Aquaculture-associated factors in QPX disease of hard clams: density and seed source

Aquaculture-associated factors in QPX disease of

hard clams: density and seed source

Susan E. Ford a,*, John N. Kraeuter a, Robert D. Barber a,George Mathis b

aHaskin Shellfish Research Laboratory, Institute of Marine and Coastal Sciences and Cook College,

Rutgers University, 6959 Miller Avenue, Port Norris, NJ 08349 USAbMathis and Mathis Enterprises, Egg Harbor City, NJ 08215 USA

Received 6 July 2001; received in revised form 4 September 2001; accepted 5 September 2001

Abstract

Quahog Parasite Unknown (QPX), a recently discovered pathogen of hard clams, Mercenaria

mercenaria, appears to be most prevalent in clams held in hatchery or nursery tanks or grown to

market in culture parks. The persistent link with cultured clams indicated that culture practices might

increase the susceptibility of clams to an opportunistic or facultative parasite. We investigated two

hypotheses: (1) that increased density would accelerate the development of QPX infections and (2)

that seed originating from nonlocal (i.e. another state) sources would be more susceptible than seed

from local sources. There was a significant trend toward higher QPX levels at higher planting

densities, but considerable variability in the data made it difficult to determine the effect of density

with a high degree of confidence. During 1995–1998, 3-year classes of clams imported as seed from

a South Carolina hatchery and grown at several sites in New Jersey acquired heavy QPX infections

and suffered mortalities of 26% to 92%. Clams of the same age from New Jersey hatcheries that had

been planted in adjacent plots for the same length of time acquired few or no detectable infections

and experienced no unusual deaths. QPX seems to be widespread in clam growing waters from

Canada to at least Virginia, but causes disease and mortality only in certain groups that may be

disadvantaged in some way, perhaps from an unfavorable genotype–environment interaction. The

fact that no mortalities have been reported in New Jersey since the purchase of South Carolina seed

was curtailed indicates that the problem was localized to these clams and has neither spread nor

0044-8486/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0044 -8486 (01 )00795 -5

* Corresponding author. Tel.: +1-856-785-0074; fax: +1-856-785-1544.

E-mail address: [email protected] (S.E. Ford).

www.elsevier.com/locate/aqua-online

Aquaculture 208 (2002) 23–38

persisted to a noticeable extent. Nevertheless, the results underscore the potential dangers of using

nonlocal stocks in molluscan aquaculture. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Pathogen; Mercenaria; Importation; Transmission; Stress; Mollusc

1. Introduction

Hard clams, Mercenaria mercenaria, are native to the east coast of North America

from the Maritime Provinces of Canada to Florida, USA. For about the past 25 years, a

hatchery-based culture industry for this species has been responsible for an increasing

component of the overall production. The United States Department of Agriculture

(USDA, 2000) reported that clam aquaculture (mostly hard clam) on the US Atlantic

coast had a value of nearly US$38 million in 1998, the last year for which data are

available. Although difficult to assess from available statistics, the best estimate is that

aquaculture is responsible for 30% to 50% of the value of total hard clam landings.

Disease-caused losses have been rare in hard clam culture (Ford, 2001), but beginning

in 1989, an infectious agent, labeled QPX for Quahog Parasite Unknown, was associated

with high mortality of clams held in a hatchery in Prince Edward Island (PEI), Canada

(Whyte et al., 1994). Descriptions of protistan parasites associated with two earlier

mortalities, in PEI, Canada in the early 1960s and in Barnegat Bay, New Jersey, USA, in

the 1970s, indicated that the same, or similar, organisms were involved (Drinnan and

Henderson, 1963; unpublished report of the Haskin Shellfish Laboratory, 1977).

Subsequently, QPX or QPX-like organisms were found in Massachusetts and Virginia

(Ragone Calvo et al., 1998; Smolowitz et al., 1998) as well as in additional locations in

the Canadian maritime provinces (MacCallum and McGladdery, 2000). Sequence

analysis of the ssRNA gene of QPX places the organism in the phylum Labyrinthulo-

mycota (Maas et al., 1999), a group that is ubiquitous in marine and estuarine environ-

ments, but which has been associated with diseases in molluscs held or grown in captivity

(Polglase, 1980; McLean and Porter, 1982; Jones and O’Dor, 1983; Bower, 1987).

QPX appears to be most prevalent in cultured clams, in clams being held in the hatchery

or nursery, or occasionally in densely set natural populations. Anecdotal evidence that

imported seed was more susceptible than locally produced clams raised early questions as to

whether the parasite might be present in hatchery/nursery reared seed clams that are often

produced in one state and shipped to another. A 2-year survey of seed from hatcheries in six

states and in cultured clams during their first year of growout, examined by pathologists in

three laboratories, failed to show QPX or QPX-like organisms in standard tissue sections

(Ford et al., 1997). Although these results indicated that hatcheries are an unlikely source for

QPX, the persistent link with cultured clams indicated that culture practices might increase

the susceptibility of clams to an opportunistic or facultative parasite. To help answer this

question, we surveyed cultured and wild clams in three northeastern states and investigated

two hypotheses: (1) that increased density would accelerate the development of QPX

infections and (2) that seed originating from nonlocal (i.e. another state) sources would be

more susceptible than seed from local sources.

S.E. Ford et al. / Aquaculture 208 (2002) 23–3824

2. Materials and methods

2.1. Density experiment

Histological examination of clams (N = 20) being grown in a tidal marsh, tributary

creek of Great Bay, New Jersey in March 1997 showed a single clam with a light,

localized QPX infection. The stock, which was purchased at 1 mm from a hatchery in

South Carolina in 1995, had experienced an estimated 30% mortality in February 1997.

This group was selected for the density experiment to maximize the probability of QPX

infection pressure in the test. The clams, which were approximately 2 years old with a

mean (F S.E.M.) shell length of 32.0F 0.86 mm, were recovered and redeployed on July

25, 1997, at three densities (215, 430, and 860 clams m � 2) in a 3� 3 Latin Square

design. The densities are referred to as 1� , 2� and 4� , respectively. The lowest

figure approximates commercial density for this size clam. The Latin Square design was

implemented to account for possible inshore–offshore/upstream–downstream differen-

ces. Each of the nine plots was 1 m2 and delineated by PVC pipe. The plots were

separated from each other by a 30-cm-wide corridor and were covered with protective

netting. The entire Latin Square was replicated in two adjacent beds in the creek, which

was about 40 m wide at that location. One bed was intertidal in a sand substrate. The

second, just offshore, was subtidal in mud substrate. The former was considered a more

stressful environment for the clams because of the wider variation in temperature.

Temperature was recorded at the intertidal bed using a continuous temperature logger.

We did not measure salinity, but the normal range is between 20 and 30 ppt with most

readings being near 25 ppt (G. Mathis, personal records).

An initial sample of 50 clams haphazardly selected from those to be deployed was set

aside for size and condition index measurements, as described below; another 30 were

processed for histopathology. This sample represented Time 0 (July 25) for all exper-

imental groups (Table 1). On October 3, 1997, the plots were sampled for the first time.

Clams were collected by hand until at least nine live individuals had been obtained from

each plot. These and all dead clams encountered in the same grabs were returned to the

laboratory for size measurement and histopathology. The same procedure was followed in

the subtidal bed at a second sampling on November 29, 1997; however, all clams, dead and

alive, were removed from the intertidal bed at this time. The intertidal portion of the

experiment was terminated because intertidal clams do not always survive the winter at this

location. The subtidal bed was retained and sampled for the last time on May 20, 1998,

when all live and dead clams were removed. Final mortality was determined from live and

dead counts, adjusted for the interim samples removed, at the final sample dates when all

clams were removed from the beds. Interim mortalities were estimated from live and dead

counts of the samples removed for histology, but were not used in statistical analyses.

At each sampling, the shell length (longest dimension) of all live clams was measured

to the nearest 0.1 mm. The valves of dead clams were measured in October and

November. Nine individuals from each plot (27 per density per bed) were shucked,

examined visually for nodules or mantle-edge swelling (which can be associated with

QPX (Smolowitz et al., 1998)) and fixed in Davidson’s fixative. The tissues of dead and

dying clams (gapers) were also fixed. Additional measurements were obtained from 25

S.E. Ford et al. / Aquaculture 208 (2002) 23–38 25

live clams from each plot (75 per density per bed), where available, at the final sample

dates. Because of high mortality levels, sufficient clams were not available for both

condition index and histopathology in all plots in the May 1998 sample. Preference was

given to providing histological samples, but least five individuals were available for the

Table 1

Growth (meanF S.E.M.) and mortality statistics of clams in the density experiment

Mortalities are based on pooled live and dead counts for three plots in each density.

T0 (July 25, 1997) T1 (October 3, 1997) T2 (November 29, 1997) T3 (May 20, 1998)

Shell length (mm)

32.0 (0.86), N= 50

Intertidal

215 m� 2 38.3 (0.76), N= 28 38.2 (0.52), N = 75

430 m� 2 37.4 (0.85), N= 27 38.1 (0.51), N = 75

860 m� 2 37.9 (0.62), N= 28 38.7 (0.58), N = 75

Subtidal

215 m� 2 37.2 (0.74), N= 27 37.8 (0.91), N = 31 40.2 (0.84), N= 30

430 m� 2 38.2 (0.94), N= 28 38.0 (0.98), N = 29 44.1 (0.66), N= 45

860 m� 2 35.1 (0.62), N= 27 37.7 (0.82), N = 29 40.1 (0.69), N= 31

Dry meat weight (g)

0.323 (0.018), N= 50

Intertidal

215 m� 2 0.486 (0.019), N= 75

430 m� 2 0.508 (0.018), N= 75

860 m� 2 0.524 (0.019), N= 75

Subtidal

215 m� 2 0.408 (0.027), N= 30

430 m� 2 0.557 (0.027), N= 45

860 m� 2 0.405 (0.021), N= 31

Condition index

0.991 (0.045), N= 50

Intertidal

215 m� 2 1.244 (0.036), N= 75

430 m� 2 1.310 (0.034), N= 75

860 m� 2 1.354 (0.051), N= 75

Subtidal

215 m� 2 0.990 (0.049), N= 30

430 m� 2 1.239 (0.045), N= 45

860 m� 2 0.995 (0.036), N= 31

Percent mortality

Intertidal

215 m� 2 8.9 21.3

430 m� 2 13.6 28.5

860 m� 2 12.2 22.9

Subtidal

215 m� 2 17.7 11.4 87.8

430 m� 2 24.7 22.8 94.0

860 m� 2 20.2 21.4 93.6


additional assays from all plots, but one low-density plot. To facilitate analysis of the

Latin Square design, a mean from the other two low density plots was substituted for

length, dry meat weight, and CI for this plot. Clams for condition index determination

were measured for shell height, length, and width, then shucked. The meats were dried at

50 jC for at least a week, then weighed to the nearest 0.0001 g. A condition index was

calculated as the ratio of dry meat weight to shell length� 100.

2.2. Geographical distribution survey

From November 1996 to June 1999, hard clam samples (N = 18 to 50 individuals per

sample) were obtained from growers in New Jersey, New York, and Connecticut. Both wild

and cultured clams were examined in order to extend the known distribution of QPX

infections in the northeastern United States The clams were processed for histological

examination as described below.

2.3. Seed origin study

Five of the New Jersey samples collected for the geographical survey consisted of

cultured clams, produced in a South Carolina hatchery. They were being grown adjacent to

cultured clams from New Jersey hatcheries in several locations, and were reported by many

growers to be suffering high mortalities. The clams were processed for histological

examination as described below. In three of these cases, the grower collected all live and

dead clams from a plot, which allowed us to estimate mortality.

2.4. Histopathology

A 5-mm-thick transverse section, taken from the hinge to the ventral margin of each fixed

clam, was embedded in paraffin, processed into tissue sections, and stained using a

modification of the Masson’s trichrome stain (Humason, 1979). Each section was com-

pletely scanned for the presence of QPX and for other parasites and pathological conditions.

When present, QPX was identified as being in one of three body compartments: gill, mantle,

and visceral mass (digestive gland, intestinal tract, kidney, heart, foot, and gonad.) The

extent of infection was classified within each compartment as focal, multifocal, or diffuse,

and rated as 1, 2, or 3, respectively. The severity of the lesion was classified according to its

diameter as mild—less than 250 Am, moderate—between 250 and 700 Am, or severe—

greater than 700 Am, or diffuse or focally extensive, and scored as 1, 2, or 3, respectively. For

each tissue, an intensity score was computed as the product of the extent and the severity

ratings, and these were summed to calculate an overall score for the clam. The maximum

possible score for an individual was 27. A sample QPX intensity was calculated as the mean

score for all individuals, including those with no detectable infections.

2.5. Statistical analysis

The data were analyzed for each sampling date using a General Linear Model ANOVA

that separated the experimental design into the components time, row, column, density,


and bed location. Analysis was conducted on length, dry meat weight, condition index,

prevalence, sample intensity, and mortality. Percent mortality and prevalence were arcsine

transformed before analysis. Differences were considered significant at a = 0.05. In only

one case (CI for row only) was there any row or column effect or interaction between rows

and columns, indicating that position within the bed was not important. Means of

significant ANOVA effects were compared by the Tukey or LSD tests. A regression

analysis was also performed to identify a possible trend between increased density and

QPX levels.

3. Results

3.1. Density experiment

3.1.1. Temperature

Temperature at the late July deployment was about 25 jC. Over the next month, the

mean remained about 25 jC, but with spikes up to 28–30 jC during daily low tide and

lows of 22–24 jC at high tide. Although we did not deploy a temperature logger in the

subtidal bed, temperatures near the surface of the intertidal site during low tide in mid day

are several degree warmer than the water. In early September, temperatures began

gradually decreasing to between 5 and 6 jC when the late November sample was

collected. The recorder was lost over the winter, but temperatures at the intertidal site

typically approach or reach 0 jC for a few weeks in mid winter, then begin to rise in early

February and reach 20–22 jC by late May, when the final sample was taken.

3.1.2. Growth and condition index

Mean shell length increased from 32 mm at the end of July to 38 mm at the first

sampling in early October, then changed little through November (Table 1). By May, it had

increased to 42 mm, suggesting either spring growth or a differential mortality of small

individuals over winter (see mortality below). There were no effects of bed location or

density at any date. Dead clams were significantly smaller than live clams in the October

and November samples (mean shell length for pooled dates = 32.5F 3.1 vs. 37.5F 4.5

mm, p < 0.0001). Dry meat weight increased over time from a mean of 0.32 g at planting

to 0.51 g in November (intertidal bed) and 0.48 g in May (subtidal bed) (Table 1), but was

not affected by location or density.

Mean condition index at all densities increased significantly between the initial, July

sample (0.991) and the final sample of the intertidal bed in late November (1.302), but the

May subtidal sample (1.075) was the same as the July sample (Table 1). Condition index

was not affected by density or bed location in any sample.

3.1.3. Mortality

Between deployment and the November terminal sample of the intertidal bed, 21% to

29% of the clams died, and the interim grab sample of the subtidal bed indicated similar

values in the subtidal bed (Table 1). The presence of numerous gapers in samples from

both beds indicated an ongoing mortality, which continued in the remaining subtidal bed


and elevated the total mortality to 88% to 94% by May. There was no significant effect of

density on final mortality in either bed.

3.1.4. Histopathology

Forty-four percent of the initial sample, taken on July 25, had detectable QPX

infections. The mean prevalence remained essentially unchanged in the October and

November samples, then fell in May, when only the subtidal bed was sampled (Fig. 1).

Fig. 1. Mean prevalence of QPX in hard clams planted at three densities in the experimental subtidal and intertidal

beds, and sampled during the autumn of 1997 and spring of 1998. Horizontal line represents the mean prevalence

when the study began on July 25, 1997. N= 27 for each sample.


There was no statistical effect of density or bed location in the autumn 1997 samples, even

though there was a trend toward higher prevalence with increasing density in the intertidal

bed (Fig. 1). A similar pattern was present in May 1998 sample of the subtidal bed, in

which prevalence in the 1� and 2� clams was markedly lower than it had been the

previous autumn, whereas that in the 4� clams was similar to the autumn level, and

significantly ( p = 0.017) higher than in the 1� and 2� plots.

In both beds and at all densities, sample intensity increased several fold during the late

summer and autumn, but showed no bed-location effect. It then decreased in May in the

subtidal bed (Fig. 2). Density had no effect in October or November, but was onlymarginally

insignificant ( p = 0.076) in the November intertidal bed. In spite of the marginally

insignificant ANOVA, the LSD (T) analysis of the sample means suggested an intensity

difference between the 4� (4.6) and 1� (1.8) density plots, but both were statistically the

same as the 2� density (2.6). At the final sample in May, the density effect mirrored that of

the November intertidal sample except that it was significant ( p = 0.048) in the ANOVA.

The LSD (T) test showed that the 4� clams had significantly higher infection levels than

did the 1� and 2� clams, whichwere the same (Fig. 2).We used linear regression analysis

to further examine the apparent trends between density and QPX sample intensity in the

November and May samples. The regression was significant for the November intertidal

( p = 0.018, r2 = 0.574) and the May subtidal ( p = 0.012, r2 = 0.619) samples, but not for the

November subtidal bed.

The appearance of QPX parasites and the lesions they caused in clam tissues resembled

that described earlier, including an intense host response (Ragone Calvo et al., 1998;

Smolowitz et al., 1998). Like Ragone Calvo et al. (1998) in Virginia, we did not see the

nodules or mantle swelling reported by Smolowitz et al. (1998) in Massachusetts. The

mantle was the tissue most often infected, although prevalences in the gill were equally high

in July 1997 and May 1998 samples (Fig. 3A). The visceral mass was the least parasitized.

Among infected clams, mantle lesions became markedly more extensive and severe during

the summer and fall of 1997 (Fig. 3B). A much less pronounced trend was observed in the

gill and visceral mass. By May, lesion severity had decreased in the mantle, remained

unchanged in the visceral mass, and increased somewhat in the gills, although the variance

about the mean for the gills was extremely high. Moribund QPX were noted in all except the

July sample, but predominated in October.

Fifteen gapers were recovered at the November sampling, seven from the subtidal bed

and eight from the intertidal bed. Diagnosable QPX was found in 86% of the former and in

all of the latter. Among the infected gapers, the mantle was the most frequently infected

tissue (86–88%), followed by the gills (36%) and the visceral mass (13% to 38%). The

sample intensity was 10.8 in the intertidal gapers and 6.4 in the subtidal group. These may be

underestimates because tissues were very poor in some animals and only those with clearly

diagnosable infections were scored.

3.2. Geographical distribution survey

During the study period, no QPX was found in the samples of cultured clams

collected at sites on the north and south shores of Long Island, New York (Table 2).

A single clam with a light, focal infection was found in a population of wild clams on


the Connecticut shore of Long Island Sound. The results of New Jersey clam analyses

are presented below.

3.3. Seed origin study

Of the 15 samples collected in New Jersey, all of which were cultured clams, 10 were

produced in New Jersey hatcheries and the remainder came from a hatchery in South

Fig. 2. MeanF S.E.M. sample intensity of QPX in hard clams planted at three densities in the experimental

subtidal and intertidal beds, and sampled during the autumn of 1997 and spring of 1998. Horizontal line

represents the mean prevalence when the study began on July 25, 1997. N = 27 for each sample.


Carolina. QPX was found in all five South Carolina samples with prevalences and sample

intensities ranging from 34% to 84% and 1.0 to 4.8, respectively (Table 2). The South

Carolina clams included one group that was only a year old, but already had a prevalence

of 40%. No New Jersey clams from that location were obtained, but adjacent New Jersey

and South Carolina groups from two other sites were examined. In one comparison

(Tuckerton, September 1997), the South Carolina group had a prevalence of 84% with a

sample intensity of 4.8. New Jersey clams of the same age and resident on a ground < 10

m away for the same length of time, had a prevalence of only 8% and sample intensity of

0.1. The second site, Dry Bay, contains extensive beds of cultured clams that are leased

and tended by most of the clam growers in southern New Jersey and contain clams of a

variety of stocks and ages at any given time. A sample of 3-year-old South Carolina clams

from Dry Bay was examined in June 1998. QPX was detected in 34% and the sample

intensity was 1.0 (Table 2). Seven samples of 2- and 3-year-old New Jersey clams from

Dry Bay were examined on five surrounding dates (Table 2). QPX was detected in a single

sample, collected in January 1998, in which 17% of the clams had very light infections

Fig. 3. Changes in mean QPX prevalence (A) and lesion intensity (B) in tissues compartments of clams sampled

during the study period. Error bars indicate standard error of the mean lesion intensity. N = 27 for each sample.


(sample intensity = 0.2). From counts of live and dead clams submitted by growers in June

and July 1998, we estimated mortality to range from 26% to 92% in the South Carolina

groups. Based on the density experiment results, we conclude that this mortality had

occurred since the previous autumn. No unusual mortalities were reported in New Jersey

clams at any location.

4. Discussion

Density has been proposed as a possible factor in QPX infections because of the strong

association between QPX and cultured clams, which are likely to be maintained at higher

densities than they are in nature (Whyte et al., 1994; Ragone Calvo et al., 1998; Smolowitz

et al., 1998; MacCallum and McGladdery, 2000). Density could increase infections in two

ways: by increasing transmission rates or by posing a stress to clams that would increase

susceptibility (Brown, 1987; Hofmann et al., 1995; Peterson, 2001). At the final samplings

of both intertidal and subtidal beds, we found a significant positive correlation between

Table 2

QPX levels in hard clams collected from three states during the study period

In New Jersey, the clam seed was originally from New Jersey and South Carolina hatcheries. In addition to these

South Carolina groups, the clams in the density experiment were from the South Carolina hatchery.

Date State Location Hatchery

location

Year

class

Sample

size

Percent

prevalence

Sample

intensity

5/8/96 NJ Dry Bay New Jersey f 1993 50 0.0 0.00

11/21/96 NJ Dry Bay New Jersey 1995 18 0.0 0.00




3/27/97 NJ Tuckerton New Jersey f 1994 50 0.0 0.00

9/29/97 NJ Tuckerton New Jersey 1995 24 8.3 0.08




9/29/97 NJ Tuckerton South Carolina 1995 25 84.0 4.80

6/8/98 NJ Dry Bay South Carolina 1995 35 34.3 0.97

6/19/98 NJ Steelmans Bay South Carolina 1996 35 65.7 0.89

7/16/98 NJ Risley Channel South Carolina 1996 26 57.7 3.19

7/16/98 NJ Risley Channel South Carolina 1997 25 40.0 2.24

1/28/97 NY Great South Bay Wild clams mixed 50 0.0 0.00

12/2/97 NY Great South Bay New York 1993–1996 35 0.0 0.00

5/26/98 NY Great South Bay New York 1993–1996 35 0.0 0.00

11/11/97 NY Oyster Bay New York 1995 34 0.0 0.00




11/11/97 CT Long Island Sound Wild clams mixed 35 0.0 0.00





density and sample intensity. It is interesting that this trend developed first in the intertidal

plot, which was considered to be more stressful to the clams. The clams were already

infected at the start of the study and the correlation was due primarily to the development

of heavier infections, which might be associated with stress, rather than to a greater

number of new infections, which would be associated with transmission. This additional

stress has not been quantified, but intertidal clams are exposed longer at low tide, have less

time to feed, and experience more extreme temperature fluctuations. For instance, hard

clams pumping rate, oxygen consumption and growth rate peak at about 25 jC (Grizzle et

al., 2001) and summer temperatures at the intertidal site often exceeded this value by up to

5 jC. On the other hand, we found no evidence that increased density affected any other

measured parameter, as has been the case with most other hard clam studies (Fegley,

2001). Although our data did not always show statistical associations between QPX levels

and density in the ANOVAs, the statistically significant correlations that we report are

consistent with an hypothesis that a combination of stress (high temperature), or more

effective transmission (high density), or both favored the development of more intense

infections.

Smolowitz et al. (1998) reported that QPX-infected clams grew more slowly and had a

lower condition index than uninfected animals and, in our study, dead and dying clams,

which had very heavy QPX infections, were significantly smaller than living ones. We

were not, however, able to demonstrate a correlation of sample condition index with either

QPX or density. Condition index is often used as an index to the metabolic condition of a

bivalve and is related to the quantity of glycogen stored or the amount of gametes

produced (Gabbott and Stephenson, 1974; Mann, 1978; Soniat and Ray, 1985). The

decline in condition index of clams between November and May could have been

influenced by QPX, but normal seasonal variation, including spawning in the late spring

and early summer, could also have played a role (Peterson and Fegley, 1986), especially

since intense QPX infections in the fall did not diminish this measure. Although we did not

quantify gonadal condition, most clams contained moderate to abundant amounts of eggs

or sperm in November, whereas about half were spawned out in May. Analysis of infection

intensity and condition index on the same individuals is perhaps the most convincing way

to demonstrate the effect of disease on condition of molluscs (Barber et al., 1988; Rogan et

al., 1991). Even though we could not show that QPX infection influenced metabolic

condition, the association of high infection levels with ongoing and subsequent mortality

and the fact that dead and dying clams had much more advanced infections than live clams

collected at the same date, and the fact that no other pathogen was prevalent in the clams,

supports the conclusion that QPX was the immediate cause of death of infected clams.

High infection prevalence and intensity in the mantle and gills (Ragone Calvo et al.,

1998; Smolowitz et al., 1998; MacCallum and McGladdery, 2000; this study) suggests that

they are the portals of entry for QPX. In fact, members of the phylum Labyrinthulomycota,

including QPX-like organisms, commonly inhabit the pallial fluid of bivalves, which bathe

these organs (Perkins, 1973; Bacon et al., 1999). The massive hemocyte accumulations

around QPX and the failure of parasites to become dispersed throughout the tissues over

time suggests that the hemocyte response may limit the spread of QPX within the clam

(Smolowitz et al., 1998), even though it may not be successful in preventing death. Even

in dead clams, infections predominated in the mantle, indicating that relatively localized,


albeit severe, lesions became lethal. This pattern contrasts with that of other molluscan

microparasites, which are spread via the circulatory system and become fairly evenly

distributed throughout the tissues as infections intensify and become lethal (Balouet et al.,

1983; Hine, 1991; Ford and Tripp, 1996; Blackbourne et al., 1998).

Our most significant finding was that clams purchased as seed from a South Carolina

hatchery and planted in New Jersey consistently had high QPX prevalences and

intensities, and experienced heavy mortalities, whereas groups from New Jersey hatch-

eries, which were the same age and had been planted in adjacent plots for the same length

of time, had few or no infections and little reported mortality. The broodstock used by New

Jersey hatcheries has been a mixture of stocks derived from hatcheries in Maine,

Massachusetts, and Virginia that have been backcrossed with native New Jersey stocks.

That used by the South Carolina hatchery was a mixture of stocks imported from several

other states, including Massachusetts, Virginia, and South Carolina. Thus, neither the New

Jersey nor the South Carolina seed was obtained solely from local broodstock and it is

impossible to definitely state what percentage of the mixture was local. In fact, both

groups were derived from broodstocks that ultimately came from many of the same

hatcheries. Although we cannot discount the possibility that QPX is imported in hatchery

produced seed, we believe the weight of evidence is against it (Ford et al., 1997). QPX

appears to be widespread in clams, including some wild stocks, in high salinity bays from

Virginia to the Maritime provinces of Canada (Ragone Calvo et al., 1998; Smolowitz et al.,

1998; MacCallum and McGladdery, 2000), although generally at low prevalence and

without causing noticeable mortality. To date, outbreaks of disease and mortality

associated with QPX have been localized and we found little or no detectable QPX in

most of the samples from New York or Connecticut. In New Jersey, QPX outbreaks were

clearly associated with out-of-state seed. Whether the problem is genetic or one of poor

acclimation, or some combination of the two, is unknown. The fact that QPX typically

does not appear in clams until they have been in the field for at least 1 year suggests that

acclimation is not the problem (Ford et al., 1997; Smolowitz et al., 1998). On the other

hand, the contribution of the same parental stocks to both the New Jersey and South

Carolina clams does not favor a strictly genetic explanation. Currently, we are participating

in a study in which clams from five geographic locations along the eastern United States

(Massachusetts to Florida), produced in the same hatchery, are being evaluated for growth,

survival, and QPX susceptibility at three sites (Massachusetts, New Jersey, and Virginia).

The result of this study should help answer these questions.

A recent QPX outbreak, apparently localized in clams from Florida being grown in

Virginia, adds to the evidence that imported clams can be highly susceptible to QPX (L.

Ragone-Calvo, Virginia Institute of Marine Science, personal communication, August

2001). Whatever the reason, these results illustrate the potential problems of using

nonlocally produced stocks, even when they appear to offer economic advantages. For

instance, New Jersey growers began buying South Carolina seed as early as 1993, but

large quantities were not purchased until 1995. They were bought because they were cheap

and could be obtained earlier in the year than could local seed. Since the 1996–1998 QPX

epizootics in South Carolina clams, however, New Jersey growers no longer purchase seed

from this source. On the other hand, hard clam seed, produced in New Jersey or in

southern hatcheries, are being moved to and from New Jersey and southern locations, as


they have been for at least 20 years. These seed are presumably from stocks that have good

survival characteristics in New Jersey. The advantage to using the southern locations is

that seed grown or over-wintered there is larger when it is returned to New Jersey for

planting in the spring, which reduces loss to predation and other sources of mortality that

are size related (Kraeuter, 2001) and shortens time to market. Notwithstanding this

practice, no mortalities of the scale associated with the 1996–1998 QPX outbreak in

the South Carolina clams have been reported before or since.

High QPX levels were found in 3-year classes of South Carolina clams from four sites in

New Jersey. In at least two of these sites, New Jersey clams were planted within 10–20 m of

these heavily infected clams, yet acquired few or no detectable infections themselves. A

large number of growers experienced heavy mortality of South Carolina clams. Because

lease sites are restricted in New Jersey, they presumably had New Jersey clams planted

nearby and as far as we can ascertain, the latter did not experience excessive mortality. Our

data are consistent with findings in other areas that QPX is widespread in clam growing

waters, but causes disease and mortality only in certain groups that may be disadvantaged in

some way, perhaps including unfavorable genotype–environment interactions. The fact that

no mortalities have been reported in New Jersey since the purchase of South Carolina seed

was curtailed indicates that the problem was localized to these clams and has neither spread

nor persisted to a noticeable extent.

Acknowledgements

We are particularly indebted to those individuals who provided samples of their clams

for analysis. Without their cooperation, much of the ancillary data we are reporting would

not have been available and the picture of the QPX outbreak would have been less clear.

We also thank Dr. Roxanna Smolowitz for valuable insights into QPX dynamics and for

comments on the manuscript. This study was funded by Grant No: 96-38500-3032 from

the Northeastern Regional Aquaculture Center to Roxanna Smolowitz and SEF. This is

Contribution No. 2001-15 from the Institute of Marine Sciences, Rutgers University and

NJAES Publication No. 32405-3-01, supported by state funds.

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Aquaculture-associated factors in QPX disease of hard clams: density and seed source

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