effects of gonadal neoplasms on oogenesis in softshell clams,mya arenaria

JOBNAME: 67#2 96 PAGE: 1 SESS: 2 OUTPUT: Mon Jun 10 12:40:57 1996/xypage/worksmart/tsp000/67596b/11

Effects of Gonadal Neoplasms on Oogenesis in Softshell Clams,Mya arenaria

BRUCE J. BARBER

Department of Animal, Veterinary and Aquatic Sciences, University of Maine, Orono, Maine 04469

Received May 25, 1995; accepted November 27, 1995

The average prevalence of gonadal neoplasms insoftshell clams, Mya arenaria, from Whiting Bay,Washington County, Maine, was 19.4% in 1994.Monthly prevalences ranged from 10 to 26.7%. Neo-plasms ranged in intensity from few, small foci of un-differentiated germ cells (Stage 1), to 50–100% of go-nadal follicles being involved (Stage 2), to invasionand metastasis with loss of tissue architecture (Stage3), indicating that the disease is progressive and le-thal. There was no relationship (P > 0.05) betweenprevalence of neoplasms and clam size between 45.7and 60.7 mm mean shell length. Clams of both sexeswere affected, but females were significantly morelikely (P ≤ 0.025) to have neoplasms thanmales. Femaleclams with neoplasms produced significantly fewer (P≤ 0.001) gametes than healthy clams. Overall reductionof gamete number was 66%, resulting from direct dis-placement of gametes by tumor cells in affected go-nadal follicles. Compared to healthy clams, clams withneoplasms exhibited a significantly lower (P ≤ 0.001)mean oocyte diameter before spawning and a signifi-cantly greater (P ≤ 0.001) mean oocyte diameter afterspawning, as the result of a general (throughout theentire gonad) inhibition of normal oogenesis andspawning. It is concluded that gonadal neoplasmshave a negative impact on the reproductive output oftheWhiting Bay clam population. © 1996 Academic Press, Inc.

KEY WORDS:Mya arenaria; gonadal neoplasm; preva-lence; intensity; oogenesis effects.

INTRODUCTION

Gonadal neoplasms (germinomas, tumors) have beenreported in several species of marine bivalve molluscs,including the softshell clam, Mya arenaria (Peters etal., 1994). Germinomas, ranging from noninvasive tometastatic, in both male and female clams, are char-

acterized by the presence of monomorphic, basophilic,undifferentiated germ cells that proliferate rather thanmature (Gardner et al., 1991). The nucleii of these cellsare often eccentric and the nucleolus is frequently dif-ficult to distinguish in the abundantly clumped chro-matin (Brown et al., 1977). Disease progression hasbeen described, based on histopathology, by Yevich andBarszcz (1977). In early cases, small foci of neoplasticcells occupy a small portion of one or more follicles.Development of oocytes or spermatocytes apparentlycontinues, even as the neoplastic cells increase in num-ber within follicles. In some cases, however, folliclesare completely packed with neoplastic cells and as aresult, sexual identity is lost. Metastasis, in which tu-mor cells have broken through basement membraneand become established at a remote site, is commonlynoted. Cell types at the metastatic sites are the sameas those seen in the follicles, except that mitotic figuresare more prevalent. These neoplasms have beentermed “germinomas” and are similar to those reportedin the quahog, Mercenaria mercenaria and other spe-cies (Barry and Yevich, 1972; Hesselman et al., 1988;Peters et al., 1994). Based on the above description,gonadal neoplasms are considered malignant.Even though M. arenaria is distributed from Labra-

dor to South Carolina on the east coast of NorthAmerica (Hidu and Newell, 1989), gonadal neoplasmshave only been reported from locations in Maine. Thiscondition was initially described by Barry and Yevich(1975) and Yevich and Barszcz (1976, 1977) in clamsfrom Long Cove, Searsport, where 1 to 22% of clamsexamined between 1971 and 1975 had tumors. Similarneoplasms were present in 3.6% of clams examinedfrom Long Cove in 1976 (Brown et al., 1977). Germino-mas were also present in 3% of clams taken from RoqueBluffs (near Machiasport) and in 35% of clams collectedfrom Dennys Bay (near Dennysville) collected in March1980 (Gardner et al., 1991). In these cases, occurrenceof neoplasms was correlated with environmental con-tamination from either hydrocarbons (Barry and Yev-ich, 1975; Yevich and Barszcz, 1976, 1977) or herbi-cides (Gardner et al., 1991). Harshbarger et al. (1979)

1 Representative cases of M. arenaria with gonadal neoplasmshave been assigned Accession Nos. RTLA 6071, 6072, and 6073 in theRegistry of Tumors in Lower Animals, Department of Pathology,George Washington University Medical Center, 2300 I Street, NW,Washington, DC 20037.

JOURNAL OF INVERTEBRATE PATHOLOGY 67, 161–168 (1996)ARTICLE NO. 0024

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reported the presence of intranuclear inclusions in tu-mor cells from clams examined by Yevich and Barszcz(1976, 1977), but a viral nature was never confirmed.Thus the etiology of this disease is presently unknown.Since 1982, the production of clams from flats in

Washington County, Maine, has declined from 240,000bushels to 24,000 bushels (Maine Department of Ma-rine Resources). Although there are several potentialreasons for this decline, the impact of disease on repro-ductive development has not been previously exam-ined. A survey of several clam flats in WashingtonCounty, Maine, conducted in September 1993 revealedthat a population in Whiting Bay had a 43% prevalenceof gonadal neoplasms (Barber, 1995). In this study, theprevalence and intensity of gonadal neoplasms in thispopulation was determined throughout 1994 and theimpact of the disease on oogenesis was quantified.

MATERIALS AND METHODS

Adult clams (n 4 11–30) were collected using a handrake from an intertidal flat (Whiting Bay, latitude 44°499 180 N; longitude 67° 099 460 W) located near Whit-ing, in Washington County, Maine. Samples were col-lected in all months in 1994 except February, when theflats were covered with ice. Water temperature (mer-cury thermometer, °C) and salinity (refractometer, ppt)were recorded at the time of collection (low tide) begin-ning May 1994.In the laboratory, clams were measured (shell

length, mm), removed from their shells, and placed inHelly’s fixative (Barszcz and Yevich, 1975). Transversesections (3–4 mm) through kidney, digestive gland, go-nad, gills, and foot were removed, placed in cassettes,and dehydrated and cleared in a Fisher Histomatic Tis-sue Processor (Model 166). Tissues were embedded inParaplast, sectioned (6 mm), and stained with Shandoninstant hematoxylin and eosin Y before being cover-slipped.Finished slides were examined with a Nikon Labo-

phot microscope (100×) for the presence of gonadal neo-plasms. In clams having neoplasms, the degree of dis-ease development was assigned using criteria similarto those defined by Peters et al. (1994). Stage 1 neo-plasms consisted of undifferentiated germs cells par-tially or totally filling one or more (but less than half ofthe total) follicles; loss of gonadal architecture wasrare. Stage 2 neoplasms were characterized by thepresence of undifferentiated germ cells in over 50% ofvisible follicles; usually there was some loss of tissuearchitecture, but invasion and metastasis were not evi-dent. In Stage 3 neoplasms, a few to all follicles wereinvolved, loss of tissue architecture was common, andinvasion or metastasis was evident. The prevalence(percentage of clams having neoplasms) and the meanintensity (Stage) of neoplasms for each monthly samplewere examined for serial randomness using the non-

parametric mean square successive difference test(Zar, 1984, p. 419).The effect of gonadal neoplasms on oogenesis was

assessed using an image analysis system consisting ofa video camera mounted on a Nikon Labophot micro-scope and connected to a Truevision Targa+ video-graphics adapter. Images of five gonadal fields fromeach female clam (600×) were enhanced, and data werecollected using Image Pro Plus software. In each of thefive fields, the number of oogonia, oocytes, and ovawere counted, and the diameter of those having a vis-ible nucleus was measured. This method has beenfound to accurately reflect oogenesis in marine bivalves(Barber and Blake, 1991; Barber et al., 1988). Themean number of gametes per field and the mean diam-eter of those gametes for clams with and without neo-plasms were compared for each sampling date usingtwo-way ANOVA (Wilkinson et al., 1992).

RESULTS

Mean shell length of clams examined ranged from45.7 to 60.7 mm (Table 1). Although these means weresignificantly different (ANOVA, P # 0.001), there wasno relationship (correlation, P 4 0.729) between clamsize and the prevalence of gonadal neoplasms over thisrange.Water temperature at the collection site increased

from 12.1°C in May to a maximum of 19.1°C in Julyand then declined to 0.6°C in December (Table 1). Sa-linity ranged from 28 to 34 ppt between May and No-vember, but was 20 ppt in December (Table 1).Thirty clams were collected each month in 1994 ex-

cept February, May, and July (Table 2). In all, 310clams from Whiting Bay were examined histologicallyover the course of the study. Of these, 60 had gonadalneoplasms, for an overall prevalence of 19.4%.1 The 60neoplasms included 22 Stage 1 cases, 33 Stage 2 cases,and 5 Stage 3 cases. At the gross level, it was possible

TABLE 1Dates That Softshell Clams, M. arenaria, Were Collected

from Whiting Bay, Mean and SE of Shell Length (mm), Wa-ter Temperature (°C), and Salinity (ppt)

Collectiondate

Shell lengthTemperature

(°C)Salinity(ppt)Mean SE

1/11/94 49.8 1.0 — —3/17/94 45.7 1.0 — —4/19/94 48.7 1.0 — —5/20/94 52.2 1.0 12.1 286/15/94 54.7 1.0 14.5 297/18/94 57.1 1.7 19.1 328/14/94 57.2 1.0 16.5 329/11/94 60.7 1.0 14.3 3310/17/94 52.9 1.0 11.1 3411/13/94 52.8 1.0 8.8 3112/13/94 52.7 1.0 0.6 20

BRUCE J. BARBER162


to distinguish clams with advanced neoplasms (Stages2 or 3) from clams without neoplasms by the shrunken,darkened, mottled appearance of their gonads. Go-nadal neoplasms were found in all months sampledand in both male and female clams (Table 2). In May,June, and July, all neoplasms occurred in femaleclams. The sex ratio of all clams without neoplasmswas not significantly different from 1:1, but the sexratio of all clams with neoplasms was significantly dif-ferent from 1:1 (x2, P ø 0.025), with predominantlymore female clams having neoplasms (Table 2).Overall, monthly prevalences of neoplasms ranged

from 10% in June and August to 26.7% in both Sep-tember and October (Fig. 1). Mean monthly intensitywas greatest (2.2) in April and May and lowest (1.0) inJune (Fig. 1). Seasonal variability of both prevalenceand mean intensity, however, was random (not signifi-cant, P > 0.25).Gonadal neoplasms had a negative impact on the

number of eggs produced per individual (Fig. 2). Inboth healthy (without neoplasms) and diseased (withneoplasms) clams, mean egg number per field (for anindividual) was greater in the months (March–June)preceeding spawning; this was especially apparent inhealthy clams. The mean number of female gametes(oogonia, oocytes, ova) per field was lower in diseasedclams than in healthy clams in all months. Two-wayANOVA revealed that there were significant (P #0.001) differences in the number of female gametesboth between samples (dates) and between healthy anddiseased clams; there was also a significant (P # 0.001)interaction between date and disease status (Table 3).The overall effect of gonadal neoplasia was a 66% re-duction in the number of female gametes produced perindividual.Gonadal neoplasms also affected the size of eggs pro-

duced, but the effect varied over the course of the oo-genic cycle (Fig. 3). Mean egg diameter in healthyclams was greatest in the months of June and July. Asharp decline in mean oocyte diameter between Julyand August indicated that most healthy clamsspawned during this period. In clams with neoplasms,mean egg diameter was greatest in August, afterhealthy clams had spawned. Two-way ANOVA re-vealed that mean egg size varied significantly (P #0.001) with date but not with disease status over theentire study; there was also a significant (P # 0.001)date–disease interaction (Table 4). On closer examina-tion of Fig. 3, it can be seen that in the months pre-ceeding spawning (January–July), mean egg diameterwas lower in clams with neoplasms than in healthyclams; the magnitude of this difference generally in-creased in relation to oogenic activity. The oppositewas true for the months immediately after spawning(August–November), when clams with neoplasms hadgreater mean egg diameters than healthy clams; thisdifference decreased with time after spawning. Whenseparate two-way ANOVA were run on these two sub-

FIG. 1. Monthly percentage prevalence (Y1; bars) and mean in-tensity (Y2; line) of gonadal neoplasms in clams, M. arenaria, fromWhiting Bay.

FIG. 2. Mean (+SE) number of female gametes per microscopicfield from clams, M. arenaria, with and without gonadal neoplasms.

TABLE 2Numbers of Clams Examined Histologically at Each Sam-

pling Date and the Distribution of Neoplasms by Stageand Gender

Collectiondate

No.examined

Stage Gender

1 2 3 M F U

1/11/94 30 4 2 0 1 4 13/17/94 30 2 4 0 2 3 14/19/94 30 1 4 2 3 3 15/20/94 29 1 4 2 0 6 16/15/94 30 3 0 0 0 3 07/18/94 11 1 1 0 0 2 08/14/94 30 1 2 0 1 2 09/11/94 30 3 5 0 4 4 010/17/94 30 4 3 1 3 5 011/13/94 30 2 2 0 2 2 012/13/94 30 0 6 0 3 2 1Totals 310 22 33 5 19 36 5

Note. M, male; F, female; U, undetermined.

EFFECTS OF GONADAL NEOPLASMS ON OOGENESIS IN Mya 163


sets of data, there were highly significant (P # 0.001)differences by date and by disease status (P # 0.004),and for the postspawning months, there was a signifi-cant date–disease interaction (P # 0.002). Thus in themonths preceeding spawning, clams with neoplasmswere producing fewer and smaller gametes than clamswithout neoplasms (Fig. 4). After spawning, clams withneoplasms contained fewer, larger gametes thanhealthy clams (Fig. 5).

DISCUSSION

There are two major types of neoplasms that occur inmarine bivalves, including M. arenaria (Yevich andBarszcz, 1976, 1977; Brown et al., 1977; Peters, 1988;Peters et al., 1994). The first type, hematopoietic neo-plasms, or disseminated sarcomas, are characterizedby unusual cells in the connective tissue, blood vessels,and sinuses of the visceral mass, muscle, and mantletissue (Peters, 1988). The cells are hypertrophied, usu-ally enlarged two to four times the diameter of normalhemocytes, and have a hyperchromatic nucleus that isoften pleomorphic. Proportions of abnormal cells in af-fected clams generally increase over time and result indeath (Cooper et al., 1982; Barber, 1990; Brousseauand Baglivo, 1991). A viral etiology has been suggestedfor this disease, but other factors, such as chemicalpollution and genetics may be involved (Oprandy et al.,1981; Oprandy and Chang, 1983; Peters, 1988). Al-

though hematopoietic neoplasms are epizooticthroughout the range of M. arenaria (Brown et al.,1977; Farley et al., 1986; Brousseau, 1987a; Reinisch etal., 1984; Morrison et al., 1993), none were seen in theclams from Whiting Bay examined in the presentstudy.The second major type of proliferative disorder re-

ported to occur in marine bivalves, including M. are-naria, is the gonadal neoplasm or germinoma (Hessel-man et al., 1988; Peters et al., 1994), the focus of thisstudy. Based on histological examination, the prolif-eration of undifferentiated germ cells first in the gonadand then into surrounding tissues suggests a progres-sive and lethal nature, although this has not been dem-onstrated (Barry and Yevich, 1975; Yevich andBarszcz, 1977; Peters et al., 1994). Initial reports ofthis disease were correlated with hydrocarbon pollu-tion (Barry and Yevich, 1975; Yevich and Barszcz,1976, 1977; Brown et al., 1977), but subsequent at-tempts to induce the disease by exposure to hydrocar-bons were unsuccessful (Gardner et al., 1991). Morerecently, higher prevalences of gonadal neoplasms inWashington County, Maine, have been correlated witha history of herbicide application on adjacent land ar-eas, suggesting that there could be a chemical etiologyfor this disease (Gardner et al., 1991). The fact, how-ever, that gonadal neoplasms have only been found inMaine despite the occurrence of M. arenaria in morechemically contaminated areas along the east coastthan Maine (Reinisch et al., 1984), suggests insteadthat the etiological agent may be of a localized, geneticnature.Gonadal neoplasms were epizootic at the Whiting

Bay site, with an overall prevalence of 19.4%. Althoughneoplasms were identified at the cellular level by ex-amination of histological preparations, it was possibleto identify clams with advanced neoplasms by the grossappearance of their gonad. In a separate sample of 100clams collected from Whiting Bay in November 1994,12 clams that had shrunken, darkened, mottled gonadswere all found to have advanced (Stage 2 or 3) neo-plasms upon histological examination; among 20 clamsthat appeared to be healthy at the gross level, 2 werefound to have very early (Stage 1) neoplasms whenexamined histologically. Thus, gross examination of

FIG. 3. Mean (+SE) diameter of female gametes from clams, M.arenaria, with and without gonadal neoplasms.

TABLE 4Two-Way Analysis of Variance for Effects of Collection

Date and Disease on Mean Egg Diameter of Individual Fe-male Clams, M. arenaria

Source of Sum of Degrees of Mean Fvariation squares freedom square value Probability

Date 3022.71 10 302.27 11.99 0.000Disease 9.14 1 9.14 0.36 0.548Date × Disease 1395.58 10 139.56 5.53 0.000Error 3555.64 141 25.22

TABLE 3Two-Way Analysis of Variance for Effects of Collection

Date and Disease on Mean Egg Number per MicroscopicField of Individual Female Clams, M. Arenaria

Source of Sum of Degrees of Mean Fvariation squares freedom square value Probability

Date 5921.54 10 592.15 18.07 0.000Disease 3327.67 1 3327.67 101.56 0.000Date × Disease 1670.54 10 167.05 5.10 0.000Error 4685.57 143 32.77

BRUCE J. BARBER164


clams might be useful for determining the approximateprevalence of gonadal tumors at a site without the timeand expense associated with histology.Based on histological observations and the distribu-

tion of cases among all three intensity stages, thisstudy also suggests that this disease is progressive andpossibly lethal. Early cases are confined to one or moregonadal follicles with little apparent effect on sur-rounding follicles, tissues, or their physiological func-

tion (Stage 1). As the cells proliferate and spread, morefollicles become involved, and gonadal architecture andfunction are affected (Stage 2). Once the tumor cellsbreak through the follicle wall and metastasize, adja-cent and distant (gill) tissues become involved (Stage3). There were considerably fewer Stage 3 cases thanStage 2 cases, suggesting that few clams survived oncethe disease reached an advanced stage. The rate atwhich this disease progresses and results in death is

FIG. 4. Gonad of female clams,M. arenaria, collected from Whiting Bay in May 1994. (A) Gonad without neoplasm (175×) and numerousdeveloping oocytes (OC). (B) Gonad (175×) with neoplasm (N); note that oocyte (OC) growth is inhibited throughout the gonad. Bar, 100 mm.



unknown. For M. arenaria with early cases of hemato-poietic neoplasia, about 60 days are required for dis-ease progression and death to occur (Barber, 1990). Atthe Whiting Bay site, it was not unusual to find deadclams with intact shells in the sediment and no visiblesign of predation. In a few instances, decomposed clamtissue was still associated with the shells. Future stud-ies will attempt to more clearly determine the mortal-ity rate associated with this disease.

All the clams examined in this study were adults(reproductively mature), and there was no evident re-lationship between prevalence of gonadal neoplasiaand shell height over the size range of 45.7 to 60.7 mm.Clams of this size from Whiting Bay are estimated tobe 7–15 years old (B. Beal, pers. commun.). The mini-mum age/size at which clams develop gonadal neo-plasms is unknown, but it would presumably be afterattaining sexual maturity, which occurs at age >1 year

FIG. 5. Gonad of female clams, M. arenaria, collected from Whiting Bay in August 1994. (A) Gonad without neoplasm (175×) afterspawning; visible cells are primary oocytes (OC) or spherical products of cytolysis (C). (B) Gonad (175×) with neoplasm (N); note that follicleswithout tumor cells contain unspawned ova (OV). Bar, 100 mm.

BRUCE J. BARBER166


and shell height >20 mm (Brousseau, 1987b). Onesample of 20 clams with a mean size of 25.4 mm wasobtained in November from a site near Whiting Baywhere neoplasms occur. These smaller clams had de-veloped gonads that appeared normal for the time ofyear (spawned out); no gonadal neoplasms were found.Thus the minimum size at whichM. arenaria developsgonadal neoplasia is probably between 25.4 and 45.7mm shell height (age 2–7 years). This may representthe time required for the etiological agent to manifestitself.Previous reports of gonadal neoplasms in M. are-

naria indicated that both male and female clams wereaffected (Yevich and Barszcz, 1976; Peters, 1994), butthe finding that gonadal neoplasms occur almost twiceas often in female clams (60%) than male clams (32%)has not been previously been noted for this species. Incontrast, healthy clams from Whiting Bay (this study)as well as those from Long Island Sound (Brousseau,1987b) had a 1:1 ratio of males to females. A similarpredominance of neoplasms in females over neoplasmsin males, however, has been reported for the quahog,M. mercenaria. Barry and Yevich (1972) examined 539individuals and found 12 neoplasms in females andonly 2 neoplasms in males. Hesselman et al. (1988)examined 1263 individuals and of the 147 with gonadalneoplasms, 61% were females, 33% were males, and 6%were not recognizable as either male or female. Thefact that gonadal neoplasms occur primarily in femalesof both clam species suggests that a common diseasemechanism might be involved.Healthy clams at Whiting Bay underwent a normal

gametogenic cycle. Mean number of eggs per micro-scope field increased from January to May and thendeclined as oocytes continued to grow. Most oocytegrowth occurred in May and June, in conjunction withincreasing water temperature. Spawning began asearly as June, based on histological observation. Thesharp decrease in mean oocyte diameter seen in Au-gust, however, indicated that most spawning activityoccurred at this time, in conjunction with decreasingwater temperature.M. arenaria at Whiting Bay exhib-ited one spawning period, which is in agreement withprevious studies of clam populations north of Cape Cod(Ropes and Stickney, 1965).Gonadal neoplasms had a direct negative effect on

the number of eggs produced. The more advanced thedisease, the greater the proportion of gonadal folliclescontaining tumor cells and the lower the proportion ofnormal follicles containing oocytes. The magnitude ofreduction in mean egg number per field between nor-mal and diseased clams at any date was thus related tothe intensity of the disease. The interaction that oc-curred between month and disease status most likelywas the result of the increase in disease intensity thatoccurred in April and May (and hence decrease in eggnumber) at the same time that healthy clams were

experiencing an increase in egg number per field. Over-all, female clams with neoplasms produced 66% fewergametes than clams without neoplasms.The effect of gonadal neoplasms on the size of eggs

produced appeared to be indirect. That is, the effect onoocytes was seen throughout the gonads of affectedclams, not merely in follicles containing tumor cells.The net effect was an inhibition of the normal oogenicprocess. The impact of neoplasms on oocyte develop-ment was related to the timing of disease acquisition,which accounts for the interaction seen between dateand disease status. For clams in which the disease ap-peared early in the oogenic process, an overall reduc-tion in mean oocyte size resulted, as growth of oocyteswas stunted (Fig. 4). If neoplasms are progressive andlethal, these clams would most likely not have lived toproduce normal ova. If any ova were produced, theywould be smaller and less likely to produce viable lar-vae than ova from healthy clams (Kraeuter et al.,1982). For clams in which the disease appeared late inthe developmental phase, the effect was an increase inmean oocyte diameter, as the otherwise normal ap-pearing ova were not spawned along with ova fromhealthy clams (Fig. 5). The mean diameter of diseasedclams in August was even larger than that of healthyclams in July, indicating that while mature ova werereleased from healthy clams as early as June, theywere retained in diseased clams. Whether or not dis-eased clams with oocytes ultimately spawned and whatthe fate of these gametes was are unknown. It is clearthat diseased clams, even if they spawned would notproduce as many eggs, and the retarded timing of re-lease would reduce the likelihood of fertilization byhealthy sperm and larval survival as water tempera-ture decreased. This interaction between disease andoogenesis is complicated by differing rates of diseaseinitiation and progression throughout the year. The ob-servation of a general inhibition of oogenesis in clamswith neoplasms suggests that the disease affects thereproductive process at some basic level, perhaps bydisrupting or blocking normal neurohormonal cycles,which are involved in the endogenous regulation of ga-metogenesis in marine invertebrates (Schroeder,1987). Disseminated sarcoma cells in M. arenaria pro-duce proteins that, besides promoting tumor growth,could be cytotoxic (Sunila, 1992), thus possibly inhib-iting oocyte development. Additionally, the high respi-ration rates of mitotically active tumor cells could pos-sibly divert energy away from oogenesis (Sunila, 1991).Estimation of the impact of gonadal neoplasms on

the reproductive capacity of the Whiting Bay clampopulation depends on several factors. This study hasestablished the high prevalence of this disease at thislocation and the serious impact that it has on oogen-esis. The disproportionate number of female clamshaving gonadal neoplasms has further implications,especially if egg production is a factor limiting overall



reproductive success. The rates of disease progressionand mortality remain to be explored. Disease is onepotential factor limiting clam production in WhitingBay.

ACKNOWLEDGMENTS

I thank the following people for their help: Brian Beal, Ken Ven-cile, Robert Bayer, Bruce Wiersma, Neil Greenberg, and Brian Bar-ber for field assistance and Dawna Beane for the histological prepa-rations. Many thanks to Paul Yevich and Esther Peters for verifyingneoplasms. Stephen Fegley and Esther Peters provided thoughtfulcomments on an earlier version of the manuscript. Finally, I wouldlike to thank Bob and Jane Bell for access to the study area. Thisproject was supported in part by the Maine Aquaculture InnovationCenter. This is Maine Agricultural and Forestry Experiment Stationexternal publication 1955.

REFERENCES

Barber, B. J. 1990. Seasonal prevalence and intensity and diseaseprogression of neoplasia in soft shell clams,Mya arenaria, from theShrewsbury River, New Jersey. In “Pathology in Marine Science”(F. O. Perkins and T. Cheng, Eds.), pp. 377–386. Academic Press,New York.

Barber, B. J. 1995. Seasonal prevalence of gonadal neoplasms inclams, Mya arenaria, in Maine and impact on oogenesis. J. Shell-fish Res. 14, 238.

Barber, B. J., and Blake, N. J. 1991. Reproductive physiology. In“Scallops: Biology, Ecology and Aquaculture” (S. E. Shumway,Ed.), pp. 377–428. Elsevier, New York.

Barber, B. J., Getchell, R., Shumway, S., and Schick, D. 1988. Re-duced fecundity in a deep-water population of the giant scallopPlacopecten magellanicus in the Gulf of Maine, USA. Mar. Ecol.Prog. Ser. 42, 207–212.

Barry, M. M., and Yevich, P. P. 1972. Incidence of gonadal cancer inthe quahog Mercenaria mercenaria. Oncology 26, 87–96.

Barry, M., and Yevich, P. P. 1975. The ecological, chemical and his-topathological evaluation of an oil spill site. Part III. Histopatho-logical studies. Mar. Poll. Bull. 6, 171–173.

Barszcz, C. A., and Yevich, P. P. 1975. The use of Helly’s fixative formarine invertebrate histopathology. Comp. Pathol. Bull. 7, 4.

Brousseau, D. J. 1987a. Seasonal aspects of sarcomatous neoplasiain Mya arenaria (soft-shell clam) from Long Island Sound. J. In-vertebr. Pathol. 50, 269–276.

Brousseau, D. J. 1987b. A comparative study of the reproductivecycle of the soft-shell clam,Mya arenaria in Long Island Sound. J.Shellfish Res. 6, 7–15.

Brousseau, D. J., and Baglivo, J. A. 1991. Field and laboratory com-parisons of mortality in normal and neoplastic Mya arenaria. J.Invertebr. Pathol. 57, 59–65.

Brown, R. S., Wolke, R. E., Saila, S. B., and Brown, C. W. 1977.Prevalence of neoplasia in 10 New England populations of thesoft-shell clam (Mya arenaria). Ann. NY Acad. Sci. 298, 522–534.

Cooper, K. R., Brown, R. S., and Chang, P. W. 1982. The course andmortality of a hematopoietic neoplasm in the soft-shell clam, Myaarenaria. J. Invertebr. Pathol. 39, 149–157.

Farley, C. A., Otto, S. V., and Reinisch, C. L. 1986. New occurrence ofepizootic sarcoma in Chesapeake Bay soft shell clams, Mya are-naria. Fish. Bull. 84, 851–857.

Gardner, G. R., Yevich, P. P., Hurst, J., Thayer, P., Benyi, S., Harsh-barger, J. C., and Pruell, R. J. 1991. Germinomas and teratoid

siphon anomalies in softshell clams, Mya arenaria, environmen-tally exposed to herbicides. Environmental Health Perspectives 90,43–51.

Harshbarger, J. C., Otto, S. V., and Chang, S. C. 1979. Proliferativedisorders in Crassostrea virginica and Mya arenaria from theChesapeake Bay and intranuclear virus-like inclusions inMya are-naria with germinomas from a Maine oil spill site. Haliotis 8,243–248.

Hesselman, D. M., Blake, N. J., and Peters, E. C. 1988. Gonadal neo-plasms in hard shell clamsMercenaria spp., from the Indian River,Florida: Occurrence, prevalence, and histopathology. J. Invertebr.Pathol. 52, 436–446.

Hidu, H., and Newell, C. R. 1989. Culture and ecology of the soft-shelled clam, Mya arenaria. In “Clam Mariculture in NorthAmerica” (J. J. Manzi and M. Castagna, Eds.), pp. 277–292.Elsevier Science, Amsterdam.

Kraeuter, J. N., Castagna, M., and van Dessel, R. 1982. Egg size andlarval survival ofMercenaria mercenaria (L.) and Argopecten irra-dians (Lamarck). J. Exp. Mar. Biol. Ecol. 56, 3–8.

Morrison, C. M., Moore, A. R., Marryatt, V. M., and Scarratt, D. J.1993. Disseminated sarcomas of soft shell clams, Mya arenariaLinnaeus 1758, from sites in Nova Scotia and New Brunswick. J.Shellfish Res. 12, 65–69.

Oprandy, J. J., and Chang, P. W. 1983. 5-bromodeoxyuridine induc-tion of hemopoietic neoplasia and retrovirus activation in the soft-shell clam, Mya arenaria. J. Invertebr. Pathol. 42, 196–206.

Oprandy, J. J., Chang, P. W., Pronovost, A. D., Cooper, K. R., Brown,R. S., and Yates, V. J. 1981. Isolation of a viral agent causing he-matopoietic neoplasia in the soft-shell clam, Mya arenaria. J. In-vertebr. Pathol. 38, 45–51.

Peters, E. C., Yevich, P. P., Harshbarger, J. C., and Zaroogian, G. E.1994. Comparative histopathology of gonadal neoplasms in marinebivalve molluscs. Dis. Aquat. Org. 20, 59–76.

Peters, E. C. 1988. Recent investigations on the disseminated sarco-mas of marine bivalve molluscs. Am. Fish. Soc. Spec. Publ. 18,74–92.

Reinisch, C. L., Charles, A. M., and Stone, A. M. 1984. Epizootic neo-plasia in soft shell clams collected from New Bedford Harbor. Haz-ardous Waste 1, 73–81.

Ropes, J. W., and Stickney, A. P. 1965. Reproductive cycle of Myaarenaria in New England. Biol. Bull. 128, 315–327.

Schroeder, P. C. 1987. Endogenous control of gametogenesis. In “Re-production of Marine Invertebrates” (A. C. Giese, J. S. Pearse, andV. B. Pearse, Eds.), Vol. IX, pp. 179–249. Blackwell Scientific, PaloAlto, CA.

Sunila, I. 1991. Respiration of sarcoma cells from the soft-shell clamMya arenaria L. under various conditions. J. Exp. Mar. Biol. Ecol.150, 19–29.

Sunila, I. 1992. Serum–cell interactions in transmission of sarcomain the soft shell clam, Mya arenaria L. Comp. Biochem. Physiol.102A, 727–730.

Wilkinson, L., Hill, M., Welna, J. P., and Birkenbeuel, G. K. 1992.“SYSTAT for Windows: Statistics,” Version 5 ed. SYSTAT, Inc.,Evanston, IL.

Yevich, P. P., and Barszcz, C. A. 1976. Gonadal and hematopoieticneoplasms in Mya arenaria. Mar. Fish. Rev. 38, 42–43.

Yevich, P. P., and Barszcz, C. A. 1977. Neoplasia in soft-shell clams(Mya arenaria) collected from oil-impacted sites. Ann. NY Acad.Sci. 298, 409–426.

Zar, J. H. 1984. “Biostatistical Analysis.” Prentice–Hall, EnglewoodCliffs, NJ.

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effects of gonadal neoplasms on oogenesis in softshell clams,mya arenaria

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