/. Moll. Stud. (1991), 58, 29-36 © The Malacological Society of London 1991

BUCEPHALID TREMATODE INFECTIONS IN MYTILIDBIVALVES FROM THE ROCKY INTERTIDAL OF

SOUTHERN CHILE

THERESA LASIAKDepartment of Zoology, University of Transkei, Private Bag XI Unitra, Umtata, Transkei 5100,

Southern Africa(Received 19 February 1991, accepted 11 June 1991)

ABSTRACT

Bucephalid trematode sporocysts were found em-bedded amidst the mantle tissue and viseral mass ofintertidal mytilid bivalves from Southern Chile. Aninvestigation into the-prevalence of infection revealedthat <1% of Perumytilus purpuratus were infectedwhereas 20-32% of the Semimytilus algosus ex-amined harboured sporocysts. No sporocysts werefound in an adjacent population of Choromytiluschorus. Prevalence of infection was related to size ofS. algosus present at a particular site, and generallyincreased with host size. The majority of parasitizedmussels appeared to be at an advanced stage of in-fection, in these animals destruction of the host'sgonadal tissue was evident. Such parasitic castrationcombined with high prevalence of infection suggeststhat the reproductive potential of this mytilid may beseverely impaired. The implications of these resultsfor the mariculture industry are also discussed.

INTRODUCTION

Despite the importance of shellfish to thenational economy of Chile, comparatively littleis known of the pathogens and parasites whichmay adversely affect the quality of these re-sources. Commercially exploited species knownto harbour parasitic infections include the 'loco'Concholepas concholepas, various species offissurellid limpets commonly known as 'lapas',the 'macha' Mesodesmium donacium, and the'almeja' Venus antigua (Bahamonde & Lopez,1962; George-Nascimento & Quiroga, 1983;Osorio & Castillo, 1984; Canas & Lozada,1987). The aforementioned studies are, how-ever, primarily of a taxonomic descriptivenature. The only host/parasite relationships tohave been examined, within an ecological con-text, are those between fissurellid limpets anddigenetic trematodes of the genus Proctoeces(Bretos & Jiron, 1980; Osorio, Diaz & Rios,

1986; Oliva & Diaz, 1988). Recent examinationof mytilid bivalves from Mehuin, near Valdivia,southern Chile, revealed that some species har-boured heavy infections of bucephalid sporo-cysts similar to those recorded by Szidat (1963).As larval trematodes of the genus Bucephalusvon Baer, 1827 have been found to parasitizemany commercially important species and, in sodoing, destroy the host's reproductive tissue amore in-depth study was initiated. This paperdescribes the results of an investigation into theprevalence and intensity of bucephalid infectionsin three mytilid species, namely Choromytiluschorus (Molina, 1782), Perumytilus purpuratus(Lamarck, 1819) and Semimytilus algosus(Gould, 1850).

METHODS

Between January and April 1990 samples of P. pur-puratus and S. algosus, encompassing the full sizerange available, were collected from sites A & B atthe northern end of Playa Universitaria, Mehuin andfrom sites C, D & E within the Marine Reserve of theUniversidad Austral de Chile at Punta Kilian (39°24'S,73°13'W) (Fig. 1). At sites A-E individual musselswere selected randomly from rocks which supportedcontiguous populations of these two mytilids. Addi-tional samples of S. algosus were also obtained fromsites F & G at Punta Pichicuyin, 1 km north of PuntaKilian (Fig. 1). Samples of C. chorus were obtainedonly from site E. Punta Kilian and Punta Pichicuyinare exposed coastal headlands which abut the PacificOcean. This area is subject to considerable tectonicactivity, and hence the topography is extremely rug-ged and varied; it consists of an assortment of cliffs,overhangs and isolated broken rocks. As none of thestudy sites were directly exposed to the full force ofthe waves, they were classed as semi-exposed. Inwinter and autumn this coast is exposed to N and NWwinds whilst in spring and summer S and SW windspredominate (Jaramillo, 1978).

The sample size, which varied from 111 to 218, was

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30 T. LASIAK

7314'

Figure 1. Map showing position of study sites A—G relative to Punta Kilian and Punta Pichicuyin, BahiaMaiquillahue, near Mehuin, Southern Chile.

dependent on the size composition of each popula-tion. Wherever possible, 30 individuals from each5 mm size class represented were examined. Thepopulation size composition of 5. algosus at sitesA—E was determined subsequently by removing andmeasuring all the mussels located within five randomlyselected 25 cm2 quadrats. To avoid complications

arising from a period of heavy settlement which tookplace between sampling periods, only mussels with atotal length of > 5 mm were measured.

All mussels were made to gape, by severing theadductor muscle prior to preservation in 10% for-malin. To determine whether or not mussels harbouredparasitic infections the soft tissues were removed from

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TREMATODE INFECTIONS OF MUSSELS 31

the shell, teased apart where necessary, and examinedwith a dissecting microscope. The presence orabsence and extent of infection were recorded. Theintensity of infection was scored on a five point scaleaccording to the amount of host tissue occupied bythe parasite: I = <5%, II = 5-25%, III = 25-50%,IV = 50-75% and V = 75-100% of tissues affected.The total length (measured to the nearest 0.5 mm)and sex of each mussel were also recorded.

The relationships between infection, site and sizeof host mussels were examined by means of two andthree-way contingency table analyses (Sokal & Rohlf,1981). Comparisons between sites were based oncommon size groups. Initial identification of the para-site was based on the multi-branched structure of thesporocysts and the morphological characteristics ofthe cercariae, specifically the presence of a posteriorhaptor bearing two slender furcae. Identification ofthe parasite was confirmed by Dr R.A. Bray (Dept.of Zoology, The Natural History Museum, London).The cercariae were similar in appearance and size tothose of Bucephalus ? chilensis described by Szidat(1963) from specimens of S. algosus. Since it is notpossible to identify bucephalids to generic level fromeither sporocyst or cercarial stages the parasite will bereferred to as a bucephalid.

RESULTS

Parasitized mussels, particularly female indi-viduals, could easily be distinguished because ofthe sharp contrast in colour between thereddish-brown mantle and the off-white to paleyellow sporocysts. This bucephalid infection re-sembled that observed in Mytilus platensis andBrachydontes rodriguezi from Mar Del Plata,Argentina (Morris, 1976). It differed markedlyfrom the bright orange sporocyst infections de-scribed by Cole (1935), Umiji, Lunetta & Leonel(1976) and Sanders & Lester (1981). Micro-scopic examination of infected mussels showedthat the sporocysts were long, inextricably tang-led, multi-branched, tubular structures whichcontained various other developmental stages,ranging from germ balls to fully-developed cer-cariae. In the early stages of infection (stages1-3) sporocysts were found only amidst themantle tissue, however, in heavily infectedmussels (stages 4 & 5) they were also found inthe digestive gland and around the labial palps.There was no evidence of infection on the gilllamellae as reported by Menzel & Hopkins(1955) and Mohan Joseph (1978). Infected mus-sels snowed varying degrees of gonadal atrophyculminating in total gonadal destruction. Inmussels with stage 5 infection, the gonadal tis-sues appeared to have been replaced totally bya densely packed mass of branching sporocysts.

Bucephalid sporocyst infections were foundto be most prevalent in the 'chorito negro', 5.algosus, between 20% and 32% of the indi-viduals collected from each site were infected.None of the C. chorus examined (N = 90)showed signs of infection and only 4 of the 734specimens of P. purpuratus examined were in-fected.

The number of 5. algosus examined withineach 5 mm size class together with the pre-valence of infection within these groupings isshown, for each study site, in Fig. 2. The dataindicate that it is the larger, and therefore prob-ably older, mussels which are most likely toharbour bucephalid infections. R x C contin-gency table analyses showed that prevalence ofinfection was not independent of host size atany of the study sites (Table 1). Three-way con-tingency analysis indicated (a) that the factorsprevalence, size and site were not jointly inde-pendent (G = 223.69; d.f. = 45; p<0.001) and(b) that the relationship between size and pre-valence varied from site to site (G = 175.04;d.f. = 21; p<0.001). Mussels collected fromsites A and E covered the widest size range andalso showed the highest prevalences of infec-tion. Re-examination of the data from sites A-E, on the basis of the population size composi-tion of 5. algosus (Fig. 3) indicated that preva-lence and mean size of host were positivelycorrelated (r = 0.95, d.f = 3, 0.01<p<0.02).

Most of the parasitized mussels examined,irrespective of their size, were at an advancedstage of infection (Fig. 4). R x C contingencytable analysis of the data indicated that stage ofinfection was independent of host size at allstudy sites (Table 2). There was no evidence tosuggest that these infections adversely affectedeither the tissue or shell weight of the mussels. •Analyses of covariance indicated that the slopesof the regression equations relating both drytissue weight and dry shell weight to total lengthof parasitized and non-parasitized mussels didnot differ significantly (Table 3).

DISCUSSION

The widespread occurrence of larval bucephalidtrematodes in commercially important bivalvesand the association of these parasites with de-struction of the host's gonadal tissues hasattracted considerable attention (Hopkins,1954; Menzel & Hopkins, 1955; Cheng &Burton, 1965; Cheng, 1967; Umiji et al., 1976;Mohan Joseph, 1978; Saunders & Lester, 1981;Morris, 1983; Turner, 1985). The prevalences

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218

10 15 20 25 30 35 40 45 50

TOTAL LENGTH

30

20 -

10

YAJU -

20 -

10 -

f\ _

1i

$yy

-

pI 1

193

10 15 20 25 30 35 40 45 50

TOTAL LENGTH

Figure 2. Histograms showing the prevalence of bucephalid sporocyst infections relative to size class ofSemimytilus algosus at sites A-G. (Solid fill and striped fill represent respectively the number of infected anduninfected mussels; N = the total number of mussels examined at each site.)

of infection reported vary from <5% (MohanJoseph, 1978; Morris, 1983; Turner, 1985) to>25% (Hopkins, 1954; Umiji etal., 1976; San-ders & Lester, 1981). The prevalence of infec-tion is dependent on various ecological factorswhich influence the probability of contact be-

Table 1. Results of R x C contingency tableanalyses comparing prevalence of infection withsize of host at the seven study sites (* indicatesthat prevalence of infection is not independent ofhost size i.e. the computed G statistic is greaterthan the critical x2 value at p<0.05 for (r-1)(c-1)degrees of freedom (d.f.) where r = number ofsize classes and c = presence/absence of infec-tion).

Site

AB0DEfQ

G statistic

96.9023.3619.3265.25

123.8017.2017.35

d.f.

7444754

Critical x2 value

14.07*9.49*9.49*9.49*

14.07*11.07*9.49*

tween host and infective miracidial stage. Fac-tors of particular importance are the nature ofthe habitat and density of intermediate host.Bucephalid infections have generally beenfound to be more prevalent in animals fromprotected areas, such as estuaries and embay-ments, than in those from open coastal waters(Hopkins, 1954; Umiji et al., 1976). The signi-ficance of these differences is often difficult toassess because the comparisons made do nottake into account differences in population sizecomposition of host animals at the various sites.A further problem is the substantial differencein time-scale over which observations are made,these vary from one-off surveys to regularmonthly sampling over several years.

The prevalence of bucephalid infectionsobserved in S. algosus from Mehuin are thehighest figures so far recorded from an exposedcoastal locality. Although the relative exposureof the sampling sites differed, because of theirposition relative to the prevailing winds and therugged broken nature of the coastline, therewas no discernible link between prevalence andexposure. The results did, however, indicatethat the prevalence of infection observed at a

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TREMATODE INFECTIONS OF MUSSELS 33

COCO

ccLJJm

CO

LLJCOCO

ODC111m2

12010080604020

012010080604020

015 20 25 30 35 40 45 50

TOTAL LENGTH

DX = 22.6+2.9XP = 28.0N = 115

EX = 23.3+7.0XP = 32.4N = 122

5 10 15 20 25 30 35 40 45 50

TOTAL LENGTH

Figure 3. Population size composition of S. algosiis at sites A - E . (Solid fill represents the infected portion ofthe population; %P = prevalence of infection at each site; and N = number of mussels per 125 cm2.)

particular site was related to the size of musselspresent. The fact that prevalence increases inconjunction with host size is probably indicativeof cumulative exposure to infection. The rela-tively high prevalence values suggest that thetransmission of bucephalids from the definitivehost to S. algosus is fairly successful throughoutthe study area. This implies that the infectivemiracidial stage may be dispersed ubiquitouslyin the inshore waters of Bahia Maiquillahue. Inview of the turbulent water motion character-istic of wave-swept shores (Denny & Shibata,1989) the successful transmission of this para-site is probably dependent on the close proxim-ity of the first and second intermediate hosts.As bucephalids generally use teleost fish assecond intermediate hosts a species which feedsin the mid-intertidal is probably the most likelycandidate to fulfil this role. The low prevalenceof infection observed in adjacent populations ofP. purpuratus indicate that this species is prob-ably less susceptible to infection because ofphysiological or genetic resistance. In view ofthe small sample size the data obtained for C.chorus are regarded as inconclusive.

The fact that most parasitized mussels have

been effectively castrated combined with therelatively high prevalence of infection suggeststhat the reproductive potential of 5. algosusmay be severely impaired. The situation isaggravated further since the larger, normallymost fecund individuals, are those most likelyto be parasitized. Precise determination of theeffect of bucephalid infections on reproductiveoutput is somewhat problematical, due partly tothe fact that S. algosus is a functional herma-phrodite which reproduces throughout the year(Gonzalez, Villagra, Hinojosa & Becerra,1980), and also because it is not known whetherinfections are transitory or persistent. Littleelse is known about the gross effects of theseparasites on their hosts. Menzel & Hopkins(1955) suggested that the growth of the oysterCrassostrea virginica may be stimulated duringearly stages of infection but retarded in the laterstages. They noted that the tissues of healthyoysters became emaciated and tasteless afterspawning, whereas bucephalid-infected animalsremained in good condition and excellentflavour throughout the summer. A detailedstudy of the scallop Pecten alba showed thatmaximum gonad weight was attained three

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34 T. LASIAK

o

0 IQ II

El iv• v

a. 15-iijm

0 5 10 15 20 25 30 35 40 45

TOTAL LENGTH

El I c• IIEl IIIH iv• v

ni I nil0 5 10 15 20 25 30 35 40 45

TOTAL LENGTH

3 10-

0 1 EB II0 III

H iv• V

•

• 1 I r nl0 5 10 15 20 25 30 35 40 45

TOTAL LENGTH

S .51U

0 I• IIa inQ IV

• V

• nlfl

G

l I10 IS 20 25 30 35 40 45

TOTAL LENGTH

2z

0 Ia IIH IIIQ IV

• v

§3 10 -

0 5 10 15 20 25 30 35 40 45

TOTAL LENGTH

0 I Da II0 inQ IV

• v

0 5 10 15 20 25 30 35 40 45

TOTAL LENGTH

0 I FO N0 IIIQ IV• V

0 5 10 15 20 25 30 35 40 45

TOTAL LENGTH

Figure 4. Histograms depicting the intensity ofbucephalid sporocyst infections observed in musselsfrom various size classes represented at sites A-G.(Details of the five stages of infection I-V are given inthe text.)

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TREMATODE INFECTIONS OF MUSSELS 35

Table 2. Results of R x C contingency tableanalyses comparing intensity of infection withsize of host at the seven study sites (conventionsas in Table 1)

Site

ABCDui

FG

G statistic

26.1012.538.568.77

21.3625.2811.22

d.f.

181296

242012

Critical x2 value

28.8721.0316.9212.5936.4231.4121.03

months later in parasitized than non-parasitizedindividuals -and that average weight of muscletissue of parasitized scallops was less than thatof healthy animals for six months of the year(Sanders & Lester, 1981). Further studies areclearly needed to establish whether or not thelack of differences in tissue and shell weight ofparasitized and non-parasitized 5. algosus is atransient feature.

In view of the mariculture potential of S.algosus in Chile (Gonzalez et al., 1980) thefinding of heavy bucephalid infections in naturalpopulations is cause for concern. As the industryinvolves the rearing of juveniles obtained fromnatural populations, considerable attentionneeds to be given to the choice of site fromwhich seed stock is obtained. An example ofthe potential problems which can be created isthe case of the experimental mussel culturepark established at Cabelo Beach, Sao Sebastiao,Brazil where 30-35% of the mussel Perna pernawere found to be infected with bucephalidsbrought in with seed stock from natural popula-tions (Umiji et al., 1976). As bucephalid-parasitized molluscs are easily recognized,periodic macroscopic examination of soft tis-sues is a precautionary measure that could beintroduced to detect infections. Mariculturists,who favour the use of condition indices, mayotherwise inadvertently maintain infected indi-viduals within their culture systems.

ACKNOWLEDGEMENTS

The Instituto de Ecologia y Evolucion of theUniversidad Austral de Chile is thanked forproviding the facilities and transport which erv-abled this study to take place. The assistance ofDr R. Bray (The Natural History Museum,London) with identification is also gratefullyappreciated.

REFERENCES

BAHAMONDE, N. & LOPEZ, M.T. 1962. Proboscido-saccus mesodesmatis n. sp. parasito de Mesodesmadonacium Lamarck. Investigaciones Zoologia deChile, 8: 43-56.

BRETOS, M. & JIRON, C. 1980. Trematodes in Chileanfissurellid molluscs. Veliger, 22: 293.

CANAS, P. & LOZADA, E. 1987. Nota sobre un trema-tode digeneo endoparasito del loco Concholepasconcholepas Brugiere 1789 (Gastropoda: Murici-dae). Parasitologia al Dla, 11: 173-176.

CHENG, T.C. 1967. Marine molluscs as hosts for sym-bioses. Advances in Marine Biology, 5: 1-424.

CHENG, T.C. & BURTON, R.W. 1965. Relationshipbetween Bucephalus sp. and Crassostrea virginica:histopathology and sites of infection. ChesapeakeScience, 6: 1—16.

COLE, H.A. 1935. On some larval trematode para-sites of the mussel (Mytilus edulis) and the cockle(Cardium edule). Parasitology, 27: 276-280.

DENNY, M.W. & SHIBATA, M.F. 1989. Consequencesof surf-zone turbulence for settlement and externalfertilization. American Naturalist, 134: 859-889.

GEORGE-NASCIMENTO, M. & QUIROGA, G. 1983. De-scripcion de una nueva especie de trematodo Proc-toeces humboldti n.sp. (Digenea : Fellodistomidae)parasito de las lapas Fissurella spp Brugiere, 1789(Mollusca: Archaeogastropoda). Parasitologia alDia, 7: 100-103.

GONZALEZ, J., VILLAGRA, J., HINOJOSA, M. &BECERRA, R. 1980. Potencialidad del cultivo enbalsa de Semimytilus algosus (Gould) 1850, en LaBahia de Concepcion (Mollusca, Bivalvia, Mytili-dae). Boletimo Instituto Oceanographia Sao Paulo,29: 187-189.

HOPKINS, S.H. 1954. The American species of trema-tode confused with Bucephalus (Bucephalopsis)haimeanus. Parasitology, 44: 353—370.

Table 3. Results of analyses of covariance on logarithmicmodels of the relationships between dry tissue weight (TW)or dry shell weight (SW) and total shell length of parasitizedand non-parasitized Semimytilus algosus.

Model d.f. Common Slope

In(TW) = bln(TL) + Ina 0.028 1,63 2.07In(SW) = bln(TL) + Ina 0.127 1,63 2.31

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JARAMILLO, E. 1978. Zonacion y estructura de la com-munidad macrofaunistica en playas de arena delSur de Chile (Mehuin, Valdivia). Studies on Neo-tropical Fauna and Environment, 13: 71-92.

MENZEL, R.W. & HOPKINS, S.H. 1955. The growth ofoysters parasitized by the fungus Dermocystidiummarinum and by the trematode Bucephalus cucu-lus. Journal of Parasitology, 41: 333-342.

MOHAN JOSEPH, M. 1978. Observations on the larvaltrematode Buccephalus sp. parasitic in the oysterCrassostrea madrasensis. Journal of InvertebratePathology, 32: 381-383.

MORRIS, M.R. 1976. Estados larvales de trematodesdigeneos en molluscos marinos Mytilus platensisd'Orb. y Brachydontes rodriguezi d'Orb. Revistadel Museo de la Plata, 13: 65-71.

OLIVA, M. & DIAZ, M. 1988. Aspectos cuantitativosde la infeccion por Proctoeces humboldti (Trema-toda: Fellodistomidae) en la lapa Fissurella crassa(Mollusca : Archaeogastropoda). Revista Chilenade Historia Natural, 61: 27-33.

OSORIO, C. & CASTILLO, M. 1984. Almejas comes-tibles (Venus antiqua, infectadas por Trematodos).Parasitologia al Dia, 8: 117-118.

OSORIO, C , DIAZ, M. & Rios, M. 1986. Prevalenciade Proctoeces humboldti George-Nascimento &Quiroga 1983 (Digenea, Fellodistomidae) en lapoblacion de Fissurella maxima Sowerby 1835(Mollusca, Gastropoda) de Los Vilos, Chile. Revistade Biologia Marina Valparaiso, 22: 157-168.

SANDERS, M.J. & LESTER, R.J.G. 1981. Furtherobservations on a Bucephalid trematode infectionin scallops (Pecten alba) in Port Phillip Bay, Victoria.Australian Journal of Marine and Freshwater Re-search, 32: 475-478.

SOKAL, R.R. & ROHLF, F.J. 1981. Biometry. Freeman& Company, San Francisco.

SZIDAT, L. 1963. Los parasitos de los mitilidos y losdanos por ellos causados. Neotropica, 9: 80-85.

TURNER, H.M. 1985. Parasites of eastern oysters fromsubtidal reefs in a Louisiana estuary with a note ontheir use as indicators of water quality. Estuaries, 8:323-325.

UMIJI, S., LUNETTA, J.E. & LEONEL, R.M.V. 1976.Infestation of the mussel Perna perna by digenetictrematodes of the Bucephalidae family, gen.Bucephalus. Anais Academia Bias Ciencias, 47(suppl.): 115-117.

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