Development of Resistance toMinchinia nelsoni (MSX) Mortality in Laboratory-Reared and

Native Oyster Stocks in Delaware Bay

HAROLD H. HASKIN and SUSAN E. FORD

DELAWARE BAYARNOLDS

BED

Figure I.-Delaware Bay showing locations of ew Jersey's natural seedoyster beds and planted (leased) grounds.

Canada, in 1915. During the epizootic,more than 90 percent of the oysters inMalpeque Bay were killed (Needler andLogie, 1947). Al though an etiologicalagent has never been positively iden-

in both laboratory-reared and nativeoyster populations in Delaware Bay.

The Delaware Bay mortalities werereminiscent of those occurring in Mal­peque Bay, Prince Edward Island,

Harold H. Haskin and Susan E. Ford are with theOyster Research Laboratory, ew Jersey Ag­ricultural Experiment Station, P.O. Box 1059,Piscataway, NJ 08854 and the Department ofZoology, Rutgers College, Rutgers-The StateUniversity, New Brunswick, N.J.

Introduction

In the spring of 1957, devastatingmortalities of oysters occurred in lowerDelaware Bay. In 1958 and 1959, themortalities were repeated, but this timeover a much wider area of the estuary.During this period, cumulative killreached 90-95 percent in the high­salinity planting areas and 50-70 per­cent on seed beds in lower-salinity re­gions (Fig. I). Only in very low­salinity areas, such as the tidal riversand creeks and on the uppermost seedbeds, did oysters escape the mortality(Haskin et aI., 1965). In 1959, simi­larly destructive kills occurred in lowerChesapeake Bay (Andrews and Wood,1967). The disease-causing organism,a haplosporidan parasite, was identifiedand named Minchinia neLsoni (morecommonly known as MSX) by Haskinet al. (1966).

Since the original outbreak of M.neLsoni, the Oyster Research Labora­tory at Rutgers University has pursuedmany lines of investigation into the na­ture of the MSX problem. All attemptsto transmit the disease under controlledlaboratory conditions have failed, sothese studies have been primarily fieldinvestigations concerned with variousepizootiological aspects of the disease.This paper describes one part of theoverall investigation: The developmentof resistance to MSX-caused mortality

54 Marine Fisheries Review

tified, a highly contagious pathogen issuspected (Frazer, 1937-38). Needlerand Logie (1947), studying Malpequedisease in the 1930's, reported the first,and to date only, instance of a popula­tion of marine invertebrates acquiringresistance to a disease. Fifteen yearsafter the initial kill, these investigatorsfound that native Malpeque Bay oys­ters, offspring of survivors of the 1915epizootic, were surviving normally; incontrast, oysters imported into the bayfrom areas which had not experiencedkill suffered heavy mortalities.

In a pattern similar to the Malpequeexperience, and during the years im­mediately following the Delaware Bayepizootic, several lines of evidencesuggested that our native oysters weredeveloping resistance to MSX: I) From1958 through 1960, successive yearclasses of nati ve spat on the flats in frontof our lower Delaware Bay Cape ShoreLaboratory, all subjected to heavy dis­ease pressure, showed increasing sur­vival with each year class. During itsfirst full year of exposure 1957 set had84 percent kill, 1958 set had 48 percentkill, and 1959 set had 29 percent kill(Haskin!); 2) Beginning in 1961, seedoysters transplanted onto the leasedgrounds showed improved survival; 3)In addition, native Delaware Baystocks from seed beds, plantedgrounds, and Cape Shore, when testedfor survival at Cape Shore, had consid­erably lower mortalities than did oys­ters imported from MSX-free east coastlocations (Haskin 2).

In later histopathological studies,lower mortalities were correlated withlighter, more local ized infections(Myhre and Haskin, 1969; Ford, 1970),suggesting that a defense mechanisminvolved the containment of the para­site in small, local, nonlethal lesions.

Lowered mortalities, nonetheless,were the earliest and certainly the mostsubstantial evidence that Delaware Bay

I Haskin, H. H. 1961. Delaware Bay oyster mor­tality project. Unpublished report to U.S. Fishand Wildlife Service for period 1 Jan. - 30 June1961.

'Haskin, H. H. 1960. Delaware Bay oyster mor­tality project. Unpublished report to U.S. Fishand Wildlife Service for period 1 July 1959 - 30June 1960.

January-February 1979

oysters had developed resistance toMSX. At this point, however, in theearly 1960's, we were not able to dem­onstrate that lowered kill in nativestocks was due to an inherited traitrather than to selection within the gen­eration of oysters under test. Also, wehad no means of sorting out the influ­ence of fluctuating disease acti vity onmortality rates from that due to selec­tion. To resolve these questions webegan laboratory rearing of stocks ofoysters with various histories of expo­sure to MSX. The test of their offspringfor resistance to MSX began with firstexposure to the disease. Our first oys­ters were spawned in 1961, and since1964 numerous stocks, with knownparentage and precise selection history,have been reared and tested on a routinebasis each year.

A second method of investigatingdevelopment of resistance to MSX hasbeen to examine data from the large­scale monitoring of many groups of na­tive oysters throughout Delaware Bayfor trends in mortality and infectionlevels over the course of the past 20years.

In this paper we present evidence thatresistance to MSX-caused mortality isheritable; that it can be expressed to ahigh degree in laboratory-reared oys­ters with rigorously selected parents;and that it has developed in native popu­lations under natural selection in Dela­ware Bay, although to a lesser measurethan is possible using the laboratoryspawning and experimental selectionprocedures.

Methods

Laboratory-Reared Stocks

Oysters to be spawned, other thanimported susceptible stocks, are par­tially conditioned in trays on the tidalflats at our Cape Shore Laboratory.Groups of about 30 oysters are finallyconditioned within the laboratory.Spawnings involved groups of malesand females, usually from 4 to 10 each.Larvae are reared using a modified Mil­ford technique (Hidu et aI., 1969) andare fed daily, with each water change,on natural phytoplankton in DelawareBay water. All bay water used in the

larval culture is passed through 60jLmplankton netting to prevent contamina­tion by wild larvae. Spat (recently setoysters) are placed in trays and held inCape May harbor, usually until Octoberwhen the setting season for native oys­ters in Delaware Bay has passed. Thenfor each stock, duplicate trays are setup, each containing from 2,000 to5,000 spat. Between March andNovember these trays are held on theflats in front of the laboratory where,being exposed on most low tides, theycan be examined frequently. Duringperiods of heavy mortality, this may beon a daily basis. In November, trays aremoved from the flats to deeper water toprevent possible ice damage. Theyhave generally been stored in a tidalcreek behind the laboratory or, morerecently, in Cape May harbor. Preda­tion is normally quite low because of: I)Frequent handling of the oysters; 2) lowtide exposure; 3) the fact that the traysare raised several inches above the bot­tom on drainage tiles. Obvious predatormortality is tallied and subsequentlyexcluded from disease mortality calcu­lations.

Mortality counts are cumulated sea­sonally and annually and groups withan ancestral history of selection byMSX (called resistant or selectedstocks) are compared with each otherand with offspring of oysters with nohistory of selection by MSX (calledsusceptible or unselected stocks). Mor­talities in paired trays of each stockrarely differ by more than a few per­centage points and when a large differ­ence does appear, it can usually be re­lated to some non-MSX stress, such asmudding. In these cases, mortalities arenot used in the calculations.

In assessing survival rates for labora­tory-reared oysters, a standard expo­sure period was established at the startof the larval rearing program. This in­terval, which begins in October of theirfirst year and ends in June as they com­plete their third year, spans a 33-monthperiod. It was chosen because mortalityrates in early studies declined sharplyafter the second summer-fall exposureperiod, indicating that most suscepti­bles had been weeded out and also thatoysters living on the Cape Shore flats

55

for this length of time begin to experi­ence kill caused by another oyster para­site, Labyrinthomyxa marina, whichgreatly complicates interpretation ofthe mortalities.

From 1964 through 1977, a total of3 I resistant stocks were spawned andtheir offspring were subjected to thecomplete test exposure. Ten of thesewere first generation resistants, 14 weresecond, 5 were third, and 2 were fourthgeneration resistant groups. Oysters arenot used as spawners unless they havesurvived the standard 33-month expo­sure to MSX. There is, therefore, aminimum of 3 years of selection forMSX resistance between generations.This means, for instance, that the totalexposure time to intense MSX activityand selective mortality of the ancestorsof the F4 generation is at least 12 years.In addition to breeding oysters selectedfor resistance to MSX mortality, wehave, each year, spawned unselectedoysters imported from various locationsalong the east coast where there is littleor no MSX activity. To date, 24 suchgroups have been tested. In most years,susceptible imports have come fromLong Island Sound, the Navesink Riverin New Jersey, and the James River inVirginia. Mortalities in these stocksserve as controls, providing a base lineagainst which survival of selectedgroups can be judged.

Monitoring Program

In New Jersey, the Delaware Bayoyster industry transplants oysters frompublic, upper bay, natural setting areas(seed beds) to individually leased grow­ing grounds in the lower bay (Fig. I).Shortly after the first epizootic, a pro­gram was established to monitornumerous bed populations throughoutthe bay for MSX prevalence and formortality. Now in its 20th year, thismonitoring program has provided de­tailed statistics from 5 major seed beds,and for 78 different plantings of oysterson the leased grounds. Each lower bayplanting has been sampled on a regularbasis for 1-5 years. On the selected seedbeds, sampling has been continuous forup to 20 years. Until 1971, sampleswere collected on a monthly basis,weather permitting. Since then, a

56

schedule of 7-8 sampl ing periods peryear, designed to coincide with criticalphases of the MSX cycle, has been ineffect.

At each station a I-bushel sample ofoysters, gapers3

, and boxes4, is col­lected with a 30-inch oyster dredge. A"recent" mortality count, based on thenumber of gapers and new boxes (thosewith little or no fouling) is made. Theinterval during which this mortality hasoccurred is estimated using knowledgeof recent fouling rates in the bay. Thismortality interval may be as short as 2weeks during the summer or as long as3 months in the winter. Interval mor­talities are then cumulated to provideseasonal and annual totals. Mortalitydue to predation by oyster drills andmud crabs, dredge damage, mudding,etc. is distinguished from disease killwhich includes that associated withMSX. All mortalities considered in thisdiscussion, except where specified,refer to the second category only. Bothpredation and disease mortalities arecomputed separately as a function ofthe total sample. It is likely that preda­tors kill some oysters which wouldotherwise die with MSX. Thus the dis­ease mortalities reported here are lowerthan if the oysters were protected frompredation as they are to some extent inthe trays. In nearly all cases, a 20­oyster sample has been fixed for his­tological study. Most of these havebeen worked up so that indices of infec­tion prevalence and intensity accom­pany the mortality statistics. These datawill be presented in forthcoming pa­pers.

Histological and mortality data fromboth monitoring program and experi­mental tray stocks have established thatthe major infective period for MSX inDelaware Bay is in June, with a secondperiod of variable, and generally lesser,activity in late summer and early fall(Haskin et aI., 1965; Ford, 1970). An­drews ( 1966) found a similar pattern forMSX in Virginia. Oysters first exposed

3"Gapers" are dead or dying oysters with meatstill in the shell.·"Boxes" are dead oysters which no longer con­tain meat, but with valves still attached at thehinges.

in June usually begin dying in late Julyor early August. Mortalities extend intoNovember, tapering off as water tem­peratures drop. Additional kill is re­corded in late winter and early springand is associated with cold weatherstresses as well as with MSX. A thirdmortality period occurs in June and Julyand is thought to be primarily the resultof infections acquired late the previousyear which remained subpatent over thewinter, then proliferated with warmingtemperatures (Andrews, 1966; Ford,1970).

Results

Laboratory-Reared Stocks

As of summer 1977, a total of 55 lotsof laboratory-reared oysters had beencarried through a 33-month testingperiod, and cumulative mortalities forall have been calculated. As indicatedearlier, these lots have been groupedinto susceptibles (24 lots) and first, sec­ond, and third generation resistantswith 10, 14, and 5 lots, respectively.For each of these 55 lots, cumulati vemortalities by season have been calcu­lated and then these mortalities aver­aged for each group. The results areshown in Figure 2. Also included arethe results for Cape Shore natives (nineyear classes).

The patterns of mortality are quiteclear in this summarizing figure. Newlyset spat first exposed to MSX in the fallmay acquire infections then. Some dif­ferential kill results the followingspring, but mortalities for all groups areclosely clustered through June. It is notuntil the oysters are exposed to a com­plete MSX infective period beginningin June that large mortalities are seen.By November, as death rates declinewith the onset of cold weather, a clearpattern of differential total kill has beenestablished, and this is maintained dur­ing the rest of the test period. Meanmortalities at the end of this initial killhave reached 73 percent in the suscep­tibles and 60 percent in Cape Shorenati ves. Resistant laboratory-rearedoysters have distinctly less kill, at37-42 percent, but show little differ­ence among generations.

At the end of the test period mean

Marine Fisheries Review

Cape Shore natives 9 81 2.7

First generation resistants 10 68 4.6

Second generationresistants 14 64 5.1

Third generation resistants 56 63

JMD

virtually all of the oysters susceptible toMSX kill had been weeded out of thepopulation. To explore this possibility,mean seasonal mortality rates havebeen examined (Fig. 3). While there isa clear lessening of mortality differ­ences among stocks as exposure and

S D M J

EXPOSURE PERIOD

JMD

20

90• SUSCEPTIBLES

Figure 2.-Cumulative mortality means for oyster stocks exposed to MSX inexperimental trays on the Cape Shore tidal flats. The 33-month exposure period isshown on the abscissa.

%

"

III

>0:Cl:-'::::>~40::::>u

~60-'Cl:...Ill:o~

50

After the first large selecti ve kill,cumulative mortality curves displaydistinctly similar slopes (Fig. 2). Thissuggested to us that mortality for allstocks might be approaching a commonrate. This would imply that differentialselective mortality had ceased and that

24 93Susceptibles

Oyster stock

mortality for the susceptible stocks hasclimbed to 93 percent, while that forCape Shore natives stands at 81 per­cent. Resistant groups show decreasingmortalities, from 68 percent in the firstgeneration, to 64 percent in the second,and 56 percent in the th ird (Table I).

Two fourth generation resistantstocks have also been tested, but theirfinal mortalities (38 percent and 95 per­cent) were so disparate that they werenot included in the figure. Additionalfourth generation stocks are currentlyundergoing testing and may provide thedata needed to establish more defini.temortality rates for this generation.

It is convenient to consider survivalrather than mortality as a measure ofresistance to kill in oyster populations.Using survival of susceptibles as a baseline (representing survival of un­selected oysters in epizootic situa­tions), we have calculated survivalratios for the various groups of selectedoysters. This is simply the ratio of thepercent survival of selected stocks tothe 7 percent survival of susceptibles atthe end of the test exposure (Table I).After nearly 3 years of intensive selec­tion against MSX in each generation,first, second, and third generation resis­tant stocks have4.6, 5.1, and 6.3 timesas many survivors, respectively, as dounselected groups. The first selection,that operating on susceptible stocks be­fore they produce the first generationresistants, raises the survival ratio morethan do the next two selections com­bined. Native Cape Shore oysters havenearly three times as many survivors asdo susceptible imports.

Table 1.-5urvival ratios for Cape Shore native andlaboratory~reared oyster stock after 33-months expo­sure to MSX at the Cape Shore. The ratio of survivorshas been calculated by comparing the percentage sur­vival of each selected group with the 7 percent survivalof susceptibles, both at the end of the last exposure.

Number Percent Ratioof Cumulative of

groups Mortality survivors

January-February 1979 57

% I•60 I

50

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•

II•

II•

0

II II! I01,1 ". iill!II a•

'I IMAR.-JUN. NO\{.- MAR. MAR.-JUN.

PERIOD

Figure 3.-Seasonal mortality means for oyster stocks exposed to MSX in experimental trays on the CapeShore tidal flats. Successive seasons during the 33-month exposure period are shown on the abscissa.Analysis of variance was used to estimate variability due to year of exposure and to stock differences, and thisin turn was used to construct 95 percent confidence intervals about each mean.

selection progress over the test period,there is, at the same time, a consistentpattern of highest kill in the suscepti­bles, followed by Cape Shore nati vesand then by laboratory-reared resistantsthroughout the entire 33 months. Forthe most part, the same pattern is fol­lowed among resistant groups, withmean seasonal mortalities progres­si vely decreasing with increasing gen­eration number.

Native Seed

The general mortal ity pattern forplanted oysters in lower Delaware Bayis similar to that for experimental stockstested at the Cape Shore Laboratory.Mortalities for monitored leased­ground plantings have been calculatedby season and year of exposure to MSXon the planted grounds. These have

58 •.4.

been averaged and then cumulated overa 3-year exposure period (Fig. 4). Dif­ferences from the experimental stocksat the Cape Shore Laboratory should benoted: 1) Seed oysters, which have littleor no MSX, are transplanted in lateMay and early June so their initial lowerbay exposure to the disease includes acomplete summer infective period.This contrasts with the laboratory­reared groups which have undergonesome selection from late fall infectionsbefore they experience a full June infec­tion period; 2) Seasonal mortality inter­vals for Cape Shore stocks span some­what different periods than do thosecalculated for planted oysters. How­ever, they include essentially the samecritical mortal ity periods (i. e., latesummer-fall, winter-spring, and earlysummer) so that comparisons may be

made without difficulty; 3) The majormortality on the planted grounds occursin late winter and early spring, a periodduring which oysters die from a combi­nation of overwintering stress as well asfrom MS X. In contrast, the greatestseasonal mortality in experimentalstocks occurs in late summer and fall,when virtually all kill has been as­sociated with MSX (Haskin et aI.,1965).

Cumulative mortalities for allplanted oysters sampled between 1960and 1977 average 36 percent after Iyear (Fig. 4). After a second and thirdyear of exposure, average total mor­talities have risen to 50 percent and 56percent, respectively. Neither seasonalmortality levels themselves, nor theirdecrease after exposure and selectionare as great as for experimental stocks.

Marine Fisheries Review

® NUMBER OF PLANTINGS SEASONAL MORTALITIES

SUM.

@

SPRING

@

SUM. FALL

CUMULATIVE MORTALITY

SPRINGSUM. FALL

------.-.--.-.-..-----­.----e/e//

S 0 M SO M J SO M

--- FIRSTYEAR -----I--SECOND YEAR---/--THIRD YEAR

EXPOSURE

J

Figure 4. -Seasonal and cumulative mortality means for native seed on the planted grounds over the period1960-77. Mortalities for all grounds sampled during a particular season and year of exposure to MSX havebeen averaged and are shown in the upper graph along with the number of plantings involved in eachcalculation. These seasonal mortal ities have been cumulated over a ]-year period and are shown in the lowergraph.

Calculating averages from pooleddata in this situation has certain disad­vantages. Mortalities are a great dealmore variable among planted groundsthan among experimental stocks with acommon selection history under test atthe Cape Shore Laboratory. This vari­ability is due to fluctuations in MSXactivity from year to year on the plantedgrounds, to variable disease pressurewithin the planted ground area, and tostress from harvest dredging (Haskin 5

).

Averaging is a useful tool for demon­strating general patterns, but it tends tomask extremes which add to a morecomplete understanding of the data.

'Haskin, H. H. 1972. Disease resistant oysterprogram - Delaware Bay 1965-1972. Unpub­lished report to National Marine Fisheries Ser­vice.

Four plantings of seed oysters on theleased grounds have been chosen to il­lustrate mortality extremes and alsosome more typical mortality levels, re­corded after the 1957-59 epizootic.None of these groups was harvestedduring the sampling period. These arecompared with experimental traystocks in Figure 5. For comparison withplanted grounds, Cape Shore tray mor­talities have been calculated beginningin July of their first summer's exposure,disregarding kill which has taken placesince the previous October, most ofwhich is not associated' with MSX.Highest kill for any ~group on theplanted grounds since the' originalepizootic was suffered 9Y James Riverseed oysters imported arid- planted ex-

~'

perimentally in 1964. At 84 percentafter 2 years, this. value falls midway

between laboratory-spawned suscepti­bles (92 percent) and native Cape Shoreset (73 percent). For native seed, high­est kill was recorded for a group of 1972plants. Their 65 percent 2-year mortal­ity equalled that for first generation lab­oratory-reared resistants (62 percent)and represents a doubling of survivalover James River imports, despite' thefact that these 1972 plants were ex­periencing the heaviest disease acti vityon record since the first epizootic (Has­kin 6). An example of a ground withrelatively low mortality was oneplanted in 1974 which lost 39 percentover 2 years. More typical for oyster

"Haskin. H. H. 1975. Control of disease in oysterpopulations of Delaware Bay. Unpublished re­port to National Marine Fisheries Service forperiod I June 1973 - 3 I May 1974.

: .) >,-;•. :

January-February /979..~ ...... .. ~~

59

% plantings is the 51 percent loss shownby a ground planted in 1968. This valuefalls close to the 47 percent for experi­mental third generation resistants atCape Shore .

When Delaware Bay seed bed oys­ters are tested at Cape Shore with ex­perimental tray stocks, a different pic­ture is seen (Fig. 6). Since 1966, 15native groups have been thus tested .Eight have come from upper bay seedbeds in the vicinity of Arnolds bed (Fig .I), and they have shown a mean 2-yearmortality of 75 percent. Nine groupsspanning the distance between Egg Is­land and Cohansey Beds have had anaverage kill of 70 percent after 2 years.These figures are almost identical to the72 percent for Cape Shore natives, andall fall between the 92 percent for I

laboratory-reared susceptibles and the62 percent for first generation resis­tants .

When oysters in lower Delaware Bayexperienced their first MSX kill, losseswere as high as 85 percent during a6-week period in the spring of 1957.During the 2 years after the initial out­break, total cumulative mortalitiesreached 90-95 percent over most of theplanted area, a level which compareswell with the 92 percent 2-year loss forlaboratory-reared susceptibles whoseparents have been imported annuallyfrom areas with little or no MSX pres­sure, and are thus comparable withDelaware Bay natives before theepizootic in 1957.

Since the early and mid-1960's,when lowered mortalities of nativestocks became obvious, no further de­cline in mortalities has been evidenced.Kill has tended, instead, to vary withfluctuations in MSX activity. But evenwhen disease pressure has been ex­tremely heavy, as it has been since1972, losses have never equalled thoseof the late 1950's. As indicated above,maximum 2-year kill for native seedsince the 1957-59 epizootic was 65 per­cent on a ground planted in 1972 (Fig.5). While histological data for the ini­tial epizootic are scarce, all availableevidence indicates that the MSX pres­sure experiencd by the 1972 plants wasequal to that of the first kill, as mea­sured by numbers of oysters infected,

JMD

o 1974 D.B. SEED

o 1972 D.B. SEED

• SUSCEPTI BlES

• CAPE SHORE NATIVES

• F1 RESISTANTS

e F3 RESISTANTS

o 1964 J.R. SEED

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Figure S.-Comparison of cumulative mortality means for experimental stockstested at the Cape Shore with Delaware Bay native seed on the planted grounds.Mortalities have been calculated for a 2-year period following exposure to initial Juneinfective period. J. R. and D. B. indicate James River and Delaware Bay seed,respecti vely.

60

>-I-...<I-Ill: 500~

w>=c...:;:)

~:;:)

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60 Marine Fisheries Review

Figure 6.-Comparison of cumulative mortality means for experimental stocks andDelaware Bay native seed, both tested at the Cape Shore. Mortalities have beencalculated for a 2-year period following exposure to initial June infective period.

JMDS

• SUSCEPTIBLES

• F, RESISTANTS

o UPPER SEED BED STOCKS

C LOWER SEED BED STOCKS

• CAPE SHORE NATIVES

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Laboratory-reared stocks, whosepattern of kill upon exposure to MSX isone in which each succeeding genera­tion has less mortality than did its par­ents, provide convincing evidence thatresistance to MSX is, indeed, heritable.Offspring display better survival be­cause susceptible individuals in theirparents' generation die from MSX be­fore that generation spawns the next.As each generation is exposed to thedisease, its gene pool is altered by"weeding out" of susceptibles, and itis this new genetic make-up with ahigher proportion of resistant genes,which is passed into the next generation. ~

Disease pressure at Cape Shore,where experimental oysters are ex­posed to MSX, is very high. Infectionprevalences commonly reach 100 per­cent prior to heavy mortalities 7

. Anyoyster which survives nearly 3 years w

>exposed to this level of disease acti vity -~should be extremely resistant. In fact, it ...

seems reasonable to believe that all oys- ::>ters which survive such pressure should ~be almost equally resistant. It is puz- uzling, therefore, to find that differential';'kill continues after oysters have beenexposed to, and selected by, MSX fornearly 3 years. Stocks which are mostsuscepti ble at the start of the test periodcontinue to show highest mortal ity ratesat its conclusion.

We propose the following explana­tion for our findings. A population ofoysters which has never been exposedto MSX contains a random distributionof those genes which will determine itscapacity to deal with the disease. Thesegenes present a continuum of abilitiesranging from total incapability inhighly susceptible individuals, throughstages of increasing ability to controlinfections, to some highly resistant oys­ters well-equipped to deal with intenseMSX pressure.

Upon first exposure to MSX, thehighly susceptible individuals, whichcomprise most of the population, die.

yet 4-7 times as many oysters survi vedas during the early epizootic.

Discussion

'Unpublished data. ew Jersey Oyster ResearchLaboratory, P.O. Box 1059, Piscataway, N.J.

January-February 1979 61

Some survivors of the initial kill alsocontain susceptible characteristics, butthese are masked to varying degrees byresistant ones. Such individuals are in­fected with MSX, however, and areweakened by Iingering infections, re­newed infections, and non-MSXstresses. There is, following the initialmassive kill, a progressive "weedingout" in which oysters with the lowestproportion of susceptible genes survi velongest. At the end of the 33-month testperiod, survivors with a preponderanceof resistant genes still have somemasked susceptible qualities.

In reproduction, genes are recom­bined and some offspring acquire agreater porportion of susceptiblecharacters than their selected parentshad. Upon first exposure to MSX, thesedie and there follows the same gradualselection of less susceptible individualsas in the parent generation.

The argument for such a recombina­tion is strengthened by seasonal mortal­ity patterns (Fig. 3). There is a consis­tently higher mortal ity during first ex­posure in any gi ven generation thanthere was in the parents during their lastseason of exposure.

With each succeeding generation,fewer susceptible traits remain to bepassed along, and MSX-caused mor­talities decrease accordingly. But evenafter three generations of rigorousselection and breeding, MSX still killsoysters in the F3 generation. At least asfar as this generation, then, there isevidence that resistance to MSX kill hasnot levelled off, but is continuing todevelop in laboratory-reared stocks.

Two important aspects of an oyster'sability to deal with MSX are the dosageof infective particles it receives and thekinds and numbers of additional stress­es with which it must cope. The failureof native oysters from widely differentareas in Delaware Bay to show differen­tial mortality, when tested at the CapeShore Laboratory, suggests that theremay be enough mixing of larvae withinthe estuary so that oysters setting in allareas are equally resistant. The lack ofdifferential mortality may, however,result from overwhelming doses of in­fective particles at Cape Shore. A quitedifferent result was obtained in a recent

62

study. In each of 3 years, oysters fromthree different seed beds, Arnolds, Co­hansey, and New Beds, were placed intrays and exposed to MSX on a plantedground (Fig. I). In each of the years,mortalities followed a consistent pat­tern: Arnolds seed suffered heaviestkill, followed closely by that fromCohansey Bed; New Beds oysters al­ways had the least mortality (Haskin,see footnote 6). This is the expectedpattern, since MS X selection pressureon the seed beds diminishes in anyupbay direction along the decreasingsalinity gradient. The differential in re­sistance of these stocks, demonstratedon the planted ground, has been com­pletely masked under the testing condi­tions on the Cape Shore flats.

Working at this laboratory, Valiulisand Haskin (1972) showed conclu­sively that the demonstration of resis­tance in oysters to the pathogenLabyrinthomyxa marina was dose-de­pendent. We tested stocks with differ­ing resistances to MS X mortality to seeif those oysters most resistant to MSXwere also most resistant to L. marinamortality .. Unlike MSX, L. marina canbe transm1tleo\in known dosage underlaboratory'·con.ditions. Valiulis andHaskin (1972) fQ.und that, with heavyparasite dosag'~s, all of the stocks diedin equally great numbers. With lowerdoses, how'ever, oysters resistant toMSX kill wer'e shown to be also moreresistant to L. marina than were MSX­susceptible stocks. This work may in­dicate that the mechanism of resistanceto MSX in the selected laboratory­reared stocks is not specific for MSX.In any event, whatever the mechanismmay be, it is suggested that resistance toMSX mortality may also be over­whelmed by increased dosage of thatparasite.

A second reason for higher kill, andperhaps for higher prevalences, at CapeShore is that it is a much less stable andprobably harsher environment than arethe planted grounds. Oysters arestacked in trays and may suffer fromcrowding. They are exposed at lowtide, are subjected to temperature ex­tremes, and are periodically infestedwith heavy accumulations of tube­building worms, particularly Polydora.

Some of these stresses may becomeespecially critical in the case of oysterswhich survive initial exposure but re­main infected with MSX. We know thatinfected oysters which are removed toMSX-free areas can support chronic in­fections for at least 3 years (Haskin andFord H

). If MSX-infected oysters aresurviving marginally under diseasepressure, the additional stress of un­favorable ambient conditions may re­sult in death.

There has been very little MSX-as­sociated kill in the upper bay except forthe first years of the epizootic and againduring severe drought in the mid­1960's, when elevated salinities per­mitted upbay MSX intrusion. Some in­fected oysters are usually present on thelower seed beds, but even when infec­tions are present, disease-related mor­tality is very low. Long-term monitor­ing shows that normal river flows willmaintain a salinity regime in whichMSX selection is effectively preventedover most of the seed area, even whenthe pathogen is flourishing in the high­er-salinity waters of the plantedgrounds (Haskin and Ford~). Lack ofselection over such a vast area meansthat there is little likelihood of furthermeasurable increase in resistance of na­tive seed. In fact, it is probable that thepresent level of resistance to kill, 3-4times that of unselected stocks, wasreached within a few years of the origi­nal epizootic.

The MSX epizootic in Delaware Baycompounded an already serious prob­lem for the oyster industry. A lengthyperiod of set failures had resulted in aserious shortage of seed for the localplanters. They had been relying heavilyon imported seed, but MSX precipi­tated a ban on imports, forcing them torely on scarce nati ve seed. The shortagecontinued into the late 1960's, wellafter MSX kill had subsided, and ham­pered the industry's recovery from the

"Haskin, H. H., and S. E. Ford. 1977. Control ofdisease in oyster populations of Delaware BayUnpublished report to National Marine FisheriesService for period 1 July 1975 to 30 June 1976."Haskin, H. H., and S. E. Ford. 1978. Control ofdisease in oyster populations of Delaware Bay.Unpublished report to National Marine FisheriesService for period I July 1976 to 30 June 1977.

Marine Fisheries Review

early kills. A series of very good setsoccurring throughout the bay between1968 and 1973 has recently providedplanters with an ample supply of seedoysters. It is this readily available, rela­tively resistant native seed, easily andeconomically planted, which has en­abled oystermen to remain in businessdespite substantial losses to MSX.Since the disease shows no signs ofdiminishing, and native oysters willprobably not become much more resis­tant, a continuing supply of inexpen­sive seed is an absolute necessity for theindustry. Only naturally produced oys­ters are abundant and inexpensiveenough to meet these needs. It is im­perative, then, that natural seed bedsand the water over them be protectedfrom any and all degrading influences.

Acknowledgments

This 20-year study required the dedi­cation of many people. Among the firstwho established the tray testing tech­nique were Walter Canzonier and SungYen Feng. Rearing of resistant oysterswas handled in turn by John Dupuy,

January-February /979

Herbert Hidu, Fred Krueger, Dean Par­sons, Stewart Tweed, and Cindy VanDover. All of the above and a largenumber of other graduate and under­graduate assistants shared in the care ofthe hundreds of tray stocks over theyears. Sampling on the oyster beds hasbeen handled by Donald Kunkle and thelate William Richards. Contributors tothe histopathology have been JohnMyhre, George Valiulis, Walter RuddDouglass, and Tom Keller. We thankJay Andrews and 1. R. Nelson fornumerous shipments of oysters fromVirginia and Connecticut, respec­tively.

All of the work has been supportedalmost continuously in the early yearsby the Bureau of Commercial Fisheriesof the U.S. Fish and Wildlife Service,followed by grants-in-aid under P.L.88-309 by the National MarineFisheries Service, NOAA, and the Di­vision of Fish, Game and Shellfisheriesof the State of New Jersey.

Literature CitedAndrews, J. D. 1966. Oyster mortality studies in

Virginia. V. Epizootiology of MSX, a protis-

tan pathogen of oysters. Ecology 47: 19-31.____ ,and J. L. Wood. 1967. Oyster mor­

tality studies in Virginia. VI. History and dis­tribution of Minchinia nelsoni, a pathogen ofoysters, in Virginia. Chesapeake Sci. 8: 1-13.

Ford, S. E. 1970. "MSX" - 10 years in the lowerDelaware Bay. (Abstr.) Proc. Natl. Shellfish.Assoc. 61:3.

Fraser, R. 1937-38. Pathological studies of Mal­peque disease. Fisheries Research Board ofCanada. Ms. Rep. BioI. Stn. 144.

Haskin, H. H., W. J. Canzonier, and J. L.Myhre. 1965. The history of "MSX" on Del­aware Bay oyster grounds, 1957-65. (Abstr.)Am. Malacol. Union Inc. Bull. 32:20-21.

- , L. A. Stauber, and J. A. Mackin.1966. Minchinia nelsoni n. sp. (Haplosporida,Hap10sporidiidae): Causative agent of the Del­aware Bay oyster epizootic. Science (Wash.,D.C.) 153:1414-1416.

Hidu, H., K. Drobeck, E. Dunnington, W.Roosenburg, and R. Beckett. 1969. Oysterhatcheries for the Chesapeake Bay region.Nat. Resources Inst., Univ. Maryland, Spec.Rep. 2.

Myhre, J. L., and H. H. Haskin. 1969. "MSX"infections in resistant and susceptible oysterstocks. (Abstr.) Proc. Natl. Shellfish. Assoc.60:9.

Needler, A. W. H., and R. R. Logie. 1947.Serious mortalities in Prince Edward Islandoysters caused by a contagious disease. Trans.R. Soc. Can., Ser. III, 41(V):73-89.

Valiulis, G. A., and H. H. Haskin. 1972. Resis­tance of Crassostrea virginica to Minchinianelsoni and Labyrinthomyxa marina. (Abstr.)Proc. atl. Shellfish. Assoc. 63:6.

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