggs are fertilized internally, but embryonic development differs between gastropods and pelecypods. The early developmental stages of snails proceed within eggs that are deposited externally in gelatinous masses. In bivalves, the zygotes initially develop in distentions of the gills (marsupia), then become free-swimming larvae or glochidia larvae that are parasitic on fish hosts. Hundreds of mollusc species are found in freshwater habitats throughout the world. Some species have been accidentally or intentionally transported across ocean barriers and introduced to new continents. Some of these introduced species have been very successful, often displacing endemic populations. A. Gastropoda (Snails)
Freshwater gastropods are normally vegetarian grazers, feeding principally on various types of attached algae that are common in shallow-water environments. For this reason, snails are mostly found in water less than 3 m deep. Some snails are scavengers, some are omnivorous, and a few species occasionally feed on live animals. Food intake increases substantially with increasing temperature. The univalve shell of gastropod is characteristically spiral shaped and provides protection for the internal organs. The shell is covered with an
Mollusc Indicators of Metals in Surface-Water
41
organic epidermis that has an important function of protecting the underlying calcium carbonate layer from erosion. B. Pelecypoda (Clams, Mussels)
The pelecypods are characterized by a bivalve shell with a hinge ligament; hence they are commonly known as bivalves. Under natural conditions, the valves gape slightly to permit protrusion of the foot and siphons. Extension of the foot allows limited movement of the animal through the substrate. Most freshwater bivalve species are members of the family Unionidae. Other freshwater families are Sphaeriidae and Corbiculidae. Unlike gastropods, all the bivalves are aquatic. They can live in all surface-water systems, but they are most abundant and diverse in large rivers. Within river systems, bivalves tend to be more abundant in the lower waters than in the small tributaries. The large river systems provide continual replenishment of food in suspended particles, favorable substrate types, and abundance of oxygen. On relatively stable mixed gravel and sand bottoms, bivalves can be very abundant. In contrast, most species are not found on bare-rock bottoms or in shifting sands and muds. The bivalve shell consists of an outer protective periostracum. Beneath this is the prismatic layer, consisting of prism-like blocks of calcium carbonate. The inner, thick, nacre layer consists of alternating laminae of calcium carbonate and organic materials. The shell can be a site of accumulation of metals and the accumulation can be varied in the different layers (Imlay 1982). The principal feeding process of bivalves is filter feeding, whereby suspended microscopic particles are removed from water passing over the gills. Particles can include zooplankton, phytoplankton, and organic detritus. Food particles are trapped by mucus on the gills, then driven by ciliary action toward the digestive tract. In temperate climates, feeding and growth are considerably more active in the summer than in the cooler months. C. Introduced Bivalve Species
Currently, the principal environmental and economic importance of freshwater bivalves is the nuisance caused by overpopulations of introduced species. Notable among these in North America are the Asian clam (Corbicula-species) and, more recently, the zebra mussel (Dreissena polymorpha) that has become enormously abundant in parts of the Great Lakes (Griffiths et al. 1989). Fouling of underwater surfaces and clogging of waterways have become major problems due to the huge population growths of these species. Much of the recent bivalve biomonitoring research has come about in part because of the needs to understand and control the overpopulation problems. Research with Corbicula has developed
42
JF. Elder and J.J. Collins
considerably in the last decade and a similar expansion of research on Dreissena may soon follow. The great adaptive success of Corbicula has allowed it to displace native bivalve species in many river systems; hence it is ecologically disruptive as well as an economic nuisance. Several physical characteristics of Corbicula account for its competitive advantage (Kraemer 1979). It has reproductive adaptability, including hermaphroditism. Like marine pelecypods, but unlike other freshwater bivalves, Corbicula has a free-swimming larval form that is independent of association with fish hosts for further development. Corbicula also has greater mobility than many other bivalves because it has more rapid and coordinated foot movements. The juveniles have abyssal holdfast thread that allows the animals to inhabit a shifting, sandy river bottom that would exclude other bivalves. Finally, Corbicula has unusual ability to withstand harsh environmental conditions, partly because of its extensive mantle fusion, heavy protective shell, and narrow pedal gape.
III. Freshwater Molluscs as Bioindicators of Metals A. Characteristics that Pertain to Biomonitoring Gastropods and pelecypods, both freshwater and marine, are commonly selected as bioindicators of metals because of certain special characteristics that render them useful for this kind of application. These characteristics are basically consistent with the following criteria, similar to those suggested by Phillips (1977, 1980). for bioindicator organisms.
1. Tolerance to a wide range of metal exposures; ability to accumulate metals without suffering mortality. 2. Sedentary habits; slow and limited range of movement. 3. Sufficient life span to allow for sampling of more than one year class. 4. Abundance in the study area. 5. Sufficient size to allow chemical analysis of tissue samples. 6. Hardiness; ability to remain healthy during sampling and laboratory incubation. 7. Relative ease of sampling and identification. 8. High metal accumulation rates. 9. Responsiveness to changes in metal exposure. Most freshwater mollusc species commonly used as bioindicators are both large and sedentary; hence they simultaneously meet criteria 2 and 5. Their size gives them an advantage for selection as bioindicators over most other freshwater groups except fish, and their limited mobility gives them an advantage over fish. The only other major aquatic taxonomic group that has similar advantages for use in biological monitoring work is the macrophyte group-the attached vascular plants. Most freshwater mollusc species also meet criteria 1, 6, and 7, although their suitability with respect to these characteristics differs considerably
Mollusc Indicators of Metals in Surface-Water
43
among species. The advantages of their sedentary nature, large size, easy identification, and amenability to sampling and handling have been emphasized repeatedly (Benfield and Buikema 1980; Czarnezki 1987; Dauble et al. 1985; Foe and Knight 1986; Forester 1980; Foster 1981; Graney et al. 1983; Green et al. 1989; Hartley and Johnston 1983; Khangarot and Ray 1988; M unzinger 1987; Slooff et al. 1983). In addition, the molluscan feeding habits (grazing, filter feeding, and deposit feeding) make them good bioaccumulators of plankton, seston, and dissolved constituents in water (Foe and Knight 1986; Foster 1981; Friant 1979; Graney et al. 1983; Hartley and Johnston 1983; Wren et al. 1983). In most species, a moderate amount of metal bioaccumulation does not lead to disease or mortality. Some freshwater mollusc species have a relatively broad tolerance range to environmental conditions, including temperature, salinity, water hardness, pH, and dissolved oxygen, but others are very sensitive. Abundance (criterion 4) of the freshwater molluscs is highly variable. It is not unusual to find hundreds or even thousands of individuals per square meter at one site and none in a nearby site in the same water body. Differences among sites in environmental conditions such as substrate, temperature, water current, or food supply can cause this kind of variability, even if the differences are rather subtle (Pennak 1978). However, such variability is common to nearly any taxonomic group; it is not a unique disadvantage of the molluscs. The need for a multiyear life span (criterion 3) applies only to certain kinds of biomonitoring. Most mollusc species have life spans of more than 1 yr (Hartley and Johnston 1983), which is sufficient for use in acute toxicity tests or short-term bioaccumulation monitoring. For investigations that require longer-lived animals, there are several suitable freshwater mollusc species, such as some of the Unionid mussels, whose life cycles span several years (Green et al. 1989). A potential limitation that applies to the use of any organism as transplanted test species is the lag time before the influence of the destination environment dominates over the influence of the source environment. Reciprocal transplant experiments with Elliptio complanta by Hinch and Green (1989) indicated that when the test animals are transplanted to a new area, their growth and metal uptake remain largely a function of characteristics of their genotype and their home habitat, rather than their new habitat, for several weeks or months. The duration of this lag time probably depends on the extent to which the environments differ, as well as genetic acclimation capacity and behaviour. The importance of genotype and source habitat in controlling responses of freshwater molluscs to a new habitat has received only limited research attention; hence the extent to which it applies to diverse species and environmental conditions remains in question. Foe and Knight (1986) observed that bioaccumulation of metals in transplanted Corbicula clams
44
IF. Elder and J.J. Collins
closely reflected bioaccumulation by native clams following an acclimatization time of a few weeks. They suggested that the source effect was not a major problem in that case. However, the possible influences of long-term source effects and genetics on responses to contaminants should be considered in the design of transplant biomonitoring studies. Test animals that are transplanted to a study site should be taken from a common source. The responsiveness to changes in metal exposure (criterion 9) depends on the bioavailability of the metals, which, in turn, is controlled by numerous environmental factors (Luoma 1983). Although the behavior, physical characteristics, and feeding habits of the organism are important influences in bioaccumulation and toxicity, a great deal of variation and complexity may be expected, even within one species, because of the environmental influences. Therefore, the influence of environmental concentrations on bioaccumulation and response is rare if ever a simple relation. The literature contains numerous reports showing both good and poor relations between biological and environmental contaminant concentrations. Further discussion of these seemingly contradictory reports follows in a later section. Again, this problem is not unique to molluscs. Biological monitoring of coastal and estuarine contamination has been accomplished in recent years by "mussel watch" programs (Bayne 1989; Farrington et al. 1987). Since their initiation in U.K and U.S.A. in the middle 1970s, the mussel watch programs have provided a long-term and large-scale approach, using bivalves as "sentinel" organisms. A relatively small number of bivalve species, particularly blue mussels (Mytilus) and oysters (Crassostrea and Ostrea) are widely distributed in coastal areas around the world. This cosmopolitan nature, and the other features common to bivalves that make them useful biomonitors, led to the development of the mussel watch programs. Although the advantages of molluscs as bioindicators are frequently mentioned in the literature, some authors have pointed out a need for caution. Most of the precautions have to do with concerns about the resistance of some species to pollutants, tending to make them less sensitive bioindicators than some other taxonomic groups (Amiard-Triquet et al. 1986; Cairns and Cherry 1983; Klerks and Weis 1987; Slooff et al. 1983; V.-Balogh 1988a). The resistance is usually attributed to metal-binding agents, such as metallothioneins, synthesized by the organism itself when exposed to elevated metal concentrations in the surrounding media. Synthesis of protective biochemical agents such as metallothioneins can increase resistance to metal toxicity. Binding-agent resistance should be taken into account in studies of bioaccumulation or toxicity (Engel 1988). Another defense mechanism of molluscs is the ability to withdraw into the shell, at least during short-term episodes of unfavourable conditions (Doherty et al. 1987a). This defence response can affect acute toxicity test responses.
Mollusc Indicators of Metals in Surface-Water
45
As the complexities created by variable factors such as metal speciation and metallothionein production are considered, it becomes evident that inconsistent results are to be expected among different studies. Such variability led Coleman et al. (1986) to suggest that more emphasis to be given to ranges than to averages in reporting bioaccumulation or effective toxic concentrations. In their discussions of invertebrates in general as bioindicators, Benfield and Buikema (1980), Gaufin (1973), and Maciorowski and Clarke (1980) have recommended the approach of examining community responses rather than effects on single species. The variability of single-species responses may not reflect the cumulative impacts of the contaminants on the interdependent network of diverse species in the community. Freshwater molluscs clearly have significant potential as useful bioindicator species, and they are likely to be used more frequently for that purpose in the future. However, it is also clear that they are not universally suitable as bioindicators, as is true of any commonly used bioindicator organism. Generally, the most effective biomonitoring approach, if practical within the study design, is to use several bioindicator species representing different trophic levels in combination with chemical and hydrologic measurements. Results from such an approach may provide the kind of extensive and complementary data that are needed for thorough understanding of the contaminant problem. B. Special Characteristics of Corbicula Clams Among the dozens of mollusc species that inhabit inland waters, one of the most frequently used molluscs for biomonitoring is the Asian clam of the family Corbiculidae. Commonly known by its genus name, Corbicula, the taxonomy of the Asian clam is not yet clearly defined. Most investigators who worked with Corbicula have identified the species as either Corbicula fluminea or Corbicula manilensis. According to Pennak (1978) and Britton and Morton (1979), both of them are the same species, but other authors (e.g., Hillis and Patton 1982) consider the species distinction to be valid. The increasing popularity of Corbicula as a bioindicator organism can partially be attributed simply to its abundance and widespread distribution. However, Corbicula does have other attributes that make it especially well suited for biomonitoring work. Some of the characteristics that give Corbicula its competitive advantages over other bivalve species are also factors that make it useful as a bioindicator. Corbicula clams are generally recognized as efficient bioaccumulators of contaminants (Graney et al. 1983; Hartley and Johnston 1983; Rodgers et al. 1980). Despite this capability for bioaccumulation, the Asian clams can survive relatively high bioaccumulation. In addition, they are tolerant of temperatures as high as 36°C (Cairns and Cherry 1983) and salinities greater than 10%0 (Gainey 1978). This resistance to contaminants and adverse
46
1.F. Elder and J.J. Collins
environmental conditions has contributed to the success of Corbicula in becoming well established in North America rivers. Corbicula has been the test organism in a number of recent toxicity studies, some of which included considerable discussion of recommended procedures. Because of the high capacity for survival, the Asian clam is often used in chronic tests in which sublethal responses are monitored. Several kinds of measurements, including reproduction, growth, and condition index (the ratio of dry weight of soft tissue to shell volume), were discussed by Foe and Knight (1986). These indices are subject to error when used alone; however, an effective toxicity test approach is to apply all of them simultaneously and to complement them with tissue analyses. Belanger et al. (1986) found that shell and tissue growths of Corbicula were sensitive indicators of zinc stress. Even at zinc concentrations less than 0.05 mg/L considered protective of aquatic life during chronic exposure (U.S. E.P.A. 1987), growth was inhibited. Some studies have made use of Corbicula larvae instead of adults for biomonitoring of contaminants (Foster 1981). This procedure parallels the relatively common practice of using oyster larvae for biomonitoring studies in marine systems. Adult Corbicula are easily maintained in the laboratory, and spawning of the captive clams may be expected after a few months of conditioning and growth (King et al. 1986). The free-swimming veliger larvae are also easy to maintain in the laboratory. They are good bioaccumulators and are sensitive to elevated metal concentrations (Foster 1981), although not as sensitive as the cladoceran, Daphnia magna, commonly used in trace metal toxicity tests (Elder 1989). In addition, the use of individuals from a single larval brood provides a set of test organisms, all from the same parents, that are of the same size, age, and life history (Foster 1981). This homogeneity minimizes variability in response due to secondary factors. IV. Bioaccumulation and Toxicity of Metals to Freshwater Molluscs Many studies have demonstrated bioaccumulation of metals in tissues of freshwater molluscs. In situations where metal concentrations in the water and( or) sediments are elevated, some bioaccumulation occurs nearly always. However, the rates of bioaccumulation and correlations to environmental concentrations are by no means consistent. Previous reviews of marine and estuarine work (Luoma 1983; Morel and Hudson 1985) have given clear evidence that bioavailability and uptake of metals are highly dependent on geochemical and biological factors. There are major differences in bioaccumulation among biological species. Even within a single species, bioaccumulation can be a function of size, age, sex, genotype, phenotype, feeding activity, and reproductive state. There are also many environmental variables other than metal concentrations that influence bioaccumulation. Some of the most important of these are concentration and type of organic
Mollusc Indicators of Metals in Surface-Water
47
carbon, water hardness, temperatures, pH, dissolved oxygen, sediment grain size, and hydrologic features of the system. Whole-body tissue analyses can also be influenced by sediments and detritus in the digestive tract at the time of collection. To clear the digestive tract, a brief depuration time (ranging from several hours to three days) in clean water may be allowed prior to analysis. However, this procedure can also lead to inaccurate results due to the possibility that excessive depuration time may permit partial elimination of metals from body tissues. A second major area of biomonitoring research with freshwater molluscs involves toxicity testing. Mortality and sublethal effects following exposure to elevated contaminant concentrations are, like bioaccumulation, subject to much variability due to changing biological and environmental conditions. However, numerous authors have reported success, relative to the bioindicator prerequisites listed earlier, using snails and bivalves in both acute and chronic toxicity tests. Although many mollusc species have served as test organisms, Corbicula has been used more than any other single genus for this purpose. Overviews of the use of freshwater molluscs as bioindicator organisms in research on metal bioaccumulation and toxicity are provided in Tables 1 and 2, respectively. Some investigations have included discussions of particular factors that influence or relate to bioaccumulation, mortality, or sublethal effects; those factors are indicated in the tables. Sublethal responses commonly observed in freshwater molluscs include changes in respiration rates, growth rates, gill-cilia beating rates, reproductive capacity, enzymatic activity, valve closure, and production of metal-binding proteins. A. Bioconcentration and Depuration The bioconcentration phenomenon results in a tendency for aquatic biota to accumulate metals in their tissues at concentrations that greatly exceed ambient water concentrations. The bioconcentration factor (BCF)-a measure of the equilibrium ratio of metal concentration in biological tissue to metal concentration in ambient water-frequently falls in a range of 10 3 to 106 . In a study of C.jluminea, Graney et al. (1983) reported BCF ranges of 17,000 to 22,000 for copper and 1,700 to 3,800 for cadmium. They cautioned, however, that metal interactions were not considered in their study and such interactions could substantially influence the bioconcentration capacity. V.-Balogh and Salanki (1984), working with the bivalve Anodonta cygnea, also reported high BCF for mercury and cadmium and noted that the BCF is higher for some tissues than for others. It was also different for the two elements, and was a function of exposure time and water concentration. Mercury BCF in the kidney was determined to be nearly 100,000 after an exposure to Hg at an average concentration of 10 jlg/L for 840 hr. Elimination, or depuration, of a substance that has accumulated in tissues is as important as uptake in determining the equilibrium tissue concentration
G
P P G
Lymnaea stagnalis
Corbicula manilensis
Anodonta grandis Amblema plicata
Lymnaea sp. Physa sp.
Anodonta grandis P S,L x Lampsilis radiata Lasmigona complanata
Corbicula manilensis
Cu
Pb
Cu, Mn, and Zn
Hg
Hg
Pb P
P
Fusconaia flava Amblema pUcata Quadrula quadrula
Cd, Co, Cr, Cu, Ni, Pb, and Zn
S
S
S
S
L
S
L
P
Anodonta cygnea
Zn
x
x
x
L
Helisoma campanulata G
x
Hg
S L
P G
Lampsilis radiata
Lymnaea stagnalis
Cu
L
Anodonta californiensis P
Zn
3
Ce, Co, Cr, Cs, Mn, Sr, and Zn
2
Tested Organism
1
Elements
x
x
x
x
x
x
4
x
9
10
x
11
x
x
x
x
x
x
x
x
12
Mathis and Cummings (1973)
Foulquier et al. (1973)
Fang (1973)
Spronk et al. 1971
Harvey 1969
Pauley and Nakatani (1968)
Reference
Clarke et al. (1976)
Smith et al. (1975)
Cox et al. (1975)
Seagle and Ehlmann (1974)
Clarke and Clarke (1974)
x
x
8
x
x
x
x
7
Spronk et al. (1973)
x
6
x
x
x
x
5
Table 1. Studies of metal bioaccumulation by freshwater molluscs (arranged chronologically).
-I'-
C/O
~ 5·
n
'~
0-
::l
l'O
(\)
0: ...,
tTl
''T1
oc
Campeloma sp.
Cd, Cu, Pb, and Zn Pb Cu Pb As, Cd, Hg, and Pb
Hg
Cd, Fe, Mn, and Zn Cd, Mn, and Pb
Cr, Cu, Hg, Pb, and Zn
Cd and Pb Cu
Anodonta anatina
Cd, Cu, Hg, Ni, Pb, and Zn
(Various) (Various) Physa integra Physastra variabilis Potamopyrgus antipodum Elliptio complanata Lampsilis radiata Anodonta cataracta Velsunio ambiguus Physa sp. Musculium transversum Anodonta piscinalis
Quadrula quadrula
Lymnaea palustris
P
G P
P
P
G P G G
G P G G P
P
6 clam species Physa sp.
Physa gyrina
P G
Unio elongatulus
Hg Cd Cd, Cu, Pb, and Zn Cd and Pb L
S
S S
S
S
L
S S
L
L
x
x
x
S S S S
x
L
x
x
x
x
x
x
x
x
x
x
x
x
x
x x
x
x
x
x
x
x
Sarkka (1979) ( Con tinued)
Jones and Walker (1979) Mathis et al. (1979)
Friant (1979)
Anderson et al. (1978) Borgmann et al. (1978) Foster and Bates (1978) Moller (1978) Price and Knight (1978) Spehar et al. (1978) Vidal (1978)
x
X
x
Enk and Mathis (1977) Manly and George (1977)
x
x
x
x
x
x
x
x
Ranzoni and Bacci (1976) Wier and Walter (1976) Anderson (1977)
~ 0
.j>.
'"
po
~ co ...
(") (1)
63'
\:
...
CoIl
S·
Sr;;-
(1)
~
0-,
0" ...
po
0(=i'
::s
(")
-
i::
L L
P
Helisoma campanulata G Stagnicola emarginata P P
Corbicula fluminea
Amblema perplicata
Mytilus viridus
(Various)
Anodonta cygnea Unio pictorum Corbicula fluminea
Corbicula fluminea
Physa integra Campeloma decisum
As, Cd, Ce, Cr, Cu, Hg, Mn, Mo, Se, and Zn
As
Cd and Zn
Cd, Cu, Hg, Pb, and Zn
Pb
Cd, Cu, Fe, Hg, Mn, Pb, and Zn Cd, Cu, and Zn
Ag, Cd, Cr, Cu, Fe, Mn, and Zn
Pb G
S
S
L
P P
S
S
S
P
G
S
P
Lampsilis radiata Elliptio complanata Anodonta grandis
As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Sn, and Zn S
S
P
Amblema perplicata
Cd and Zn
2 S
1
Anodonta californiensis P Unio novahollandae
Tested Organism
Fe and Mn
Elements
x
x
3
x
x
x
4
x
x
5
x
6
Table 1. (Continued)
x
Newman and McIntosh (1983a)
Joy et al. (1983)
Graney et al. (1983)
Salanki et al. (1982)
x
'"
S'
n
x
'-
c...
~
~
G
..,
c...
~
"T1
'-
x
Spehar et al. (1980)
Rodgers et al. (1980)
Heit et al. (1980)
Adams et al. (1980)
Swinehart and Smith (1979)
Reference
g,
x
x
12
Menasveta and Cheevaparanapiwat (1981) Newman and McIntosh (1982)
11
Contents I. Introduction .................................................... II. Characteristics of Freshwater Molluscs .............. . . . . . . . . . . . . . . . A. Gastropoda (Snails) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Pelecypoda (Clams, Mussels) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Introduced Bivalve Species .................................... III. Freshwater Molluscs as Bioindicators of Metals .................... A. Characteristics that Pertain to Biomonitoring . . . . . . . . . . . . . . . . . . . . B. Special Characteristics of Corbicula Clams. . . . . . . . . . . . . . . . . . . . . . . IV. Bioaccumulation and Toxicity of Metals to Freshwater Molluscs. . . . . . A. Bioconcentration and Depuration .............................. B. Differential Bioaccumulation Among Tissues and Organs. . . . . . . . . . C. Relation to Individual Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . D. Bioaccumulation in Relation to Environmental Concentrations .... E. Effects of Metal Mixtures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Biochemical Effects and Metallothioneins . . . . . . . . . . . . . . . . . . . . . . . . G. Metals as Molluscicides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Comparisons with Nonmollusc Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary........................................................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37 39 40 41 41 42 42 45 46 47 58 59 61 64 65 66 66 68 69
I. Introduction During the past several decades, studies from a variety of locations have demonstrated widespread occurrence of metals in surface waters at concentrations significantly higher than background levels. Elevated concentrations are not limited to certain water types or polluted areas; they appear in all types of systems and in all geographic areas. It is clear that metals enter the aquatic systems from diverse sources, both point and nonpoint, and they can be readily transported from one system to another. Transport routes *U.S. Geological Survey, 6417 Normandy Lane, Madison, WI 53719. tU.S. Geological Survey, 227 N. Bronough St., Suite 3015, Tallahassee, FL 32301.
© 1991 by Springer-Verlag New York Inc. Reviews of Environmental Contamination and Toxicology, Vol. 122. 37
38
J.F. Elder and 1.1. Collins
include atmospheric, terrestrial, subterranean, aquatic, and biological pathways (Elder 1988; Salomons and Forstner 1984). Many trace metals have toxic effects on some biological species at concentrations common in water bodies that are contaminated to some degree. Elevated metal concentrations can cause severe reduction or elimination of intolerant species, thereby having a significant effect on the diversity and trophic structure of the biological community (Peterson 1986). Bioaccumulation-the uptake of contaminants from the surrounding media or food and storage of the contaminants in body tissues-can lead to latent toxic responses in the affected organism or toxicity to organisms higher in the food chain. Among the effects of such propagation through the food chain, there is the possibility of human health hazards. Concerns about contaminants in the aquatic environment have led to extensive research on chemical behavior and biological effects of such contaminants. Although many questions about sources, distribution, partitioning, and transport can be addressed by hydrologic monitoring and chemical analyses of environmental samples, other questions having to do with fates of the metals and their effects on the ecosystem can be addressed only with biological research, or more appropriately, research that makes use of combined biological, chemical, and hydrologic data. Biological research of contaminant problems in aquatic systems may take anyone of several forms. One of the most common general approaches is the use of bioindicator organisms. This involves measurement of contaminant effects on aquatic biota that are exposed to the contaminants either in the natural environment or by some simulation of the natural setting. The organisms may be natives or introduced for the purpose of the study. The exposure time may be short-term or long-term, depending on whether the research is primarily focused on acute effect or chronic effects. Three general approaches involving the use of bioindicator organisms are commonly applied to the assessment of contaminant effects in aquatic systmes. These approaches are: 1. TisS1Je analysis. Organisms collected from exposed environments are used for chemical analysis of tissues to detect bioaccumulation. The procedure involves the use of either native organisms or introduced organisms that are contained at the study site for a limited time. The procedure can also be used in the laboratory, where the test organisms in controlled enclosures are exposed to known concentrations of contaminants. 2. Toxicity testing. Organisms are exposed experimentally to contaminants and their toxicological responses to such exposure are monitored. Acute toxic effects are usually measured as the rate of mortality caused by the exposure and chronic effects are manifested by a variety of sublethal responses. Testing usually involves exposure to a gradient of contaminant concentrations, ranging from a concentration that is not expected
Mollusc Indicators of Metals in Surface-Water
39
to produce response to one that is equal to or greater than any concentrations that are likely to be found in nature. If possible, the test systems include controls, in which no contaminants are present. A great variety of testing procedures are employed. 3. Ecological surveys. Existing biological communities in aquatic systems are examined and described where contaminati-on is suspected. Commonly measured ecological variables include community structure, biomass, species diversity, and metabolic functions such as primary productivity or respiration. If there are significant trends over time and (or) space, these may reflect stresses caused by contaminants. Molluscs have been used extensively as bioindicators of pollution in estuarine and marine systems (Bayne et al. 1985), but much less in freshwater systems. Like the marine species, freshwater mussels and snails have the potential to be very useful for biomonitoring studies. This potential has been realized with increasing frequency in recent years, as reflected in the literature on inland surface-water contaminant studies. Although much more limited in terms of species diversity than their marine counterparts, the freshwater molluscs are widely distributed in nearly all types of surface-water systems, and they are frequently very abundant. The purpose of this review is to examine current information on concepts and principles of metal biogeochemistry and toxicity as elaborated by studies with freshwater molluscs. There is a limited but expanding body of literature describing work with freshwater molluscs. Because of the limited literature, there are information gaps. Some processes of metal biogeochemistry that have been thoroughly studied in marine animals, or in other freshwater groups, have been rarely, if ever, demonstrated in studies of freshwater molluscs. In recognition of this limitation, this review is restricted almost entirely to freshwater work to focus on a relatively narrow scope. It is hoped that a synthesis of this work will facilitate evaluation of the current status of research in this area and the most promising and needed directions for future research. Important advantages and disadvantages of the use of freshwater molluscs as indicators of metal contamination are also discussed. As stated in an earlier review (Elder 1988), some results from research on metals in aquatic systems are controversial; comparisons among different studies reveal apparent inconsistencies. Even if the focus is narrowed to studies of freshwater molluscs as bioindicators of metals, some of the same inconsistencies persist. One of the objectives of this review is to examine the inconsistencies and offer possible explanations for them.
II. Characteristics of Freshwater Molluscs The taxonomy of molluscs and other freshwater invertebrates is described by Pennak (1978). Included in that discussion are detailed descriptions of morphologicaL behavioral, developmental, and ecological characteristics of
40
J.F. Elder and 1.1. Collins
each taxonomic group. The following is a synthesis of mollusc characteristics that are particularly important with respect to their use as contaminant bioindicators. Molluscs are found in almost all types of freshwater habitats, from the smallest ponds to large lakes and rivers. Only in extremely cold alpine lakes and streams, acidic waters, or grossly polluted areas, they are likely to be absent. Even in cold climates, they can be successful if there are seasonal warm periods; they can survive the cold periods by burrowing into bottom mud or organic debris and hibernating. Mollusc distribution can be limited by very low concentrations of dissolved salts, especially calcium carbonate, which is important for shell construction. Because of their requirement for calcium carbonate, molluscs are more likely to live in alkaline waters and they are rare in waters more acid than pH 6. Because of rather high dissolved oxygen requirements, they are also excluded from anoxic or near-anoxic environments. The principal predators of freshwater molluscs are fish. However, only a few fish species, such as suckers, perch, and whitefish, depend on molluscs for a significant portion of their diet. Snails are also eaten by ducks, shore birds, and sometimes amphibians. Clams and mussels are commonly eaten by terrestrial mammals, particularly muskrats. Some mollusc species are hermaphroditic (each individual has male and female sex organs and is capable of self-fertilization). More commonly, the sexes are separate. Eggs are fertilized internally, but embryonic development differs between gastropods and pelecypods. The early developmental stages of snails proceed within eggs that are deposited externally in gelatinous masses. In bivalves, the zygotes initially develop in distentions of the gills (marsupia), then become free-swimming larvae or glochidia larvae that are parasitic on fish hosts. Hundreds of mollusc species are found in freshwater habitats throughout the world. Some species have been accidentally or intentionally transported across ocean barriers and introduced to new continents. Some of these introduced species have been very successful, often displacing endemic populations. A. Gastropoda (Snails)
Freshwater gastropods are normally vegetarian grazers, feeding principally on various types of attached algae that are common in shallow-water environments. For this reason, snails are mostly found in water less than 3 m deep. Some snails are scavengers, some are omnivorous, and a few species occasionally feed on live animals. Food intake increases substantially with increasing temperature. The univalve shell of gastropod is characteristically spiral shaped and provides protection for the internal organs. The shell is covered with an
Mollusc Indicators of Metals in Surface-Water
41
organic epidermis that has an important function of protecting the underlying calcium carbonate layer from erosion. B. Pelecypoda (Clams, Mussels)
The pelecypods are characterized by a bivalve shell with a hinge ligament; hence they are commonly known as bivalves. Under natural conditions, the valves gape slightly to permit protrusion of the foot and siphons. Extension of the foot allows limited movement of the animal through the substrate. Most freshwater bivalve species are members of the family Unionidae. Other freshwater families are Sphaeriidae and Corbiculidae. Unlike gastropods, all the bivalves are aquatic. They can live in all surface-water systems, but they are most abundant and diverse in large rivers. Within river systems, bivalves tend to be more abundant in the lower waters than in the small tributaries. The large river systems provide continual replenishment of food in suspended particles, favorable substrate types, and abundance of oxygen. On relatively stable mixed gravel and sand bottoms, bivalves can be very abundant. In contrast, most species are not found on bare-rock bottoms or in shifting sands and muds. The bivalve shell consists of an outer protective periostracum. Beneath this is the prismatic layer, consisting of prism-like blocks of calcium carbonate. The inner, thick, nacre layer consists of alternating laminae of calcium carbonate and organic materials. The shell can be a site of accumulation of metals and the accumulation can be varied in the different layers (Imlay 1982). The principal feeding process of bivalves is filter feeding, whereby suspended microscopic particles are removed from water passing over the gills. Particles can include zooplankton, phytoplankton, and organic detritus. Food particles are trapped by mucus on the gills, then driven by ciliary action toward the digestive tract. In temperate climates, feeding and growth are considerably more active in the summer than in the cooler months. C. Introduced Bivalve Species
Currently, the principal environmental and economic importance of freshwater bivalves is the nuisance caused by overpopulations of introduced species. Notable among these in North America are the Asian clam (Corbicula-species) and, more recently, the zebra mussel (Dreissena polymorpha) that has become enormously abundant in parts of the Great Lakes (Griffiths et al. 1989). Fouling of underwater surfaces and clogging of waterways have become major problems due to the huge population growths of these species. Much of the recent bivalve biomonitoring research has come about in part because of the needs to understand and control the overpopulation problems. Research with Corbicula has developed
42
JF. Elder and J.J. Collins
considerably in the last decade and a similar expansion of research on Dreissena may soon follow. The great adaptive success of Corbicula has allowed it to displace native bivalve species in many river systems; hence it is ecologically disruptive as well as an economic nuisance. Several physical characteristics of Corbicula account for its competitive advantage (Kraemer 1979). It has reproductive adaptability, including hermaphroditism. Like marine pelecypods, but unlike other freshwater bivalves, Corbicula has a free-swimming larval form that is independent of association with fish hosts for further development. Corbicula also has greater mobility than many other bivalves because it has more rapid and coordinated foot movements. The juveniles have abyssal holdfast thread that allows the animals to inhabit a shifting, sandy river bottom that would exclude other bivalves. Finally, Corbicula has unusual ability to withstand harsh environmental conditions, partly because of its extensive mantle fusion, heavy protective shell, and narrow pedal gape.
III. Freshwater Molluscs as Bioindicators of Metals A. Characteristics that Pertain to Biomonitoring Gastropods and pelecypods, both freshwater and marine, are commonly selected as bioindicators of metals because of certain special characteristics that render them useful for this kind of application. These characteristics are basically consistent with the following criteria, similar to those suggested by Phillips (1977, 1980). for bioindicator organisms.
1. Tolerance to a wide range of metal exposures; ability to accumulate metals without suffering mortality. 2. Sedentary habits; slow and limited range of movement. 3. Sufficient life span to allow for sampling of more than one year class. 4. Abundance in the study area. 5. Sufficient size to allow chemical analysis of tissue samples. 6. Hardiness; ability to remain healthy during sampling and laboratory incubation. 7. Relative ease of sampling and identification. 8. High metal accumulation rates. 9. Responsiveness to changes in metal exposure. Most freshwater mollusc species commonly used as bioindicators are both large and sedentary; hence they simultaneously meet criteria 2 and 5. Their size gives them an advantage for selection as bioindicators over most other freshwater groups except fish, and their limited mobility gives them an advantage over fish. The only other major aquatic taxonomic group that has similar advantages for use in biological monitoring work is the macrophyte group-the attached vascular plants. Most freshwater mollusc species also meet criteria 1, 6, and 7, although their suitability with respect to these characteristics differs considerably
Mollusc Indicators of Metals in Surface-Water
43
among species. The advantages of their sedentary nature, large size, easy identification, and amenability to sampling and handling have been emphasized repeatedly (Benfield and Buikema 1980; Czarnezki 1987; Dauble et al. 1985; Foe and Knight 1986; Forester 1980; Foster 1981; Graney et al. 1983; Green et al. 1989; Hartley and Johnston 1983; Khangarot and Ray 1988; M unzinger 1987; Slooff et al. 1983). In addition, the molluscan feeding habits (grazing, filter feeding, and deposit feeding) make them good bioaccumulators of plankton, seston, and dissolved constituents in water (Foe and Knight 1986; Foster 1981; Friant 1979; Graney et al. 1983; Hartley and Johnston 1983; Wren et al. 1983). In most species, a moderate amount of metal bioaccumulation does not lead to disease or mortality. Some freshwater mollusc species have a relatively broad tolerance range to environmental conditions, including temperature, salinity, water hardness, pH, and dissolved oxygen, but others are very sensitive. Abundance (criterion 4) of the freshwater molluscs is highly variable. It is not unusual to find hundreds or even thousands of individuals per square meter at one site and none in a nearby site in the same water body. Differences among sites in environmental conditions such as substrate, temperature, water current, or food supply can cause this kind of variability, even if the differences are rather subtle (Pennak 1978). However, such variability is common to nearly any taxonomic group; it is not a unique disadvantage of the molluscs. The need for a multiyear life span (criterion 3) applies only to certain kinds of biomonitoring. Most mollusc species have life spans of more than 1 yr (Hartley and Johnston 1983), which is sufficient for use in acute toxicity tests or short-term bioaccumulation monitoring. For investigations that require longer-lived animals, there are several suitable freshwater mollusc species, such as some of the Unionid mussels, whose life cycles span several years (Green et al. 1989). A potential limitation that applies to the use of any organism as transplanted test species is the lag time before the influence of the destination environment dominates over the influence of the source environment. Reciprocal transplant experiments with Elliptio complanta by Hinch and Green (1989) indicated that when the test animals are transplanted to a new area, their growth and metal uptake remain largely a function of characteristics of their genotype and their home habitat, rather than their new habitat, for several weeks or months. The duration of this lag time probably depends on the extent to which the environments differ, as well as genetic acclimation capacity and behaviour. The importance of genotype and source habitat in controlling responses of freshwater molluscs to a new habitat has received only limited research attention; hence the extent to which it applies to diverse species and environmental conditions remains in question. Foe and Knight (1986) observed that bioaccumulation of metals in transplanted Corbicula clams
44
IF. Elder and J.J. Collins
closely reflected bioaccumulation by native clams following an acclimatization time of a few weeks. They suggested that the source effect was not a major problem in that case. However, the possible influences of long-term source effects and genetics on responses to contaminants should be considered in the design of transplant biomonitoring studies. Test animals that are transplanted to a study site should be taken from a common source. The responsiveness to changes in metal exposure (criterion 9) depends on the bioavailability of the metals, which, in turn, is controlled by numerous environmental factors (Luoma 1983). Although the behavior, physical characteristics, and feeding habits of the organism are important influences in bioaccumulation and toxicity, a great deal of variation and complexity may be expected, even within one species, because of the environmental influences. Therefore, the influence of environmental concentrations on bioaccumulation and response is rare if ever a simple relation. The literature contains numerous reports showing both good and poor relations between biological and environmental contaminant concentrations. Further discussion of these seemingly contradictory reports follows in a later section. Again, this problem is not unique to molluscs. Biological monitoring of coastal and estuarine contamination has been accomplished in recent years by "mussel watch" programs (Bayne 1989; Farrington et al. 1987). Since their initiation in U.K and U.S.A. in the middle 1970s, the mussel watch programs have provided a long-term and large-scale approach, using bivalves as "sentinel" organisms. A relatively small number of bivalve species, particularly blue mussels (Mytilus) and oysters (Crassostrea and Ostrea) are widely distributed in coastal areas around the world. This cosmopolitan nature, and the other features common to bivalves that make them useful biomonitors, led to the development of the mussel watch programs. Although the advantages of molluscs as bioindicators are frequently mentioned in the literature, some authors have pointed out a need for caution. Most of the precautions have to do with concerns about the resistance of some species to pollutants, tending to make them less sensitive bioindicators than some other taxonomic groups (Amiard-Triquet et al. 1986; Cairns and Cherry 1983; Klerks and Weis 1987; Slooff et al. 1983; V.-Balogh 1988a). The resistance is usually attributed to metal-binding agents, such as metallothioneins, synthesized by the organism itself when exposed to elevated metal concentrations in the surrounding media. Synthesis of protective biochemical agents such as metallothioneins can increase resistance to metal toxicity. Binding-agent resistance should be taken into account in studies of bioaccumulation or toxicity (Engel 1988). Another defense mechanism of molluscs is the ability to withdraw into the shell, at least during short-term episodes of unfavourable conditions (Doherty et al. 1987a). This defence response can affect acute toxicity test responses.
Mollusc Indicators of Metals in Surface-Water
45
As the complexities created by variable factors such as metal speciation and metallothionein production are considered, it becomes evident that inconsistent results are to be expected among different studies. Such variability led Coleman et al. (1986) to suggest that more emphasis to be given to ranges than to averages in reporting bioaccumulation or effective toxic concentrations. In their discussions of invertebrates in general as bioindicators, Benfield and Buikema (1980), Gaufin (1973), and Maciorowski and Clarke (1980) have recommended the approach of examining community responses rather than effects on single species. The variability of single-species responses may not reflect the cumulative impacts of the contaminants on the interdependent network of diverse species in the community. Freshwater molluscs clearly have significant potential as useful bioindicator species, and they are likely to be used more frequently for that purpose in the future. However, it is also clear that they are not universally suitable as bioindicators, as is true of any commonly used bioindicator organism. Generally, the most effective biomonitoring approach, if practical within the study design, is to use several bioindicator species representing different trophic levels in combination with chemical and hydrologic measurements. Results from such an approach may provide the kind of extensive and complementary data that are needed for thorough understanding of the contaminant problem. B. Special Characteristics of Corbicula Clams Among the dozens of mollusc species that inhabit inland waters, one of the most frequently used molluscs for biomonitoring is the Asian clam of the family Corbiculidae. Commonly known by its genus name, Corbicula, the taxonomy of the Asian clam is not yet clearly defined. Most investigators who worked with Corbicula have identified the species as either Corbicula fluminea or Corbicula manilensis. According to Pennak (1978) and Britton and Morton (1979), both of them are the same species, but other authors (e.g., Hillis and Patton 1982) consider the species distinction to be valid. The increasing popularity of Corbicula as a bioindicator organism can partially be attributed simply to its abundance and widespread distribution. However, Corbicula does have other attributes that make it especially well suited for biomonitoring work. Some of the characteristics that give Corbicula its competitive advantages over other bivalve species are also factors that make it useful as a bioindicator. Corbicula clams are generally recognized as efficient bioaccumulators of contaminants (Graney et al. 1983; Hartley and Johnston 1983; Rodgers et al. 1980). Despite this capability for bioaccumulation, the Asian clams can survive relatively high bioaccumulation. In addition, they are tolerant of temperatures as high as 36°C (Cairns and Cherry 1983) and salinities greater than 10%0 (Gainey 1978). This resistance to contaminants and adverse
46
1.F. Elder and J.J. Collins
environmental conditions has contributed to the success of Corbicula in becoming well established in North America rivers. Corbicula has been the test organism in a number of recent toxicity studies, some of which included considerable discussion of recommended procedures. Because of the high capacity for survival, the Asian clam is often used in chronic tests in which sublethal responses are monitored. Several kinds of measurements, including reproduction, growth, and condition index (the ratio of dry weight of soft tissue to shell volume), were discussed by Foe and Knight (1986). These indices are subject to error when used alone; however, an effective toxicity test approach is to apply all of them simultaneously and to complement them with tissue analyses. Belanger et al. (1986) found that shell and tissue growths of Corbicula were sensitive indicators of zinc stress. Even at zinc concentrations less than 0.05 mg/L considered protective of aquatic life during chronic exposure (U.S. E.P.A. 1987), growth was inhibited. Some studies have made use of Corbicula larvae instead of adults for biomonitoring of contaminants (Foster 1981). This procedure parallels the relatively common practice of using oyster larvae for biomonitoring studies in marine systems. Adult Corbicula are easily maintained in the laboratory, and spawning of the captive clams may be expected after a few months of conditioning and growth (King et al. 1986). The free-swimming veliger larvae are also easy to maintain in the laboratory. They are good bioaccumulators and are sensitive to elevated metal concentrations (Foster 1981), although not as sensitive as the cladoceran, Daphnia magna, commonly used in trace metal toxicity tests (Elder 1989). In addition, the use of individuals from a single larval brood provides a set of test organisms, all from the same parents, that are of the same size, age, and life history (Foster 1981). This homogeneity minimizes variability in response due to secondary factors. IV. Bioaccumulation and Toxicity of Metals to Freshwater Molluscs Many studies have demonstrated bioaccumulation of metals in tissues of freshwater molluscs. In situations where metal concentrations in the water and( or) sediments are elevated, some bioaccumulation occurs nearly always. However, the rates of bioaccumulation and correlations to environmental concentrations are by no means consistent. Previous reviews of marine and estuarine work (Luoma 1983; Morel and Hudson 1985) have given clear evidence that bioavailability and uptake of metals are highly dependent on geochemical and biological factors. There are major differences in bioaccumulation among biological species. Even within a single species, bioaccumulation can be a function of size, age, sex, genotype, phenotype, feeding activity, and reproductive state. There are also many environmental variables other than metal concentrations that influence bioaccumulation. Some of the most important of these are concentration and type of organic
Mollusc Indicators of Metals in Surface-Water
47
carbon, water hardness, temperatures, pH, dissolved oxygen, sediment grain size, and hydrologic features of the system. Whole-body tissue analyses can also be influenced by sediments and detritus in the digestive tract at the time of collection. To clear the digestive tract, a brief depuration time (ranging from several hours to three days) in clean water may be allowed prior to analysis. However, this procedure can also lead to inaccurate results due to the possibility that excessive depuration time may permit partial elimination of metals from body tissues. A second major area of biomonitoring research with freshwater molluscs involves toxicity testing. Mortality and sublethal effects following exposure to elevated contaminant concentrations are, like bioaccumulation, subject to much variability due to changing biological and environmental conditions. However, numerous authors have reported success, relative to the bioindicator prerequisites listed earlier, using snails and bivalves in both acute and chronic toxicity tests. Although many mollusc species have served as test organisms, Corbicula has been used more than any other single genus for this purpose. Overviews of the use of freshwater molluscs as bioindicator organisms in research on metal bioaccumulation and toxicity are provided in Tables 1 and 2, respectively. Some investigations have included discussions of particular factors that influence or relate to bioaccumulation, mortality, or sublethal effects; those factors are indicated in the tables. Sublethal responses commonly observed in freshwater molluscs include changes in respiration rates, growth rates, gill-cilia beating rates, reproductive capacity, enzymatic activity, valve closure, and production of metal-binding proteins. A. Bioconcentration and Depuration The bioconcentration phenomenon results in a tendency for aquatic biota to accumulate metals in their tissues at concentrations that greatly exceed ambient water concentrations. The bioconcentration factor (BCF)-a measure of the equilibrium ratio of metal concentration in biological tissue to metal concentration in ambient water-frequently falls in a range of 10 3 to 106 . In a study of C.jluminea, Graney et al. (1983) reported BCF ranges of 17,000 to 22,000 for copper and 1,700 to 3,800 for cadmium. They cautioned, however, that metal interactions were not considered in their study and such interactions could substantially influence the bioconcentration capacity. V.-Balogh and Salanki (1984), working with the bivalve Anodonta cygnea, also reported high BCF for mercury and cadmium and noted that the BCF is higher for some tissues than for others. It was also different for the two elements, and was a function of exposure time and water concentration. Mercury BCF in the kidney was determined to be nearly 100,000 after an exposure to Hg at an average concentration of 10 jlg/L for 840 hr. Elimination, or depuration, of a substance that has accumulated in tissues is as important as uptake in determining the equilibrium tissue concentration
G
P P G
Lymnaea stagnalis
Corbicula manilensis
Anodonta grandis Amblema plicata
Lymnaea sp. Physa sp.
Anodonta grandis P S,L x Lampsilis radiata Lasmigona complanata
Corbicula manilensis
Cu
Pb
Cu, Mn, and Zn
Hg
Hg
Pb P
P
Fusconaia flava Amblema pUcata Quadrula quadrula
Cd, Co, Cr, Cu, Ni, Pb, and Zn
S
S
S
S
L
S
L
P
Anodonta cygnea
Zn
x
x
x
L
Helisoma campanulata G
x
Hg
S L
P G
Lampsilis radiata
Lymnaea stagnalis
Cu
L
Anodonta californiensis P
Zn
3
Ce, Co, Cr, Cs, Mn, Sr, and Zn
2
Tested Organism
1
Elements
x
x
x
x
x
x
4
x
9
10
x
11
x
x
x
x
x
x
x
x
12
Mathis and Cummings (1973)
Foulquier et al. (1973)
Fang (1973)
Spronk et al. 1971
Harvey 1969
Pauley and Nakatani (1968)
Reference
Clarke et al. (1976)
Smith et al. (1975)
Cox et al. (1975)
Seagle and Ehlmann (1974)
Clarke and Clarke (1974)
x
x
8
x
x
x
x
7
Spronk et al. (1973)
x
6
x
x
x
x
5
Table 1. Studies of metal bioaccumulation by freshwater molluscs (arranged chronologically).
-I'-
C/O
~ 5·
n
'~
0-
::l
l'O
(\)
0: ...,
tTl
''T1
oc
Campeloma sp.
Cd, Cu, Pb, and Zn Pb Cu Pb As, Cd, Hg, and Pb
Hg
Cd, Fe, Mn, and Zn Cd, Mn, and Pb
Cr, Cu, Hg, Pb, and Zn
Cd and Pb Cu
Anodonta anatina
Cd, Cu, Hg, Ni, Pb, and Zn
(Various) (Various) Physa integra Physastra variabilis Potamopyrgus antipodum Elliptio complanata Lampsilis radiata Anodonta cataracta Velsunio ambiguus Physa sp. Musculium transversum Anodonta piscinalis
Quadrula quadrula
Lymnaea palustris
P
G P
P
P
G P G G
G P G G P
P
6 clam species Physa sp.
Physa gyrina
P G
Unio elongatulus
Hg Cd Cd, Cu, Pb, and Zn Cd and Pb L
S
S S
S
S
L
S S
L
L
x
x
x
S S S S
x
L
x
x
x
x
x
x
x
x
x
x
x
x
x
x x
x
x
x
x
x
x
Sarkka (1979) ( Con tinued)
Jones and Walker (1979) Mathis et al. (1979)
Friant (1979)
Anderson et al. (1978) Borgmann et al. (1978) Foster and Bates (1978) Moller (1978) Price and Knight (1978) Spehar et al. (1978) Vidal (1978)
x
X
x
Enk and Mathis (1977) Manly and George (1977)
x
x
x
x
x
x
x
x
Ranzoni and Bacci (1976) Wier and Walter (1976) Anderson (1977)
~ 0
.j>.
'"
po
~ co ...
(") (1)
63'
\:
...
CoIl
S·
Sr;;-
(1)
~
0-,
0" ...
po
0(=i'
::s
(")
-
i::
L L
P
Helisoma campanulata G Stagnicola emarginata P P
Corbicula fluminea
Amblema perplicata
Mytilus viridus
(Various)
Anodonta cygnea Unio pictorum Corbicula fluminea
Corbicula fluminea
Physa integra Campeloma decisum
As, Cd, Ce, Cr, Cu, Hg, Mn, Mo, Se, and Zn
As
Cd and Zn
Cd, Cu, Hg, Pb, and Zn
Pb
Cd, Cu, Fe, Hg, Mn, Pb, and Zn Cd, Cu, and Zn
Ag, Cd, Cr, Cu, Fe, Mn, and Zn
Pb G
S
S
L
P P
S
S
S
P
G
S
P
Lampsilis radiata Elliptio complanata Anodonta grandis
As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Sn, and Zn S
S
P
Amblema perplicata
Cd and Zn
2 S
1
Anodonta californiensis P Unio novahollandae
Tested Organism
Fe and Mn
Elements
x
x
3
x
x
x
4
x
x
5
x
6
Table 1. (Continued)
x
Newman and McIntosh (1983a)
Joy et al. (1983)
Graney et al. (1983)
Salanki et al. (1982)
x
'"
S'
n
x
'-
c...
~
~
G
..,
c...
~
"T1
'-
x
Spehar et al. (1980)
Rodgers et al. (1980)
Heit et al. (1980)
Adams et al. (1980)
Swinehart and Smith (1979)
Reference
g,
x
x
12
Menasveta and Cheevaparanapiwat (1981) Newman and McIntosh (1982)
11
