Tolerance of the Asiatic clam Corbicula spp. to lethal level of toxic stressors—A review

Environmental Pollution 51 (1988) 269--313

Tolerance of the Asiatic Clam Corbicula spp. to Lethal Levels of Toxic Stressors--A Review

Francis G. Doherty Syracuse Research Corporation, Merrill Lane, Syracuse, NY 13210, USA

& Donald S. Cherry Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA (Received 12 May 1987; revised version received 1 July 1987: accepted 27 October 1987)

ABSTRACT Studies assessing the tolerance q / the Asiatic clam. Corbicula spp., to a ~:arietv ~[" chemical, em,ironnlental, and physical stressors are summarised and re~,iewed. A majority qf the studies were conducted ( 1 ) in the laboratory, (2) with .jm~enile and adult stages and (3) with on O' one stressor per treatment. Trends in the data demonstrate that ( 1 ) equivalent median lethal concentrations were ,~enerated h7 studies using hoth static andJtow-through techniques, (2) suhstantial increases in the levels ~?['mortality among treated clams were obtained hi' extending exposure ~htrations: (3) incorporation q[ recovery periods into experimental de.s'i~ns permitted the development ~?[ latent mortalities among treated clams; (4) toh, ranee among larvae was stage depemk, nt while tolerance amonq adults was not: (5) hi~her levels ¢?[ mortality were ohtained when tests were conducted at higher temperatures: and (6) testing ht the presence q[.,;uhstrate resulted in a decrease in the le~,els q[ mortality among treated clams. The intplications of these findings are dLs'eussed in relation to ~[.]brts to mitigate theJouling ~?ff'e('ts qf Asiatic clams in inch~strial coolin¢ water systems h)' exposure to toxic chemicals. 269 Era,iron. Poll. 0269-7491/88/$03"50 ,(~ 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain

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Francis G. Doherty, Donald S. Cherry

INTRODUCTION The Asiatic clam, Corbicula spp., has been a macroinvertebrate biofouling pest of canals, dredging sands used in the manufacture of concrete, and power plant and industrial cooling water systems in North America for over 40 years (Ingram, 1959; Sinclair & Isom, 1963; Eng, 1979; Mattice, 1979; Morton, 1979; Page et al., 1986). The establishment and discovery of fouling populations of Corbicula were only marginally preceded by, and occasionally coincided with, documentation of this bivalve's presence within a river drainage basin. For example, see the series of reports by Rodgers et al. (1977), Graney et al. (1980), and Cherry et al. (1986) on Corbicula in the New River, Virginia, and those of H o r n i n g & Keup (1964), Diaz (1974), and Dreier & Tranquilli (1981). Other studies have established that individuals of this genus demonstrate high rates of reproduction and growth (McMahon & Williams, 1986). These characteristics probably contribute to Corbicula's successful competitive interactions with endemic bivalves and present a new organism for beneficial aquaculture efforts in North America. The history of its continental migration (early 1930s to late 1970s), the ecological ramifications of its presence in North America, its fouling propensity, and its potential as an aquaculture and toxicity test organism have all been well documented (Burress et al., 1976; Foster & Box, 1976; Goss & Cain, 1977; Cherry et al., 1980; Foster, 1981; Harvey, 1981; McMahon, 1982; Counts, 1986) and do not serve as the focus of this review. Rather, the objective of this review is to summarise toxicity testing methodologies used for exposing Asiatic clams to toxic stressors and the results of those tests on the toxicity of chemical, physical, and environmental agents to the Asiatic clam. Current technology attempts to minimise and control Asiatic clam fouling in power plant and industrial cooling water systems through mechanical, physical, and chemical approaches that are often used in conjunction with one another. Mechanical measures to alleviate fouling entail the use of strainers and traps to minimise the probability of the initial fouling event, backflushing of clogged lines, and manual removal of dead shells and living specimens of Corbicula. Removal of living adults is especially important because it reduces the rate of recruitment of larvae that ultimately reinfest sites within a plant (Goss & Cain, 1977; Harvey, 1981). While complete removal of clams from the infested areas can be achieved manually and by backflushing, this approach is costly in terms of equipment shut-down and lost productivity. Physical measures used to minimise bivalve fouling include electric currents, electromagnetic fields, gamma radiation, heated water, and ultrasonic vibrations (Tilly et al., 1978; Goss et al., 1979; Morton, 1979). Although these methods have been effective in small-scale laboratory tests, they may be inappropriate for clam control in

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large-scale once-through industrial cooling water systems. Facilities currently under construction can be designed to provide the capabilities of physical bivalve control options, but facilities already in operation must often rely on mechanical and chemical control options. Chemical approaches used to minimise bivalve fouling consist of the addition of biocidal chemicals to the circulating process water supply. These agents can be introduced either by dissolution of a gas or powder in the raw water supply at the point of intake or by release from surface coatings (e.g. paints) and chemical impregnated pellets. The advantages to addition of biocide to the raw water supply at the point of intake include the scheduling of treatments when the potential for fouling is greatest (e.g. during C o r b i c u l a breeding seasons) and manual control of the biocide concentration. Biocide released from coatings or pellets occurs year-round with a rapid reduction in the concentration of biocide below that required to effectively control clam fouling. Previously, the practical life of toxic coatings on marine structures was thought to be as brief as 1 to 1.5 years (Benson et al., 1973). Regardless of the approach, the use of biocides in once-through cooling systems is limited by state and federal regulations prohibiting the release of toxic substances to receiving waterways. An effective and environmentally acceptable molluscicidal treatment programme, therefore, must provide both eradication of C o r h i c u l a fouling within a plant and non-detectable effects on the biota of receiving waters following release of the biocide containing effluent from the fouled facility. The purpose of this review is to present the first major overview of Asiatic clam control by biocidal agents and to provide direction in the design and conductance of toxicity tests assessing the molluscicidal efficacy of chemical agents to be used in mitigating the fouling effects of the Asiatic clam in power plant and industrial cooling water systems.

REVIEW OF M E T H O D O L O G I E S Exposure of adults to stressors in the laboratory Studies in which adults (and juveniles) were exposed to lethal levels of toxic stressors in the laboratory ranged in duration from < 1 min (Tilly et al., 1978) to as long as 80 days (Evans et al., 1979). Tests included both static and flow-through designs. Some studies included recovery periods (_< 10 days) within the original exposure system in which treated clams were observed for latent mortalities beyond the exposure period (Doherty et al., 1986). In one study, clams exposed to a stressor for brief durations (4 and 24h) were transferred to floating cages in a holding pond for an extended period (60 days) of post-exposure observation (Marking et al., 1977). Stressors were

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Francis G. Doherty, Donald S. Cherry

often chemicals identified by nondescriptive c o m m o n or trade names rather than by descriptive chemical names. Where available, chemical stressors referred to in the literature by c o m m o n or trade names are cross referenced with their chemical names in Table 1. Static, short-term studies were conducted with limited numbers of clams (9 to 20) exposed to chemical stressors in glass jars with solution volumes ranging from 3 to 151itres (Chandler & Marking, 1975, 1979; Bills et al., 1977a,b; Marking & Chandler, 1981). Tolerance of Corbicula to changes in pH, salinity, and temperature was also determined in static tests (Habel, 1970; Coldiron, 1975; D a u m et al., 1978; Gainey, 1978). Test solutions in

TABLE 1

Chemical Identities or Synonyms of those Compounds Listed by a Common, Generic, or Trade Name in Subsequent Tables Common name

4-AP Acrolein Bayer 73 Cytox 2410 DMA-2,4-D Dow EC-7 DRC-1339 DRC- 1347 DRC-2698 Formalin Furanace GD-174 GivGuard BNS Juglone Malachite green Methiocarb/Mesural PA-14 Pro-Noxfish Rhodamine B Rotenone Sal 1 Slimicide 364 Slimicide C-41 TBTO TFM Thonite

Chemical identity or synonym

4-aminopyridine propenal 2-aminoethanol salt of 2,5-dichloro-4'-nitrosalicylanilide n-dodecyl guanidine acetate, 95% diethylamine salt of [2,4-dichlorophenoxy] acetic acid pentachlorophenol 3-chloro-4-methyl benzeneamine hydrochloride 3-chloro-4-methyl benzeneamine N-[3-chloro-4-methylphenyl] acetamide 37% formaldehyde 6-hydroxymethyl-2-[2-(5-nitro-2-furyl) vinyl] pyridine 2-[digeranylamino] ethanol fl-bromo-fl-nitrostyrene 5-hydroxy- 1,4-naphthoquinone 4-[P-(dimethylamino)-~ phenylbenzylidene]-2, 5-cyclohexadiene-l-ylidene dimethyl ammonium chloride 3,5-dimethyl-4-[methylthio] phenol methyl carbamate ~t-alkyl [C4-CI 5]-omega-hydroxypoly[oxyethylene] Rotenone, 2.5%; sulfoxide, 2"5% Red fluorescent, xanthene dye Tubotoxine 2',5-dichloro-3-tertbutyl-6-methyl-4'-nitrosalicylanilide fl-bromo-fl-nitrostyrene, 10%; hydrocarbon solvents and stabilising agents, 90% /Lbromo-fl-nitrostyrene, 9"2%; methylene bis thiocyanate, 4-9%; hydrocarbon solvents and stabilising agents, 85-9% tributyltin oxide 3-trifluoromethyl-4-nitrophenol isobornyl thiocyanoacetate

Tolerance of Corbicula spp. to lethal levels of toxic stressors

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some static tests were continuously mixed by magnetic stirrers (Belanger el al., 1986a,b). Attempts to maintain consistent target doses in static tests included a static renewal approach (Davis & Lyons, 1986) or adding stressor to the exposure chambers during the course of the study (Rodgers et al., 1980). Tolerance of C o r b i c u l a to atmospheric exposure or dessication was also assessed under 'static' conditions by leaving clams on dry enamel trays (Dudgeon, 1982) or holding in a closed atmosphere dessicator (McMahon, 1979a). Tolerance of Asiatic clams to gamma radiation was determined in 250ml screw-top, wide-mouth bottles (Tilly et al., 1978). Some studies incorporated variables or modified approaches for generating toxicity data in static tests. Marking & Chandler (1978) utilised substrates (sand and mud) in their test containers. Horne & Mclntosh (1979) notched the ventral margins of the test specimens' shells to ensure constant exposure to the stressors. Mussalli et al. (1986) leached stressor from ground pellets to prepare test solutions. Other studies conducted in the laboratory utilised a flow-through design in which solutions of dissolved stressor in the exposure chambers were continuously renewed. In these studies, solutions of stressor were delivered either by gravity flow through an apparatus similar to that of the Mount & Brungs (1967) proportional diluter or by peristaltic pump. Volume of proportional diluter exposure chambers ranged in size, from 19 to 50 litres. Flow rates ranged from 4"4 litres min i to 3"6 litres h - 1 resulting in solution turnover rates of approximately once every 6 to 12 h (Chandler & Marking, 1975, 1979; Anderson et al., 1976; Bills et al., 1977a, b; Marking & Chandler, 1978). In contrast to studies using standard exposure chambers and proportional diluters, Harrison et al. (1981, 1984) exposed clams to stressor in 20-1itre plastic trays. The gravity dependent flow rate from a mixing chamber to the trays was 6"5 to 7.0 litres hDesigns that did not rely on gravity dependent flow of test solutions utilised pumps to accomplish the transfer of solutions. H o r n e & Mclntosh (1979) conducted tests in a modified 8-1itre dessicator with a regulated water flow of ~ 3 0 0 m l h - 1 . The shells (ventral margins) of these clams were notched with a triangular file to permit constant exposure to the stressor. Rodgers et al. (1980) pumped stressor solutions at a rate of ~ 2-0 ml min and diluent water at a rate of 0.5 to 0"8 litres rain ~ into side by side linear artificial streams. Each stream was divided in half along its length to facilitate testing with and without substrate. Water depth was ~ 50 ram. Graney (1980) utilised a similar approach. While other investigators dissolved stressor in dilution water to prepare concentrated stock solutions, Mussalli et al. (1986) leached stressor from pellets by pumping well water through containers with different quantities of pellets to deliver a continuous flow of stressor to clams in 6-1itre glass tanks. Flow rates were either 250 or

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Francis G. Doherty, Donald S. Cherry

350 ml min-1. Belanger et al. (1986c) and Doherty et al. (1986) pumped test solutions (1.6 litres m i n - 1 and 7 litres d a y - 1, respectively) to oval artificial streams (13- to 20-1itre volumes) in which clams were being exposed to stressor. A current was m a n u f a c t u r e d in the streams by rotating paddlewheels to increase environmental realism. A few studies incorporated elaborate acclimation and exposure duration schedules in their study designs. Evans et al. (1979) assessed the ability of Corbicula to tolerate salinities ranging from 2 to 24ppt by gradual acclimation over a period of 80 days. Mattice & Dye (1976) assessed longterm heat stress, long-term cold stress, and heat shock after 1 to 4 weeks acclimation to 5, 10, 15, 20, 25, 30, or 32°C in 190-1itre aquaria with constantly flowing well water. Mattice et al. (1982) simultaneously exposed Corbicula to elevated temperature and chlorine. They used a uniquely designed exposure chamber (28"5 × 10.0 × 6-0 cm) of --~ 1"7 litre capacity that used a series of baffles to prevent layering of ambient and higher temperature test water. The test design included a 20-min tank acclimation period, a 15min ambient water flow acclimation period, and a 10-min exposure to the test temperature followed by a 30-min simultaneous exposure to chlorine and the test temperature. Exposure of adults to stressors in the field

Haines (1979) studied the use o f Corbicula as a clarifying agent of the effluent from an algal production facility utilising secondarily treated wastewater. Clams were spread out in a monolayer on epoxy-coated wire trays suspended off the bottom of a 515-1itre fibreglass tank (surface area = 1-56m 2, water depth = 33cm). H o r n e & McIntosh (1979) exposed Asiatic clams to diluted sewage in a side channel of the Blanco River, Texas. Clams were held in 1/4 in square mesh cages partially buried in the substrate. Graney (1980) exposed Asiatic clams to heavy metals in epoxy-coated aluminium fishery troughs (3"5 x 0"7 × 0"3 m) on the banks of the New River, Virginia. The artificial streams were layered with New River sediment and supplied with New River water by a pair of 1-hp submersible pumps (7-5 litres min-1). Metals were dosed from stock solutions by peristaltic pumps. Burress (1982) placed 100 Asiatic clams in a cage in a pond to be treated with a single dose of synergised Rotenone. Additional cages with 20 clams each were placed in the treated pond on each of four consecutive days beginning three days following treatment. Smithson (1981, 1986) created anoxic conditions in the intake basin of a power station to alleviate Corbicula fouling problems within the plant. Belanger et al. (1986c) utilised oval artificial streams in a river side study exposing clams to zinc. Doherty et al. (1986) diverted the chlorinated process water in a manufacturing plant

Toh, rance

~fCorbicula spp. to lethal let~els ~[" toxic stressors

275

through small bore tubing to steel cylinder chambers within the plant to expose adults to chlorine under in-plant conditions. Exposure of larvae to stressors

All studies in which the larvae of Corbicula spp. were exposed to stressors were conducted in the laboratory. All tests were static in design except for the flow-through system of Sickel (1976). Exposure containers ranged in volume from that of a depression in a glass slide ( --~1 ml) to 500-ml beakers. Numbers of larvae exposed to stressor ranged from 10 per millilitre to 500 per litre per treatment. Larvae were obtained for testing either by (1) sacrificing the adult and manually removing larvae from the gills of the adult (Foster, 1981; Harrison et al., 1981, 1984), (2) collecting larvae upon release by the adult (Davis & Doherty, 1985; Belanger et al., 1986a; Davis & Lyons, 1986), or (3) straining recently released larvae from substrate occupied by gravid adults (Sickel, 1976). Those investigators that sacrificed adults to obtain larvae for testing found it necessary to distinguish between developmental stages of larvae during testing. Larvae collected via the other methods were consistent in their stage of development. Adult Corbicula diets

Habel (1970) fed test clams 5ml of an algal culture consisting of 94% Dictyosphaerium and 6% Oseillatoria at a concentration of 1.26 x 106cellslitre -1. Coldiron (1975) did not provide food to clams held in aquaria in the laboratory other than for the 'rich growth of several species of algae, protozoa and other microorganisms (in the stock aquaria) ... and by direct addition of algae scraped from the walls of the stock aquarium'. Mattice & Dye (1976) fed clams a powdered mixture of Biorell tropical fish flakes and trout chow at a rate of 0.01 g dry weight of food per clam per day. Marking & Chandler (1978) fed clams small amounts of boiled trout chow and cereal leaves (Daphnia food) weekly. Chandler & Marking (1979) fed clams trout chow and boiled cereal grass leaves during holding (7 days) and post-exposure observation periods. Evans et al. (1979) provided a nutrient medium of strained spinach to clams for 8h prior to testing. Horne & Mclntosh (1979) did not feed clams during either pre-experimental or experimental phases. Cherry et al. (1980) reported feeding clams a finely ground mixture of trout chow and tropical fish food daily. Harrison et al. (1984) did not feed clams during either acclimation or testing. Belanger et al. (1986a, b) fed clams a daily diet of 1000 cells per millilitre of green algae ( C h l a m y d o m o n a s rheinhardti) cultured in Bold's Basic Medium (Carolina Biological Supply Co., Burlington, NC). Belanger et al. (1986c) relied on the

276

Francis G. Dohero', DonaM S. Cherry

naturally suspended algae in dilution water pumped directly from the New River, Virginia, to nourish clams in a field-located study. Doherty et al. (1986) fed clams aliquots of dense cultures of C. rheinhardti every other day. Mussalli et al. (1986) did not feed clams during an 8-day study other than for that present in the filtered lake water used as dilution media. Davis & Lyons (1986) fed clams a daily diet of an unspecified laboratory cultured algae.

Larval Corbicula diets Foster (1981) reported that larvae of the Asiatic clam could be cultured in the laboratory for up to 1 month by periodically renewing the culture water and providing a nutrient source. He suggested that S c e n e d e s m u s and trout chow at 1 mg m l - 1 was acceptable.

Test endpoints All of the studies under review were cited because mortality levels for Asiatic clams exposed to a toxic stressor were reported. Some studies focused on lethality as an endpoint while others were primarily concerned with other points of reference. These points included bioaccumulation (Graney, 1980), ecological distribution patterns ( H o r n e & McIntosh, 1979), foot immobilisation (Anderson et al., 1976), gaping (Anderson et al., 1976; Rodgers et al., 1980), siphoning behaviour (Rodgers et al., 1980; Belanger et aL, 1986a,b,c), blood osmotic pressure (Gainey, 1978), gill ciliary action (Daum et al., 1978), per cent distribution of water (McMahon, 1979a), and growth (Belanger et al., 1986a,b,c). Mortality data were expressed either as the per cent mortality observed at a specified level of stressor or as a calculated theoretical level of mortality. For those studies in which per cent mortality levels for multiple exposure durations and concentrations were reported, subjective judgement was used with respect to which data sets would be summarised in this review. The reader is referred to the original study citation for a complete set of a study's data. The calculated levels of mortality were the median lethal concentration (MLC) for a specified test duration or a median lethal time (MLT) for a specified stressor concentration. These values represent the concentration or time, respectively, at which 50% of the test population succumbed to the stressor. Measured concentrations of stressor were clearly reported in --~50% of the studies cited in this review. Descriptions of experimental methods in the remaining studies did not adequately specify whether stressor concentrations were measured or calculated. In most instances, the criterion for death in an adult was a lack of valve adduction in gaping individuals or clams whose shell valves had been gently pried apart.

Tolerance of Corbicula spp. to lethal levels o f toxic stressors

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Occasionally, some investigators examined gill cilia activity as an indication of a clam's condition at the time the individual was sacrificed. Mortality in larvae was determined by the absence of ciliary and muscular movement.

Miseellaneous Additional aspects of culturing and testing methodologies were addressed by some investigators but the reports were too sporadic to devote separate sections to their discussions. Data on some of these points are presented in Tables 2 to 8 (e.g. water chemistry, temperature, substrate composition, and life stage). The reader is referred to the original documents for specific details on each experimental design.

REVIEW OF T H E T O L E R A N C E OF C O R B I C U L A

TO STRESSORS

Halogens (Table 2) Reports on the toxicity of halogens to adult Asiatic clams were found in three studies assessing the toxicity of two halogens. Cairns & Cherry (1983) were unable to generate mortalities among test individuals after 4days' exposure to chlorine (20mglitre -1 TRC) but did report a MLC of 0.69 mg litre- ~T R C after 10 days of treatment. Chandler & Marking (1979) also found the Asiatic clam to be resistant to chlorine during short-term exposures. Doherty et al. (1986) reported high levels of mortality at low levels of halogen after extended periods of exposure and a lack of difference in efficacy between chlorine and bromine. Exposure of clams to either T R C or total residual bromine at equivalent molar concentrations of ~ 0.65 and 1.46 mglitre 1, respectively, produced MLCs of ~ 20days.

Heavy metals (Table 3) Studies assessing the toxicities of heavy metals to adults addressed the toxicities of cadmium, copper, tin, and zinc. Tin is apparently ~ 1.5 orders of magnitude more toxic than copper on a weight:weight basis and ~2.8 orders of magnitude on a molar basis. Harrison et al. (1981) generated a 7day M LC of 3'6 mg litre- l with copper while M ussalli et al. (1986) generated an 8-day MLC of 0.024mglitre-~ with tin. In a series of 28- and 30-day studies, Asiatic clams appeared to be more susceptible to c~dmium and copper than to zinc (Graney, 1980). Exposure to 0"055 mglitre-~ cadmium (4-89x lO-Tmoleslitre-~) for 30 days or 0.057mglitre-~ copper (8-97 x 10 7moleslitre ~) for 28 days resulted in 14.6% and 46'6%

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mortality, respectively, among treated groups. Exposure to 0.835 mg litre zinc (1-28 × 10 - 6 moleslitre-1) for 28days produced negligible mortality (6%). A direct comparison of mortality levels between studies conducted with cadmium, copper, and zinc by Graney (1980) is difficult because of widely divergent test temperatures (20, 24 and 14°C, respectively).

Organic, non-heavy metal inorganic, and miscellaneous stressors (Tables 4 and 5) The toxicities of a wide range of materials (> 30) to the Asiatic clam were gleaned from more than a dozen studies. Mortality of adult Asiatic clams was observed in groups exposed to concentrations of stressor ranging over an --~ 106-fold span. Clams were most tolerant of exposure to acetone (MLC >20000mglitre-~), Betadine solution (MLC >30000mglitre-1), and ethanol (MLC > 60 000 mg litre- 1) after 4 days of treatment (Chandler & Marking, 1979). Clams appeared to be least tolerant of exposure to Antimycin (0-005mglitre~). Exposure for 30days resulted in 100% mortality after a 70-day recovery period (Marking & Chandler, 1978). The toxicity of Slimicide C-41, a patented molluscicide approved for use in oncethrough cooling systems, demonstrated efficacy at concentrations intermediate to those of the materials cited above. Davis & Lyons (1986) reported 100% mortality among adult clams exposed to 100mglitre -~ Slimicide C-41 for 1 day.

Environmental stressors (Tables 6 and 7) Adult Asiatic clams were found to be intolerant of salinity at levels in excess of 22 ppt (Gainey, 1978; Evans et al., 1979), temperature in excess of 35JC (Habel, 1970; Coldiron, 1975; Cherry et al., 1980), pH of <3"0 or >9"0 (Daum et al., 1978), and aerial exposure in excess of 2 weeks (McMahon, 1979a; Dudgeon, 1982). These are generalisations though dependent on fluctuations among environmental variables, acclimation and exposure durations, and size of clam. One investigator cited histological evidence and ecological distribution patterns to assess the tolerance of Asiatic clams to pH. Kat (1982) presented evidence suggesting that individuals of Corbicula inhabiting acidic environments experienced mortalities resulting from dissolution of the shell in the umbonal region due to low pH water.

Tolerance of larvae to stressors (Table 8) Larvae of the Asiatic clam were relatively tolerant of the 10 stressors they were directly exposed to in studies not exceeding 72 h in duration. Benthic

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larval stages were least tolerant of exposure to copper and demonstrated the greatest resistance to exposure to Slimicide 364. Harrison et al. (1981, 1984) reported a M L C of 60ppb after exposure to copper for 24h. Davis & Doherty (1985) reported mortality levels not exceeding 83% 24h after exposure to Slimicide 364 at a p r o d u c t concentration of 200mglitre-1 (20mglitre-1 of the active ingredient, fl-bromo-fl-nitrostyrene). Chlorine was intermediate between these extremes in its toxicity to larvae. Belanger et al. (1986a) discovered that exposure ofgravid adults to stressor resulted in an increase in larval mortality. They reported that exposure of adults to 108 asbestos fibres per litre resulted in 28% mortality among larvae released by treated adults whereas larvae from control clams experienced only 11% mortality.

REVIEW OF T H E I N F L U E N C E OF E X P E R I M E N T A L VARIABLES ON T H E TOXICITY OF STRESSORS

Test type There were minimal differences in toxicity test results when exposure of clams to solutions of stressor was accomplished through either static or flow-through designs (Table 4).Comparisons were made between studies generating a M L C after 4days of exposure to stressor. The differences between MLCs generated in static and flow-through tests conducted with the same stressor ranged from 1.05- to 1.78-fold (Chandler & Marking, 1975, 1979; Bills et al., 1977a,b). Stressors appeared to be more toxic when tested under flow-through conditions in four studies while more toxic under static conditions in two studies. In all instances, confidence intervals of the MLCs generated in either static or flow-through studies overlapped. Results obtained in laboratory artificial streams and in-plant field studies also demonstrated minimal differences (Table 2). Doherty et al. (1986) obtained similar levels of mortality a m o n g C o r b i c u l a exposed to comparable levels of chlorine in tests conducted in both the laboratory and the field (Table 2). These observations suggest that results obtained in acute testing of potential molluscicides are not a function of the manner in which clams are exposed to stressor. Flow-through systems might be preferable when highly volatile or unstable compounds that degrade rapidly are being tested. Static-renewal designs may be an alternative to flow-through tests when a study's duration is extended beyond 1 week.

Exposure duration Numerous studies demonstrated that increases in the exposure duration

Tolerance of Corbicula spp. to lethal levels of toxic stressors

297

resulted in substantial reductions in the concentrations of stressor required to produce a MLC. Cairns & Cherry (1983) reported a lack of mortality in clams exposed to 20 mg litre- 1 total residual chlorine (TRC) for 4 days but a M L C of 0"69 mg litre- 1 T R C after 10 days of exposure (Table 2). Harrison et al. (1981) obtained a 300-fold reduction in the M L C for copper-exposed adults by increasing the exposure duration from 7 to 42 days (Table 3). Davis & Lyons (1986) reported higher levels of mortality at lower concentrations of Slimicide C-41 when exposure duration was increased from 1 to 3 days (Table 7). Numerous studies reported reductions in the M L C as great as 15fold when exposure durations were extended from 1 to 4 days at a test temperature of 16°C (Tables 3 and 4). Rodgers et al. (1980) reported a 48-fold reduction in the M L C when exposure duration of clams to copper and zinc was extended from 1 to 4days at temperatures ranging from 15 to 24°C (Table 3). Marking & Chandler (1981) reported an 83-fold reduction in the MLC when exposure of clams to PA-14 was extended from 1 to 4days and conducted at 22°C (Table 4). Comparable observations were reported in studies with larvae (Table 8). Harrison et al. (1981, 1984) reported that a 50fold reduction in copper concentration produced comparable levels of mortality among larvae when exposure duration was increased from 1 to 8 h. Sickel (1976) reported a 3"8-fold reduction in the MLC when exposure of larvae to chlorine was extended from 8 to 24 h. Standard protocols describing procedures for the testing of toxic effects of stressors in aquatic organisms state that all organisms other than daphnids and midge larvae should be exposed to toxicant for at least 96 h and possibly as long as 8 days (ASTM, 1980). The purpose of such a test is to assess the acute toxicity of aquatic pollutants as an initial step in defining acceptable levels of a material in aquatic systems. The majority of studies cited in this review reflect this philosophy in that many studies assessing the toxicity of chemical agents to Corbicula were only run for 4 days. Tests assessing the molluscicidal efficacy of agents though are conducted for the purpose of identifying compounds to control clams rather than protect clams. Since there is a change in the objective for conducting the test, the test should be designed for the purpose for which it is intended. Extending exposure durations is an appropriate variation from standard protocols because all investigators conducting tests beyond 96 h in duration reported substantial reductions in the concentration of stressor required to produce a MLC or 100% mortality. Specifically, data presented by Harrison et al. (1981) demonstrated the lack of an incipient M L C in copper-exposed groups of clams after 42 days of treatment with continuing reductions in the MLC after each 7-day observation period. Similar conclusions could be drawn from survivorship curves presented by Doherty et al. (1986). Asiatic clams exposed to chlorine

298

Francis G. Doherty, DonaM S. Cherry

failed to show a cessation in mortalities through a 32-day treatment period at concentrations ranging up to ~ 1 m g l i t r e - ~ T R C . These results are significant in that they permit reductions in the quantity of molluscicide that may ultimately be required resulting in a reduction in the potential for harmful effects by a molluscicide on non-target organisms in the receiving stream.

Recovery periods Incorporation of recovery periods into experimental designs demonstrated that residual mortalities among stressor-exposed clams were obtained that enhanced the apparent efficacy of the treatment applied. Marking & Chandler (1978) demonstrated that mortality levels among clams exposed to Antimycin for 30 days was a function of the length of the recovery period as well as the toxicant concentration and exposure duration (Table 4). Coldiron (1975) reported data demonstrating that exposure and recovery durations assessing the tolerance of clams to elevated temperatures were inversely related. For a given test temperature, 100% mortality was obtained among exposed clams at shorter exposure durations when recovery periods were lengthened (Table 5). Cairns & Cherry (1983) and Davis & Lyons (1986) both observed higher levels of mortality among clams exposed to potassium and Slimicide C-41, respectively, when treated clams were observed for latent mortalities after the treatments had been terminated (Table 7). Testing the molluscicidal efficacies of existing chemical compounds is currently a matter of enumerating mortalities. The approach does not define the toxic mode of action of the agent within the organism. Without any knowledge on the rate of movement of toxicant from the water column to biological site of reaction, there is no way to predict at what point in time mortality might occur. Recovery periods are an important aspect of molluscicidal efficacy testing because it provides the time necessary for the body burden of toxicant to have reached a steady state in the ultimate storage sites within the organism's body and to produce mortality if present at the necessary levels. The presence of latent mortalities beyond the duration of exposure permits reductions in either exposure concentration or duration providing additional protection to non-target organisms in receiving streams.

Clam size and larval stage It was not clearly evident whether tolerance of Asiatic clams exposed to stressors is a function of the size of an adult although tolerance was clearly related to the developmental stage of larvae. Doherty et al. (1986) could not

Tolerance o l ( ' o r b i c u l a .spp. to lethal I{'t¢'L~ ol to.\ic strc.vsors

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discern a difference in sensitivity to chlorine between 7 to 11 and 15 to 21 mm adults (Table 2) even though Mussalli el al. (1986) determined that juveniles were at least an order of magnitude more sensitive to tin than adults (Table 3t. Previously, Dudgeon (1982) had reported that smaller clams ( < 10mmt were less tolerant of aerial exposure when tested either in the presence or absence of substrate than clams > 1 0 m m in shell length (Table 6). In contrast, Tilly et al. (1978) reported that small clams (9mm) were more tolerant of exposure to gamma radiation than 25ram clams (Table 7l. Despite the lack of clear-cut differences in sensitivity, juvenile stages should be the primary, non-larval stage targeted for treatment. McMahon (1977) points out that the detrimental effects produced by Asiatic clams are most prevalent when shell size exceeds the diameter of cooling condensor pipes. If treatment programmes are not initiated until newly spawned larvae have been given the opportunity to develop, treatment must begin before disruption of water flows by enlarged shells are encountered. Harrison et al. (1981, 1984) reported that the tolerance of larvae to copper increased as larvae developed from trochophore to planktotrophic veliger to benthic juvenile (Table 8). Davis & Lyons (1986) demonstrated that late veligers were more tolerant of exposure to Slimicide C-41 than early veligers (Table 8). There was no opportunity to directly compare the sensitivities of larvae and sexually immature juveniles to the same stressor. However, 10 to 25 mm clams were more tolerant of exposure to Slimicide C-41 after 1 day of treatment (Table 7) than late veligers (Table 8; Davis & Lyons, 1986). Despite the lower tolerance of fully developed larval stages, tests assessing the efficacy of molluscicides against larval stages should be conducted with the benthic stage. Doherty et al. (1987a) reported that fully shelled larvae were the predominant stage released by a naturally spawning adult. Treatment concentrations developed based on tests conducted with the most sensitive trochophore stages may be inadequate to control the more tolerant shelled benthic stage.

Temperature Susceptibility of clams to a stressor was increased with higher test temperatures. Graney (1980) observed generally higher levels of mortality in clams exposed to either cadmium or copper at 2Y~C than at 9 C (Table 3). Marking & Chandler (1981) reported MLCs of >500, 550 and 300 mg litre i after 24 h of treatment and 250, 130 and 3.6 mg litre-- 1 of PAl4 after 96h of exposure of clams at temperatures of 12, 16 and 22C, respectively (Table 4) producing > 2-, 4"2- and 83-fold reductions in MLC levels obtained. An increase in the rate of reduction in the MLC at higher temperatures is not unsubstantiated. Other studies conducted at 16C

300

FrancL~" G. Doherty, Donahl S. Cherry

produced reductions in the MLC of as much as 15-fold (Tables 3 and 4) while studies conducted at temperatures ranging from 15 to 24 C resulted in a 48fold reduction in the MLC when exposure was extended from 1 to 4 days (Rodgers et al., 1980). McMahon (1979a) reported that clams succumbed to aerial exposure approximately twice as fast at 30'~C than at 20~C in high humidity (Table 6). Smithson (1981, 1986) found that treatment of cooling water intake tanks with sodium metabisulphite and cobalt chloride produced > 90% mortality when temperatures exceeded 24'C whereas mortality levels were < 5 0 % when temperatures were < 18'~C. A similar pattern of mortality was observed when hydrogen sulphide was added to the sodium metabisulphite/cobalt chloride treatment (Table 7). Doherty et al. (1986) observed differences in susceptibility of clams exposed to halogens in studies conducted both in the laboratory and in the field (Table 2). Clams succumbed to exposure to halogens in the laboratory at least twice as fast when the collection temperature for clams was considerably less than the test temperature ( < 1 0 C and >18~C, respectively) versus comparable collection and test temperatures ( ~ 20 to 22°C). Clams were also more sensitive to chlorine in 28-day studies conducted in the field during the summer as water temperatures were rising from 20 to 25°C as opposed to clams in a study conducted in autumn when water temperatures were falling from 20 to 12~:C. Only one study reported a lack of association between temperature and the toxicity of a stressor. Mattice et al. (1982) concluded that exposure of adult clams to chlorine and elevated temperatures for 30 min did not produce greater mortality than was produced by exposure to chlorine or elevated temperature individually for 30 min. It is apparent from these studies that demonstration of an interactive association between elevated temperatures and a chemical stressor requires durations much greater than 30 min. These reports are of crucial importance in the development of effective molluscicides. These data suggest that the rate of mortality among clams exposed to a toxic stressor increases at higher test temperatures. The data also suggest that the test temperature need not be lethal itself as attempted by Mattice et al. (1982). Substantially elevated levels of mortality were achieved at 20 to 25°C in comparison with that achieved at test temperatures < 20°C. An appropriate recommendation for molluscicidal efficacy testing appears to be to test compounds in the laboratory at no less than 20°C and in the field during early to midsummer periods when water temperatures are rising towards their yearly maximum. In-plant control attempts may be feasible in winter or early spring seasons if molluscicide application can be augmented with a cycling of heated water through the system with an absolute temperature of > 20°C.

Tolerance of

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Subslrate Trends in the results of toxicity tests conducted in the absence or presence of substrate suggest that the presence of substrate during testing ameliorates the toxicity of a stressor to the Asiatic clam. Graney (1980) reported higher levels of mortality among clams exposed to cadmium or copper in the absence of substrate in 14 of 16 comparisons (Table 3). Marking & Chandler (1978) observed higher levels of mortality more often in the absence of substrate (eleven comparisons) than when substrate was present (two comparisons) in studies in which the toxicity of Antimycin to Asiatic clams was assessed. There were no differences in mortality in five instances (Table 4). Dudgeon (1982) demonstrated that both small ( < 1 0 m m ) and large {> 10mm) clams survived lengthier periods of aerial exposure in the presence of substrate (Table 6). In contrast, Rodgers el al. (1980) and Cairns & Cherry i1983) reported that there was no effect on toxicity due to the presence of substrate in their studies with heavy metals and potassium, respectively (Tables 3 and 7). It is not known if higher levels of mortality in the absence of substrate was due to a lack of physical material that served to reduce the bioavailability of stressor or because of stress from being tested in an artificial environment. Since Asiatic clams are suspension feeders rather than deposit feeders, any adsorption of chemical to non-suspended sediment will reduce the quantity ofchemical available to Asiatic clams. It is not known though if the presence of suspended sediment would increase or decrease the bioavailability of the toxicant under study. Since suspended sediment concentrations can vary both spatially and temporally and slack areas in industrial water pipes allow the deposition of suspended sediments permitting colonisation by clams, efficacy testing of molluscicidal candidates should be conducted at a variety of suspended sediment concentrations and both in the presence and absence of substrate.

Behaviour Doherty el al. (1986) noted comparable M LTs for groups of clams exposed to 1.02mglitre t T R C for 32 days and clams exposed to --.0.25mglitre T R C for 14days followed by 0-96 and 0 9 9 m g l i t r c l TRC for 18days (Table 2). The authors suggested that levels of chlorine in excess of 0-25 mg litre-~ T R C induced valve closure. Clams were weakened after 2 weeks of treatment, opened their valves, and began to succumb to chlorine exposure. H o r n e & Mclntosh (1979) attempted to minimise the behavioural valve closure response of Asiatic clams by notching their shells to permit constant exposure to the stressor (Table 5). The wdidity of this approach

302

Francis G. Doherty, DonaM S. Cherry

could not be assessed because the experimental design did not include clams with intact shells. Doherty et al. (1987b) reported a dose-response relationship between solutions of cadmium or zinc and the length of time that the valves of exposed adults remained parted. If such a response by Asiatic clams exists on exposure to other stressors such as molluscicides, the toxic effects of exposure to lower doses would be of greatest interest. Such an approach would obviously be compatible with the need to minimise potential impacts of molluscicide on non-target organisms in receiving streams. Interactions between t w o or m o r e s t r e s s o r s

Exposure of clams to stressors both individually and in combination with others resulted in a variety of interactive effects between stressors. Horne & Mclntosh (1979) obtained comparable levels of mortality when clams were exposed to low oxygen or a mixture of low oxygen and ammonia. Exposure of clams to ammonia alone resulted in a higher level of mortality than either low oxygen or low oxygen and ammonia in combination (Table 5). Rodgers et al. (1980) reported that exposure of clams to copper produced greater levels of mortality than exposure to zinc or a combination of copper and zinc (Table 3). Doherty et al. (1986) reported that a mixture of chlorine and bromine produced levels of mortality in juveniles comparable to that produced by exposure to either chlorine or bromine at similar molar concentrations of halogen. Mortality levels in adults exposed to a mixture of chlorine and bromine were intermediate between those produced by exposure to either chlorine or bromine (Table 2). Smithson (198l, 1986) obtained greater levels of mortality when clams were exposed at 18°C to a mixture of sodium metabisulphite/cobalt chloride/hydrogen sulphide versus the same mixture lacking hydrogen sulphide. Similar levels of mortality were observed when tests were conducted at > 21°C (Table 5). Mattice et al. (1982) reported a lack of any interactive action between elevated temperature and chlorine. The combination did not produce greater levels of mortality than was produced by temperature alone (Table 6). In some instances, studies were not designed to define interactive effects between stressors. Davis & Lyons (1986) exposed clams to Slimicide C-41 (Table 5), a mixture of two organic biocides (Table 1) but did not expose clams to the individual components. In studies conducted with larvae, Davis & Lyons (1986) obtained higher levels of mortality among larvae exposed to Slimicide C-41 than Davis & Doherty (1985) with Slimicide 364 (Table 8), a product with only one of the ingredients of Slimicide C-41 (Table 1). No data were available on the toxicity of the second stressor alone to larvae. Exposure bftest organisms to two stressors simultaneously can result in a

Tolerance

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wide variety of interactive effects (Klaassen & Doull, 1980). Additive effects, which are the most common, produce a situation in which the combined effect of the two stressors is equal to the sum of the individual stressors alone. Synergistic and antagonistic effects occur when the combined effect is much greater or lesser than, respectively, the effect of each stressor alone. Potentiation occurs when one stressor lacks a toxic effect but produces a much greater toxic effect in a second stressor when administered in concert. Obviously, any situation that produces potentiation or synergism between two stressors in Asiatic clams is an attractive approach in controlling Asiatic clam fouling. Such an effect would permit reductions in exposure concentration or duration, thereby reducing the potential for harmful effects on non-target organisms in receiving streams as was discussed previously. Demonstration of interactive effects, though, requires tests with each of the stressors individually and then in combination. Until appropriately designed studies are conducted, the potential interactive effects between chemical stressors or between a chemical and environmental stressor cannot be assessed. Studies reported to date appear to warrant further research with stressors administered in combination with elevated temperatures for multiday or week durations.

Water chemistry Insufficient data precluded defining the role of water chemistry on the toxicity of stressors to Asiatic clams. A single study addressed the role ofpH on effects of heavy metals in Asiatic clams. Graney (1980) reported mortality levels in clams exposed to cadmium and copper at acidic (5-0) and alkaline (7.8) pHs. While there were slightly higher levels of mortality in acidic solutions, the increase was minimal ( < 13.3%) and the trend inconsistent (Table 3).

DISCUSSION AND S U M M A R Y

Overview The Asiatic clam, C o r b i c u l a spp., is highly susceptible to a wide variety of chemical, environmental, and physical stressors. Available data demonstrate that, as with any organism, susceptibility is dependent on the concentration of stressor, the duration of exposure, and many other potential toxicity mediating variables. Among the ~ 50 stressors for which data are presented, very few were actually tested as potential molluscicides. Although chlorine was apparently the only stressor approved for such a

304

Francis G. Doherty, DonaM S. Cherry

purpose in steam-electric power generating plants through the early 1980s (Isomet al., 1986), our review uncovered only four studies addressing the toxicity of chlorine to Asiatic clams. Previously, Hall & Burton (1981) could only cite a Tennessee Valley Authority report (Gooch et al., 1978) and an unpublished abstract (Isom, 1976) as evidence for the efficacy of chlorine against Asiatic clams. The lack of experimentally derived efficacy data for chlorine exacerbates the environmental and industrial liabilities of chlorine use. The indiscriminate toxicity of chlorine to aquatic life resulted in the limitation of release of chlorine to the environment in power plant discharges to < 2 h day-1 (USEPA, 1980) although local variances by state agencies have been granted (Cherry et al., 1986). Chlorine not only presents environmental liabilities but detracts from its extensive use in cooling water systems due to its corrosive action on pipes. Corrosion is common in steel and stainless steel pipes experiencing chronic exposure to chemicals. It is defined as the deterioration of a metal that reacts with its environment by eroding, crevicing, pitting, and stress cracking. Iron reacts vigorously with hydrochloric acid forming ferrous chloride (FeC12) in solution with the evolution of hydrogen (Henthorne, 1972; Fontana, 1986). Chlorides tend to become caught in minor cracks and fittings and eventually create chloride stress crack erosion. This process is hastened by decreases in pH and increases in temperature, chlorine concentration and duration of treatment. Although it has been demonstrated that 0"5 to 1.0 mg litre- 1 TRC produces 50% mortality among exposed adult clams after 20 to 30 days of treatment (Doherty et al., 1986), the potential corrosive consequences on pipes are unknown. Pruett (1980) recommends the use of PVC pipe because of the deleterious effects of chlorine upon stainless steel pipes although this is obviously impractical in many industrial installations. The uncertainty in gaining approval for release of chlorinated discharges because of the lack of data defining the lowest necessary dose and the environmental and industrial liabilities resulting from its use detract from reliance on chlorine as the preferred molluscicide. Where corrosion may not be a problem, an industry may elect to use chlorine for clam control and dechlorinate the plant effluent to meet environmental regulations. Assessment of the molluscicidal efficacy of most other compounds appears to be limited to a single study or report (Foster, 1981; Smithson, 1981; Mattice et al., 1982; Davis & Doherty, 1985; Davis & Lyons, 1986; Mussalli et al., 1986; Smithson, 1986). Heavy metals (e.g. copper, lead, and tin) have occasionally been discussed as potential molluscicides as components of controlled release surfaces (Goss et aL, 1979; Cherry et al., 1980), but should be avoided as biocides in once-through cooling systems because of their bioaccumulatory potential and environmental liability. These concerns focus attention on organic chemicals as molluscicidal

Tolerance o f Corbicula spp. to lethal levels ¢~l toxic stressors

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agents. Data reported for Slimicides 364 and C-41 demonstrate that organic molecules can be both effective molluscicides and approved for use in oncethrough cooling systems by federal agencies. The efficacy of these materials may be ultimately enhanced after additional testing assessing the effects of increasing exposure duration at lower concentrations, the potential for latent mortalities during recovery periods, and exposure at elevated temperatures defines the most appropriate treatment regimens. Variables enhancing the toxicity of a moiluscicide to Corbicula Data from studies summarised in this review demonstrate that there are a variety of options for increasing the efficacy of chemical treatments intended to minimise the fouling effects of Corbicula. Two studies demonstrated substantial increases in the toxicity of a stressor by simply increasing the exposure duration. Harrison et. al. (1981) and Doherty et al. (1986) reported that mortality among adults exposed to stressors (copper and chlorine, respectively) continued with duration of exposure and did not plateau. Consequently, if lengthier exposure periods are employed, lower concentrations of stressor can be applied to a system to produce high levels of mortality. Such an approach lengthens the duration over which molluscicide is released to receiving streams but concentrations may be lowered to levels within the assimilative capacity of the receiving stream. Data in this review also demonstrate that a treatment need not continue until eradication is achieved. Several studies reported that latent mortalities following cessation of treatment were extensive. While standard toxicity tests do not routinely incorporate recovery periods, molluscicidal candidates are judged on their ability to produce high levels of mortality at the lowest possible concentration. Reducing the concentrations of stressor to which Asiatic clams are exposed is even more critical in successfully controlling Corhicula fouling because of behavioural valve closure responses by Asiatic clams in the presence of toxic substances. Doherty et al. (1987h) demonstrated that valve parting time in adult Asiatic clams was inversely related to heavy metal concentration. Clams can only be controlled when a biocide has the opportunity to reach vital tissues. One avenue of research that has received little study involves the possible interactive effects between stressful levels of environmental variables and exposure of Corhicula to molluscicidal chemical additives. It is known that natural environmental variables such as temperature, pH, and salinity serve to limit the geographical distribution of Corhicula and have been investigated as potential non-chemical (as opposed to halogens, organics, or heavy metals) molluscicidal alternatives (Tables 5 and 6). Sickel (1986) reviewed several case histories of mass mortalities among Corhicula

306

Francis G. DoherO', DonaM S. Cherry

populations. He listed excessively high or low temperatures, low dissolved oxygen, silt, acidic waters, pollution, bacterial or viral infections, parasites, predators, competition, and genetic changes as factors contributing to mass mortalities among Corbicula. Foe & Knight (1986) observed an increase in mortality among clams exposed to a gradient in barnacles fouling the surfaces of clam shells. The potential interactions between chemical stressors and water quality are poorly understood with respect to Corbicula control. Certainly more research is needed in this area. Only one study attempted to determine the potential for combining the stressful effects of an environmental variable, high temperature, with a molluscicidal agent, chlorine (Mattice et al., 1982). Unfortunately, the temperature (10 to 46~'C) and chlorine (5 to 10mg litre- 1 TRC) levels tested by these investigators are environmentally unacceptable to surface waters receiving an effluent with these characteristics. Their lack of success in demonstrating an interactive effect between elevated temperatures and chlorine may have been a result of the brevity of the trial (30 min). Doherty et al. (1986), though, provided evidence for an interactive effect between temperature and toxicant. They observed a higher rate of mortality among clams exposed to chlorine in the cooling water of a manufacturing plant when temperatures were rising from 20 to 2YC in comparison with tests conducted when temperatures were falling from 20 to 12~C. These results should not be unexpected in the light of a study by McMahon (1979h). He reported that 0 2 consumption increased in clams acclimated to 10, 20, or 3ffC and subsequently exposed to temperatures ranging from 5 to 25C. Clams exposed to temperatures in excess of 2 5 C experienced severely depressed 0 2 consumption rates. If elevated 02 consumption rates are a barometer of higher metabolic rates, Asiatic clams may be more sensitive to toxicants for several reasons. These include a more rapid rate of uptake of biocide, a decrease in the duration over which an individual can subsist under valve closure, some other toxicity enhancing factor, or a combination of two or more factors. Two areas of potential investigation that have yet to be addressed with respect to the efficacy of molluscicidal treatments are the influence of water chemistry and physiological condition of clams. Bioavailability and toxicity of many chemicals have been related to alkalinity, hardness, and pH (Tovell et al., 1974; Sano, 1976; Mauck et al., 1977). Studies cited in this review reported differences in the toxicity of agents dependent on the presence of substrate during the exposure period. Physiological condition of a clam has yet to be included in test designs as a variable when the tolerance to stressors is assessed. The lack of this data is reflected in the myriad of diets provided to clams before, during, and after testing by the investigators cited in this review. Investigators in other areas have shown that diet of test organisms

Tolerance

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307

can significantly alter the M LC obtained in toxicity tests with fish and invertebrates (Mehrle et al., 1977; Winner et al., 1977). In addition, lwama & Greer (1980)demonstrated that a mild state of bacterial kidney disease in coho salmon increased the susceptibility of individuals exposed to sodium pentachlorophenate.

Cooling water treatment strategies Control of Asiatic clam fouling by treatment of cooling water systems has been undertaken with either the larvae or adult as the target organism. Cherry et aL (1986) scheduled 4-week chlorination periods at the Celco Plant, Virginia, to coincide with peak spawning periods of the Asiatic clam in the New River. Smithson (1981, 1986), on the other hand, targeted juveniles and adults in the intake basins for treatment during regularly scheduled outages. This dichotomy in philosophies is not reflected in the relative volumes of toxicity data for adults and larvae. Tests in which adults or juveniles were exposed to stressor were conducted ~ 6 times more often ( ~ 8 5 % of all tests) than tests with larvae. Despite the importance of controlling the larval stages of Corhicula to minimise fouling by the adult, only one study was found that addressed the toxicity of chlorine to Asiatic clam larvae (Table 8). Assessment of molluscicidal efficacies should be conducted against the benthic larval stages of Corhicula and the sexually immature juveniles. Doherty et al. (1987a) demonstrated that the vast majority of larvae naturally released by a gravid adult is the most advanced larval stage. It has been previously established that this stage is the most tolerant of all larval stages so that any treatment effective for benthic larvae would be effective for all stages. Treatment periods may then be targeted to either coincide with spawning cycles or the warmest period of the summer season. When control of the fully shelled juvenile or adult is required, treatment practices should attempt to incorporate longer exposure durations at lower doses of molluscicide and elevated water ternperatures. Control of Asiatic clam fouling by exposure to molluscicidal chemicals may be a viable option that can be effectively employed in a comprehensive treatment strategy. Chemicals previously used as molluscicidal agents have included heavy metals as well as halogens. This review, though, has demonstrated that there are a number of organic chemicals that demonstrate toxicity in Asiatic clams. Development of efficacious organic molluscicides must address the relationship between exposure concentration, exposure and recovery durations, and water temperature in its toxicity to Asiatic clams. The manipulation of elevated water temperatures during exposure of clams to biocidal agents in conjunction with the potential seasonal vulnerability of the Asiatic clam during its life cycle also deserves

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further research. Viable molluscicidal agents will be those that are toxic to clams at concentrations not likely to adversely impact non-target organisms in receiving streams.

A C K N O W L E D G E M ENTS The authors are grateful to Ms. C. Furrow for typing the tables.

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