FISHERIES RESEARCH BOARD OF CANADA
MANUSCRIPT REPORT SERIES
No. 993
Incidence and Effect of the Copepod ClOcr., Mytilicola orientalis Mori in the Pacific Oyster of British Columbia
by F. R. Bernard
Biological Station, Nanaimo, B.C.
August 1968
This series Includes unpublished preliminary reports and data records not Intended for general dlstrlbullon. They should not be referred 10 In publications with. outclEKIIOTlcefromlhelssulngBoardeslablishmentand without clear Indication of their manuscript status.
FISHERIES RESEAR~H BOARD OF ~ANADA
MANUSCRIPT REPORT SERIES
No. 993
Incidence and Effect of the Copepod Mytilicola orientalis Mori in the Pacific Oyster of British Columbia
by F. R. Bernard
Biological Station, Nanaimo, B.C.
August 1968 Introduction
Discussion has surrounded the occurrence of a small red copepod in the distal portion of the intestine of various commercial species of oysters and mussels. This organism has been considered a dangerous parasite causing mortality or loss of condition, while other workers put forward the view that it is nonpathogenic and has little effect upon the host.
This report summarizes data from several years' study of the occur rence, seasonal infection pattern, and effect of Mytilicola orientalis upon the Pacific oyster.
Mori (1935) described a new species of copepod, Mytilicola orientalis fromCrassostrea~(Thunberg)and~crassitestaLischkecollectedin the Inland Sea of Japan. Noting affinity to M.. intestinalis Steuer, Mori sep arated the new species on the basis of the more prominent lateral thoracic protuberances in the female and lack ofa protuberance on the first thoracic segment in the male. Wilson (1938), apparently overlooking Mori's work, redescribedthespeciesfromPugetSound,namingitM..~.Odlaug(1946) reported M.. orientalis in both Pacific and Native oysters from Washington, also in bay mussels (~edulis), little-neck clams (Pjotothaca llsmi.Q!l.2.) ~~~u~~:~~~ni~u~~e (g~~i~~~;~a fm~~~~~t(~:w c:ii ~~;n~;~~).first noted the
There is some doubt concerning the commercial significance of M.vtil icola infestations of shellfish stock. Odlaug (1946) demonstrated that infec ted Q.. lurida had a lowered condition factor; however, he concluded that M.. edulis was the preferred host as mussels from the same locality invariably had consistently higher infection rates. Sparks (1962) claimed that certain metaplastic changes occurred in the gut epithelium of infected oysters with a tendency to fibrosis of underlying tissues, while in some cases the appendages of the copepod penetrated the gut wall. Sparks (1962) in a study to determine whether infection by M.. orientalis was a mortality factor in Pacific oysters, concluded that it was not. Sparks et al. (1964) demonstrated a below-average Condition Index for infected oysters. Wilson (1938) did not consider Mytilicola a true parasite as the mouth parts are not adapted for biting or sucking tissue fluids.
Korringa (1950, 1952) attributed large losses of Dutch mussels (M.. edulis)toheavyinfestations·ofM..intestinalis. Later (1953) he concluded that the copepod interferes with the health and growth of the host. Dollfus (1951) considered the mortality of the Dutch mussels was caused by a virus or bacterium, conceding that infected bivalves may be weakened and more suscept ibletosecondaryinfection,butthatdeathcouldnotbedirectlyattrib',1ted to MytilicQla. Bolster (1954) recorded the devastation of mussel beds in Whitstable, England and attributed the cause to Mytilicola, remarking that infected mussels are apt to break away from the byssus. Cole and Savage (1951) reported 100% of mussels from Blyth, England were infested by M.. intestinalis and connected this with a pronounced loss in condition. M. orientalis is found in the recurrent intestine and rectum of the host; pressing the ventral surface in close contact with the gut wall, the dorsal processes aiding in maintaining position. Adults, especially females, are found with the head orientated towards the anterior portion of the host digestive system. No individuals have been collected in the style-sac or direct intestine, but occasionally very young stages «1 mm) have been found in the stomach, indicating means of access. Wilson (1938) states that M.. ostreae (= M.. orientalis) is attached to the inner wall of the stomach, with as many as twenty being taken from a single oyster. Little activity is displayed by Mytilicola while in the intact gut, but vigorous peristaltic contractions begin as soon as the animal is removed.
Effect on host
Spark (1962) demonstrated that the presence of Mytilicola caused metaplastic changes in the gut wall of the host. No evidence of this has been foundunderBritishColumbiaconditions.Instarvedoysters(Q..~),or individuals that have been held out of water for two or more days, the copepods are often found partially extruded from the rectum and it is likely that the thoracic protuberances may cause a mechanical lesion of the gut epithelium, but no fibrosis or epithelial changes have been noted inhistologicalprepara tions from recently gathered infected oysters or mussels.
It has generally been reported in the literature that the presence of Mytilicola in lamellibranchs is associated with a lowered condition factor. ThishasnotbeendemonstrableforQ..~underBritishColumbianconditions. Calculation of the condition factor for 92 infected anduninfectedoysters from Ladysmith Harbour revealed no significant difference for the two groups (Table I).
Condition factors of oysters from Ladysmith Harbour. Chew et al. (1965) consider that the presence of Mvtilicola appears tohavelittleeffectuponthedimensionalgrowthof~.~,butthatthe Condition Factor is loweredl however, no relationship between infection and mortality could be determined.
Distribution on the British Columbia coast is sporadic and limited to areas where young oysters imported from Japan, or infected stock, have been introducedl natural spatted areas are free from infection. Figure 1 shows infected localities on the British Columbia coast. Figure 2 is a detailed distributional study of Ladysmith Harbour undertaken in 1965-1967. In all cases, centres of original plantings of infected oysters (see Fig. 1) contain numerous Mytilicola, while adjacent areas are free, though the oyster popu lation is continuous due to natural spawning. This wO'Jld indicate that the free-living infective stage is absent or very short. Over the three year dis tributionalsurvey, there have been no radical changes in the pattern of in fection in Ladysmith Harbour or in a selected control location, Hammond Bay.
Seasonal infection rates
Once established, Mytilicola maintains, in British Columbia situations; a steady population level with a peak period followed by a decline in numbers. Studies of per cent infection of Pacific oysters and bay mussels from Ladysmith Harbour for the period 1964-1966 (Fig. 3) reveal the July-August peaks in num bers of Mytilicola. For comparative purposes a similar study was undertaken in Hammond Bay, a shallow inlet 14 miles north of Ladysmith. Results are sum marized in Figure 4. Percentage infections are consistently higher for mussels thanoysters,b'Jtthecyclicperiodscoincide. While there are considerable seasonal variations, the annual mean remains remarkably constant. Data for the periods of maximum (May to July) and minimum (January to March) infestive rates are given in Figure 5.
Host size and infestation rate
Indications are that small and large oysters have a slightly lower level of infestation than medium-sized oysters, but differences are insignifi cant. Figure 6 summari zes information from infected oysters collected from Denman Island in June 1965. A high percentage of oysters were highly infected, onelOcmindividualharboring15copepods.
to be no connection between the incidence of Denm3n of Mvtilicola. Table II summarizes the results of Per cent infestation by Mvtilicola of Henry Bay oysters.
Host selectivity
Examination of various intertidal bivalves from Ladysmith Harbour indicate that the bay mussel (~. edulis) is the most frequently selected host, followed by the oyster. This supports Odlaug's (1946) findings that~. orien kill incidences were higher in ~. edulis than in Q.. lurida. A number of nat ive oysters from various localities have been found harboring MvtUi.!<9l.2.. Chewet a1. (1964) report both Q.. lurida and the Californian mussel~. califor nianus as being hosts to the copepod.
Examination of molluscs from Ladysmith Harbour for incidence of Mytilicola
Species Number examined
9.linocardi~"D.uttalli 74 Y..€n~~~ 71 Ocenebrai~ 40 ~~ 50 M.v:tl.~~ 500 Crassostreaw.~ 700 Q.tl.I:ll~ 18 ~staminea 53 Tresus£.~ 12 M.v.e. s.r.'W.a_r.i..~ 24
Environmental factors
Hepper (1955) has shown that there is a marked difference in percent age infection with regard to host position. Mussels raised from the sea bed are less infected than those on the substrate. In the case of mussels on floats the difference is even more marked. Figure 7 is a diagrammatic repre sentation of Kent's wharf, Ladysmith Harbour where detailed examination for ~bearingmusselswasundertaken.Thelowerportionofthefigure shows graphically the position/per cent infestation for mussels on the wharf. There appears to be little effect from host density. Beds with mussels and oysters far apart are as heavily infected as those where the hosts were close together. Raising mussels from the substrate dramatically lowers the incidence of~.
Infestation of1transplanted oysters
To obtain an indication of the infective rate and seasons of maximum spread of Mvtilicola, oysters from an uninfested area (Pendrell Sound) were moved to a heavily infested bed in Ladysmith Harbour. Oysters were sampled at monthly intervals and two control groups keptl one in a basket raised above the infested bed and another some distance away among copepod-free oysters.
While the numbers of samples are too small for any valid conclusion, they do indicate that the period of maximum infections is in June-July. The twocontrolgroupswerefreeof~,indicatingthatthefree-living stage is not capable of travelling any distance, and is probablynegatively phototropic.
TableIV.Intidenceof~~intranspiantedoysters
No. samples M:i:li.li.~ No. of infested oysters
Sept. 8 Oct. 8 20 10 10 May 5 July 15 5 Aug. 6 2 July (Control group) July (Control group) Morphometric studies
Frequency/size measurements were taken between 1964 and 1966 at Ladysmith Harbour. Figure 8 gives the frequency histogram for the p9riod June July-August 1965, showing a distribution with a main mode falling in the 8-9 mm size consisting of mature females and another mode at 5-6 mm for adult males. Figure 9 summarizes the study for the period January-February-March of the same year, the modes being much closer together because fewer females are carrying egg-cases. The apparent dimorphism is more evident in Washington. Chew et al. (1965) report that two distinct size classes were found throughout the year for ~. orie~ collected from Humboldt Bay and Yaquina Bay oysters. Those bet ween 2 and 5 ffi1\ were mostly male, while those specimens between 6 and 11 mm were female.
Reproduction and life cycle
Copulation takes place within the host, the female copepod bearing a pair of egg cases. Sparks (1962) concludes that reproduction of the copepod takes place in two peaks with infective stages in the water during early spring and late summ9r. While there is a peak of reproductive activity, in June-August, Ladysmith Harbour and Hammond Bay Mm.l~1J!. from British Columbia appear to have a more even cycle. The situation compares with that in Germany reported by Meyer and Mann (1951); working with ~. intestinili§. they recorded a maximum percentage of egg-bearing females in August. Little developm9ntal work has been done on the Mytilicolidae in general. Pesta (1907) has illustrated certain postembryonicstagesof~.~.
Summary
1. Distribution of~. orientalis is limited to those areas that have received plantings of Japanese seed or infested oysters, nearby areas beingfreeofthe copepod.
2. The distribution pattern indicates that the free-living infective stage must be very short in this species.
3. When established, the~. orientalis reaches a peak in July-August, then gradually decreases.
4. There is considerable seasonal variation in population levels, but the corresponding annual means are remarkably constant.
5. There is little effect of host density upon percentage infestation, but mussels raised f;rom the sea bed are less infested than those resting on the substrate. 6. A lowering of the Condition Factor of infested oysters (~. 919.ll) could not be demonstrated.
7. No lesions or metaplastic changes of the epithelia of the hostJs intestine could be observed.
Bolster,G.C.1954.Thebiologyanddispersalof~~ Steur, copepod of parasite of mussels. Fish. Invest. Fish. Exp. Sta. Con. V.K. 11, ]&(6),30 p.
Caspers, H. 1939. Uber Vorkommen und Metamorphose von Mytilicola ~ Steur (Copepoda paras.) in der SudlichenNordsee. Zoo1. Anz. n61 161-171.
Chew, K. K., A. K. Sparks, and S. G. Katkansky. 1963. Preliminary report on the incidence and pathology of Mytilicola in oysters. Prog. Rep. No.2, Dept. of Health, Ed. & Welfare, Washington.
Chew,K.K.,A.K.Sparks,andS.G.Katkansky. 1964. FirstrecordofM.Y1ll icolaorientalisintheCaliforniamussel~californianusConrad. J.Fish.Res.Bd.Canada,21(1)1205-207.
Chew, K. K., A. K. Sparks, S. G. Katkansky. 1965. Preliminary results on the seasonal size distribution of Myti li cola orientalis and the effect of this parasiteontheconditionofthePacificoyster,~919.ll.J. Fish. Res. Bd. Canada, 22(4). 1099-1101.
Cole,H.A.,andR.E.Savage. 1951. The effect of the parasitic copepod Mytilicola intestinali.§. (Steur), upon the condition of mussels. Jour. of Parasit., Vol. 41, No. 34, p. 156-160.
Dollfus,R. 1951. ThecopepodMytilicolaintestinalisSteur,peut-il'etre la Cause d'une maladie epidemique des moules. Rev. des Trav. (de l'Office Sci. et Tech. des Peche Merit.) 17, p. 81-85.
Hepper, B. T. 1955. Environmental factors governing the infection of mussels, ~~,byMytilicolaintestinalis.Fish.Invest.UKII,
Koringa, P. 1950. De aanual van de parasiet Mvtilicola intestinalis op de Zeeuwse Mossel cultuur. Vissevig-Niews, 7.
1952. Epidemiological observations on the mussel parasite Mytilicola intestinalisSteur, carried out in the Netherlands, 1951. Ann. BioI. Copen., 8, p. 182-185.
1953. Epidemiological observations on the mussel parasite Mytilicola intestinalis Steur, carried out in the Netherlands, 1952. Ibid, p. 214-224. Meyer, P. F., and H. Mann. 195L Recherches allemandes relative au ''Mytilicola'' copepode parasite de la moule, existant dans les Whatten allemands 1950-5L Rev. des Trav. (de l'Office Sci. et Tech. des Peche Merit), No. 17, p. 63-75.
Odlaug,T.0.1946.TheeffectofthecopepodMytilicola~uponthe Olympia oyster, Ostrea lurida. Trans. Amer. Micr. Soc., VoL 65, p. 311 317.
Pesta, O. 1907. Die Metamorphose von Mytilicola intestinalis Steuer. Ztschr. wissensci. ZeoL 88. 79-98.
Quayle, D. B. 1964. Distribution of introduced marine Mollusca in British Columbia waters. J. Fish. Res. Bd. Canada, 4(5): 1155-118L
Sparks, A. K. 1962a. Metaplasia of the gut of the oyster Crassostrea 9i9..a2. (Thunberg) caused by infection with the copepod Mytilicola orientalis Mori. Jour. Ins. Parasit., 4, p. 57-62.
1962b. Some preliminary observations on the incidence of infection and
f~t~~;o~;~~ ;i~ f~~~~e~f(~~:s~~~~~~;i~~~~~~n~e;~3~ c~~ath~i:e~~l~~a~~ri , of the United States. I.C.E.S., Copenhagen, Oct. 1962.
Wilson, C. B. 1938. A new copepod from Japanese oysters transplanted to the Pacific coast of the United States. Journ. Wash. Acad. Sci., VoL 28, p.284-288. -9-
Appendix 1
Distribution of!1. orientalis in Pacific oysters and Bay mussels from British Columbia and seasonal counts
Number Percentage Locality Date examined infestation
RedondaIsland June 1965 50 0 July 1966 80 0 Malaspi~a Inlet October 1965 42 12 September 1966 58 0 TexadaIsland April 1965 12 0 July 1966 46 0 Pender Harbour May 1964 57 3 Boundary Bay June 1966 50 6 Seal Island January 1964 20 18 Comox March 1964 22 2 April 1966 46 11 Baynes "Sound 18 2 May 1967 29 4 Nanoose April 1963 40 7.5 May 1965 62 14 June 1966 50 8 Ladysm~,th Harbour March 1963 50 6 April 1963 50 8 May 1963 40 2.5 June 1963 30 13 August 1963 50 4 September 1963 37 2.7 October 1963 30 10 November 1963 30 6.7 March 1964 50 6 April 1964 32 5 May 1964 50 11 June 1964 50 14 April 1965 50 6 May 1965 37 8 June 1965 50 14 July 1965 50 20 August 1965 50 22 September 1965 42 11 October 1965 50 8 December 1965 50 9 January 1966 50 4 Mar~h 1966 62 6 Appendixl-continued
Number Perventage Locality Date examined infestation
Ladysm~,th Harbour Apri11966 50 9 June 1966 50 11 August 1966 32 22 September 1966 49 18 October 1966 50 12 December 1966 50 2 Gowichan March 1965 50 3 Victoria May 1965 14 o Sooke April 1963 38 2.6 " (Brooks) April 1963 40 2.5 Sooke June 1965 50 18 Uselesslnlet May 1965 14 1 BarkleySound August 1966 23 o Appendix 2
Ladysmit.h Harbour percentage infestation of Pacific oysters
Number Percentage examined infestation
Ivy Green Park
Wedge Point
SibellBay
Sharp Point Appendix 3
Condition factors of infested and Mo. orientalis-free Pacific oysters from Ladysmith Harbour
May 1965 - Number tested 25 May 1965 - Number tested 27
Infested
Shell volume 610 Shell volume 691 Interior volume 439 Interior volume 597 Total weight 470 Total weight 448 Dry weight 82 Dry weight = 75 Condition factor = ~ Condition~= ill
JUly 1965 - Number tested 19 July 1965 - Number tested 21
Shell volume 703 Shell volume 1225 Interior volume 402 Interior volume 603 Total weight 399 Total weight 409 Dry weight 72 Dry weight 91 Condition factor gg ConditiQ.!Ltill.2l: ~ Appendix 4
PercentageinfestationofBaymusselsby~.~ Number Percentage Locality examined infestation ------ Ladysm~,th Harbour January 1964 20 12 February 1964 24 9 March 1965 60 14 April 1965 72 12 May 1965 50 12 June 1965 50 18 July 1965 50 24 August 1965 47 26 September 1965 63 20 October 1965 60 13 November 1965 50 9 December 1965 50 9 Hammon?, Bay January 1965 40 5 Febmary1965 40 7 March 1965 40 5 April 1965 40 3 May 1965 40 7 June 1965 37 11 July 1965 40 21 August 1965 40 18 September 1965 40 96 Appendix 5 Percentage infestation of Bay mussels in relation to position on beach. May September 1965. Pendre11 Sound mussels Number Percentage Station No. examined infestation IfN1ST Height Int. 81 12 2 Int. 97 18 3 Int. 66 16 4 Int. 72 20 5 Int. 80 16 6I.WST Int. 60 11 7 Distance off bottom 1ft 40 5 8 4" 40 3 9 4" 40 3 10 5" 40 2 11 7" 40 0 12 7" 40 0 13 7" 40 0 14 6" 40 0 15 11" 60 0 • MYTI!JCOLA PRESENT o MYTILICOLA FREE o OYSTER SEED PLANTING Figure 1. DistributionofMYlilicogorientalisinBritishColurnbia, relative to Japanese oyster seed plantings. Figure 2. Percentage distributionofM:i1J,lico1a orienta1is in Ladysmith Harbour for 1965. • -----0--- _ ---~ .....6 0-'- ~ .,~ .,~ .~ "';1 o N NOI1V1S3.:1NI 3!>VlN3::>~3d ----""0- o o C\I NOI!\flS:3.:lNI :3!)\flN:3::>~:3d 20 LADYSMITH HARBOUR 15 MAY-JULY 10 ~, •.D------0-- _,_-- ~ - - _-D JAN-MARCH O~" HAMMOND BAY 15 10 MAY-JULY JAN.- M~~C:~ ----.o.""", " ','-0-_------0 1963 1964 1965 1966 Figure 5. Annual means of percentage infestation of Pacific oysters in (upper figure) Ladysmith Harbour, and (lower figure) Hammond Bay. ~I ~~ 11/1 2~ \...------.------~ ~ i! .~ E H 10 I o g 6 ~3.lSAO ~3d 'VlO~lll.lA.W 'ON 3~'V~3"'V Z 0115 ~ ~ ~ ~IO ! 7 8 STATION Flgure7.RelationshlpofpercentagelnfestationofBaymusselsbyt!1.~and posltionofhost. Graph shows percentage lnfestation at each station. 40 JUNE - AUGUST 1965 30 20 FlgureB.S1ze-frequencyofM.~fromLadysmlthHerbour,June-August. JANUARV- MARCH 1965 20 10 4 ~ 6 7 MVTlLICOLA SIZE (mm) F1gure9.S1ze-frequ.ncyofM.~s.fromL.dysm1thH.rbour,J.nu.ry-M.rch.