Population dynamics of the monogenean Anoplodiscus cirrusspiralis on the snapper, Pagrus auratus

International Journal for Parasitology 28 (1998) 571-577

Population dynamics of the monogenean Anq&~&.sczks cirrusspiralis on the snapper, Pagrus auratus A.P. West* and F.R. RoubaltS ‘/‘Department

*Fisheries Research Institute, of Parasitology, The University

Cronulla.

N.S. W., Australia Brisbane, Queensland,

of Queen&and,

Australia

Received 14 November 1997; received in revised form 15 January 1998; accepted 16 January 1998

Abstract Populationsof Anoplodiscuscirrusspiraliswere monitored for 1 year on taggedindividual snapperin experimental cageskept in a large on-shorepond with flow-through filtered seawater. The cageswere stockedMh SW& and large fish at either low or high initial density. Irrespectiveof sizeand density, snapperwith light initial inf&tions maintained light infections,whereasfish with heavy initial infectionsshowedlhrctuationsin parasitepopulation sðro~hout the year. Thesedata indicatethat somesnapperhave an innate resistanceto infection by A. cirrusspirrrlis, with lit& evidence for acquiredimmunity induced by heavy infection. Parasite longvity was greater on the pectoral bn than cavdai fin, andgreateron largethan smallfish irrespectiveof fish density;longevity wasgreateron susceptiblel%bthan on resistant fish. Recruitmentand mortality ratesweregreater on the pectoral Sn and in low density cages,but werenot influenced by fork length. 0 1998Australian Society for Parasitology.Publishedby ElsevierScienceLtd. Ke.v words:

Monogenea; Anoplodiscus;

Resistance; Population; Longevity; Sparidae; Pagrus; Aquaculture; Australia

1. hdmhtion

The culture of marine fish has suffered economically from epizootics and chronic infection by ectoparasitic monogeneans [l]. Anoplodiscus cirrusspirulis affects the fins and nasal lamellae of snapper, Pagrus auratus @loch & Schneider) (family Sparidae) cultured in Australia [2]. Ahopjlohiscus tai damages the fins, and, consequently, the commercial value of P. major (synonym of P. auratus, see [3]) cultured in Japan [4]. Populations of A. cirmsspirulis on fish in the wild are small [2, 51, but large populations develop on snapper in captivity [2]. However, there are no data for the long-term

$Corresponding author. E-mail: [email protected].

changesin populations of A. cirrusspirulis on indi-

vidual fish in captivity. In this study we examined changes in populations of A. cirrusspiralis on lar’ge and small snapper held in captivity at high and low stocking densities in 1 m3 experimental cages over a period of 1 year. Susceptibility of snapper to infection was examined with respect to initiaS parasite burden, size of fish and stocking density. Several studies have shown that fish within the same population vary in their susceptibility to monogenean infection (e.g. [6]). However, the majority of these studies have been done with spe&es of Gyrodactylus (e.g. [6, 71) which reproduce: upon the host by polyembryony [8]. The majority of monogeneans, including A. cirrusspiraith, shed their eggs into the water column and infect fish by a shortlived, normally free-swimming oneomiracidium.

SOO20-7519/98 $19.00+0.00 0 1998 Australian Society for Parasitology. Published by Elsevier Science Ltd. Printed in Great Britain PII: SOO20-7519(98)000149

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2. Materials

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and methods

2.1. Experimental facility

Juvenile snapper were collected by handline from the Sydney region and kept for l-2 months in 5000 L tubs onshore at the Fisheries Research Institute at Cronulla, with flow-through, sand-filtered sea water obtained from the bottom of the adjacent Pt. Hacking estuary. The fish were placed in four l-m3 cages for 1 year and fed on a commercial diet. The cages were covered with knotless 15 mm plastic mesh and were floated in a 850 000 L on-shore pool which received sand-filtered sea water from Pt. Hacking. Automated measurements of salinity and temperature in the large pond were made daily. Salinity varied between 30 and 35p.p.t. and temperature increased from a weekly average of 14.7”C at the end of August 1992 to 24.8”C in early February 1993 to decline to 14.5”C in June 1993; weekly averages rarely increased or decreased by more than 1.5% The large pond in which the experimental cages were situated contained 200 snapper (l-2 kg) used in a feeding trial, and although infected with A. cirrusspiralis these fish were not assessed for infection levels. This had no bearing on the present experiment because we were interested in the susceptibility to infection and the size of parasite populations on individual fish in the cages; we were not interested in the total number of parasites within the pond. The position of cages in the pond was rotated to ensure each cage had equal access to infective stages, and different sides of the cage were regularly exposed to the air to kill epibionts. 2.2. Experimental

design

Small (mean = 189, range = 168-199 mm fork length (FL)) and large (277, 202-341 mm FL) snapper were stocked in two of the four cages at low density (three small, three large) and in the other two cages at high density (12 small, 12 large). Four large and four small snapper in the high density cage, and all snapper in the low density cage were collected at intervals of 2 weeks with a soft hand net, anaesthetised in benzocaine (40p.p.m.) and the number of A. cirrusspiralis on the left side pectoral fin and caudal fin surface counted under a

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dissecting microscope. Fish were returned to their respective cages. One low density and 1 high density cage were first sampled on 21/S/ 1992; the other low and high density cages on 25/g/1992. The same fish were always examined and were marked at the start of the experiment with yellow dart tags anchored beneath the dorsal fin (referred to herein as the experimental fish). The other snapper in the high density cages had blue dart tags. At the start of the experiment the number of A. cirrusspiralis were counted and the fish then bathed in fresh water to remove all worms [2]. The fish were placed in their respective cages. Fish mortality did not occur in the low density cages, but in the high density cages about 10% of fish died over the year or their tags were lost because of nipping by other fish; tags were replaced when necessary. There were always at least twice as many fish in the high density cages as in the low density cages. Parasites on tagged fish were counted at the end of the experiment. 2.3. Parasite mortality

counts,

longevity,

recruitment

and

Initial observations showed that 40 p.p.m. benzoCaine did not affect the survival of the worms. At first, all A. cirrusspiralis on the body and fins of the snapper were counted, but this resulted in prolonged handling of the fish, so A. cirrusspiralis on the left pectoral fin and caudal fin surface were counted. The pectoral and caudal fins harbour the vast majority of worms, and A. cirrusspiralis does not favour either side of the fish [2]. During the first 6 months of the experiment parasites were assigned to four size classes: (a) small, recently settled juveniles ~500 pm, (b) juveniles 500-900 pm with poorly developed vitellaria, (c) small adults 9001500 pm with well developed vitellaria, and (d) adults > 1500 pm (for examples see [9]). In the second half of the experiment only the number of parasites on the fins was recorded. Recruitment was expressed as the average number of new infections per 2-week period for each fish over the first 6 months. Date of infection for categories a and b was taken as the time of sampling, whereas infection date for categories c and d were taken as 1 and 2 weeks, respectively, before a particular sample date. West and Roubal (unpublished) found that the

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parasite settles on the body surface but migrates soon afterwards to the fins, and attains sexual maturity at about 3 weeks after settlement. Mortality was expressed as the average number of parasites per fish that died per 2-week period over the first 6 months of the experiment. The date of death was the midpoint between successive samples during which the parasite disappeared. In heavy infections, individual parasites in different cohorts could not be discerned once they matured, so we assumed that the first parasites that settled were the first to die. The fate of individual parasites could be followed with more accuracy in light infections. Longevity was determined as the time in weeks between settlement and death for individual parasites. Statistical tests were done with SAS (Version 6.03, SAS Institute, Cary, N.C.), and a significance level of 0.05 was used. Normality of the data was assessed by the Shapiro-Wilk statistic and frequency plots of the data before and after transformation.

3. Reswlts At the start of the experiment there was no significant correlation (v = 0.26, P = 0.06, n = 56) between the number of A. cirrusspiralis (average = 17, O-93, S.D. = 19.24, n = 56) and FL (av. = 233, 168-341 S.D. = 54.6mm) over all fish in the cages. Furthermore, there was no significant Tdbie 1 Infection by Anoplodiscus Cage

Density

I 2

IOW

3

4

cirrusspiralis

Size

low

high high large small low high

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difference among cages in either the number of parasites (F = 1.87, P = 0.15,3,52 d.f.) (Table 1) or in the FL of snapper (F = 0.22, P = 0.88. 3,56d.f.) (Table 2). The fluctuations in number of A. cirrwspiralis on experimental fish from the four cages are shown in Fig. 1. The graphs show that some fish acquired heavy parasite burdens (>50 worms) whereas others had few or no parasites for most of the experiment. These susceptible fish occurred in all cages and were from both large and small size groups. At the end of the experiment, neither cage (F= 0.44, P = 0.73, 3,25d.f.), fish size (F= 3.83, P = 0.06, 1,25 d.f.) nor cage x size interaction (F = 0.02, P = 0.97, 3,25d.f.) had a significant effect on number of A. cirrusspiralis (Table 1). However, there was a highly sign&ant correlation (r = 0.88, P < 0.001, II = 29) between initial and final count of A. cirrusspiralis due to the consistent level of susceptibility by individual snapper throughout the experiment (Fig. 2). There was a general increase in number of parasites on susceptible fish over the course of the experiment, but no fish showed a continuous increase in infection throughout the experiment; either the levels remained low throughout or there was an increase, decrease and subsequent increase in infection. Experimental fish which had acquired moderate to large numbers (>20) of prasites did not rid themselves completely of infection at any stage. Furthermore, there was no pattern of sim-

in different cages at the beginning and end of experiment

N

Mean

2” 6 24 24 28 28 8 48

26.8 21.18 11.25 22.0 12.0 21.9 16.2

7

Initial count Range o-14 l-78 o-93 O-41 O-93 o-41 O-78 G-93

SD

N

Mean

na 29 22.5 10.2 23.5 12.2 26.4 18.0

6 6 14 7 20 13 12 21

33.8 36.5 43.3 26.0 45.9 22.5 35.2 37.5

Final count Range O-103 9-99 I--ml09 o-72 O-109 o-72 O-103 O-109

S.D

37.4 33.8 38 23.9 .x.2 14.0 34.0 34.4

Density, low-three small, three large fish per cage, high-12 small, 12 large fish per cage; FL, fork length; N, sample size, na, not available; S.D. standard deviation; Size, large fish, mean=277mm, range 202-341 mm FL, small fish, 189mm, 16%-199mm FL.” Initial data for four additional fish incomplete.

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Fig. 1. Populations of Anoplodiscus cirrusspirah counted at 2 week intervals on individual Pagrus auratus in experimental cages. Fish were either small or large and held at either low density (3 small, 3 large fish) or high density (12 small, 12 large fish). Low density cage a, large and small fish and high density cage b, large and small fish first sampled 21/8/1992; low density cage b, large and small fish and high density cage a, large and small fish first sampled 25/9/1992.

Table 2 Fork length (FL, mm) of Pagrus auratus in different cages at the beginning and end of experiment Cage

Density

1

low low high high

2 3 4

Size

N 6

large small low high Difference = final FL-initial

Mean

Initial FL Range

S.D.

N

Mean

Final FL Range

SD.

N

48.2 45.1 57.9 55.4

6 24 24

6

232.1 244.0 236.8 226.6 277.4 188.6

188-320 192-316 168-341 174-324 202-341

57.7 56.7 58.1 52.4 43.6

6 6 24 24 18

287.8 318.8 305.4 308.2 332.5

243-360 255-370 206385 24&370 240-385

168-199

12

264.1

206-314

44.1 30.4

18 12

12

238.3

48

231.7

188-320 168-341

8.74 54.9

12 18

303.3 306.3

243-370 206385

47.3 55.5

12 18

6 24 24 30 30

55.0

Mean 55.2 74.8 60.6 55.2

50.4 71.5 65.0 58.8

Difference Range

SD.

1578 5CLllO 35-124 38-104 IS72 33&124

22.2 21.8 25.1 25.3 15.5 25.6

15-110

23.4

35-122

24.6

FL; see Table 1 for legend.

ultaneous decline in number of parasites among fishes within or between cages. Snapper which were initially in the small category showed a significantly (F= 13.58, P = 0.0013, 1,22d.f.) greater growth than large fish (Table 2),

but there was no significant effect of cage (F = 1.25, P = 0.32, 3, 22d.f.) or cage x size interaction (F = 0.78, P = 0.52, 3, 22 d.f.) on increase in FL. There was no significant correlation (r = -0.25, P = 0.18, n = 30) between amount of increase in

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FL and number of A. cirrusspiralis at the end of the experiment. The average longevity of A. cirrusspiralis on experinle&al fish during the first 24 weeks of the experiment was 9.12 weeks (2-20 weeks, S.D. = 3.9 weeks, n = 306). The longevity of the parasite was significantly greater (F= 16.88, P < 0.001, 1,302d.f.) on the pectoral fin (av. = 9.46 weeks) than on the caudal fin (av. = 6.59 weeks), and was significantly greater (F = 14.56, P c 0.001) on large fish (av. = 9.99 weeks) than on small fish (av. = 7.45 weeks), but did not differ (F = 1.08, P = 0.3) between high density (av. = 8.87 weeks) and low density (av. = 9.22 weeks) cages (Table 3). There was no significant interaction between cages and fish size (F = 3.1, P = 0.08). More parasites were recorded in the low density cages than in the

Table 3 Longevity (number of weeks), rate of recruitment and rate of mortality of Anoplodiscus Rate is given as number per 2-week period Size

high

cirrusspiraiis

N

Longevity Mean Range

SD.

N

Recruitment Mean Range

SD.

N

caudal pectoral

217 89 201 105 164 53 37 52 37 269

9.22 8.87 9.99 7.45 9.74 7.62 11.1 7.3 6.6 9.5

3.22 5.14 3.7 3.7 2.97 3.44 5.8 4.0 4.3 3.7

12 16 14 14 6 6 8 8 28 28

2.12 0.84 1.72 1.06 3.1 1.13 0.68 1.01 0.24 1.15

1.9 0.94 1.9 1.7 2.25 0.76 0.39 1.3 0.33 1.3

12 12 13 11 6 6 7 5 19 24

large small large small large small

See Table 1 for legend.

2-17 2-20 2-20 2-18 2-17 2-16 2-20 2-18 2-17 2-20

575

during first 24 weeks of experiment.

Fin

low high low

57-577

high density cages during the first 24 weeks of the experiment. Recruitment and mortality were expressed as the average number of new infectiotls and rmmber of deaths per fish, respectively, during a %-week period. Recruitment was greater (F= 13.03, P
Fig. 2. Population size of Anoplodiscus cirrusspiralis on the same fish at the start and end of the experiment. Straight line indicates equal population size at start and end of experiment.

Cage

28 (1998)

O-5.7 O-3.5 O-5.67 O-3.5 0.4-5.7 G2.2 (rl (r3.5 O-l.12 o-4.75

Mortality Mean Range _____..--___ 1.63 O-4.42 0.67 0.17.-2.2 1.38 0.17.-4.42 0.88 o-2.2 2.45 0.2.5-4.42 0.8 O-l.3 0.46 0.17-0.75 0.97 0.3-22 0.16 (Ht.73 1.02 o-3.92

SD. 1.5 0.6 1.54 0.6 I .74 0.47 0.24 0.77 0.21 1.12

516

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Fish, irrespective of size, were assigned to two classes of susceptibility to infection: low I 15, high > 15 A. cirrusspiralis at sample 19. Trends in susceptibility were well established by then, yet few fish had lost tags or had died in high density cages. Over all cages, the longevity of A. cirrusspiralis’was significantly greater (F = 26.9, P < 0.001) on fish with high susceptibility (av. = 9.48, 2-20, S.D. = 3.69 weeks, n = 269) than on fish with low susceptibility (av. = 6.03,2-l& SD. = 3.71 weeks, n = 35). When the analysis was restricted to the low density cages, the longevity of A. cirrusspiralis was still significantly greater (F = 37.4, P < 0.001, 1,215 d.f.) on fish with high susceptibility (av. = 9.6, 2-17, S.D. = 3.0 weeks, n = 199) than on fish with low susceptibility (av. = 5.11, 2-10, S.D. = 2.74 weeks, IZ = 18).

4. Discussion Levels of infection by Anoplodiscus spp. on sparid fish in the wild are low [5, lo], but captivity favours heavy infection of some snapper by A. cirrusspiralis (see [2]). The snapper in this study were held in captivity prior to stocking the cages, and some fish became heavily infected (see Fig. 2). Following treatment with fresh water at the start of the experiment to remove all A. cirrusspiralis, those snapper with low initial infections maintained low infections throughout the experiment, whereas snapper with heavy initial infections again acquired heavy infections. These data show that any prior infection in the wild or infection before the start of the experiment by A. cirrusspiralis did not confer subsequent protection. Of particular interest is the finding that some snapper in each cage did not acquire any or only a few A. cirrusspiralis. These fish could not behave in such a way that they avoided the infective oncomiracidia of A. cirrusspiralis for 1 year. Snapper swam around inside the cages, and the cages were rotated and moved in the pond. It is more likely that these fish were exposed to infection but the larvae failed to settle or detached shortly after settlement. It was difficult to discern the small larvae on the surface of anaesthetised snapper, so no estimate of establishment success by A. cirrus-

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spiralis was possible. The mechanism(s) that prevent infection is unknown, but may be due to differences among snapper in the quality or quantity of chemical or physical cues in the mucus or epidermis to which larvae are exposed initially. High density of epithelial mucous cells in salmonids correlated with low susceptibility to Gyroductylus derjuvini [(see [ll]). Differences among mucus of fish are known to affect host-finding by monogeneans [ 121, and chemical analogues of urea affect hatching of Acanthocotyle lobianchi (see [13]). Intraspecific variation in the properties of the mucus or epithelium of snapper may be sufficient to prevent settlement by A. cirrusspiralis. If these differences are controlled genetically (e.g. [14]), selective breeding and other genetic manipulations may be used to confer protection. The decline in parasite populations on individual fish did not occur at the same time on all fish within a cage or among cages, which suggests that adverse environmental factors did no cause the declines, since these effects would be more widespread among the cages, There was never a loss of all parasites, i.e. some adults and/orjuveniles were usually present on the fish during these declines. A more likely explanation is natural parasite mortality and variation in parasite recruitment. Nigrelli and Breder [ 151 found that Epibdellu melleni avoided those regions on the skin of moonfish, Vomer setapinnus, that were previously infected, and referred to a localised “skin immunity”. Anoplodiscus cirrusspiralis did not avoid those parts of the fin of snapper previously infected. Species of Anoplodiscus erode the epidermis of the fins and attach to the basement membrane [2,4, 161, and in heavy infections cause a wide area of proliferated epithelium in the vicinity of the parasite with extensive cellular infiltration beneath the attachment site; this tissue reaction does not extend to the other side of the fin [2]. Roubal and Whittington [16] suggested that adult A. australis were attached permanently to the fins, but West (personal observation) found that some adults move position on the fin once or twice per week and the epidermis heals quickly once the parasite has moved. The available evidence does not support the hypothesis that a localised tissue response affects A. cirrusspiralis adversely.

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The rate of parasite recruitment (number per 2week period) was greater on the pectoral fin than the caudal fin, and was greater in the low density cages irrespective of fish size. The same trend was evident in the mortality rate due to the greater number of parasites present. After settlement, the developing parasite moves from the body surface to the fins. The preference for the pectoral fin has been reported before [2], but it is not clear why one fin is preferred over another. Greater recruitment in the low density cages can be explained if each cage was exposed to a similar number of infective larvae so that individual fish in the low density cages were exposed to and acquired more parasites. The populations of parasites on the large snapper kept within the on-shore pond provided a background level of infective larvae of A. cirrusspiralis.

Once infected, parasites lived longer on the pectoral fin and on large fish, but longevity was independent of fish density. The nature and significance of fin-specific characteristics are not known, but presumably large fish provide more of these resources than do small fish. Alternatively, large fish develop some form of tolerance that permits A. cirrusspiralis to live longer. The cellular and other factors that contribute to variation in the susceptibility of captive snapper to A. cirrusspiralis have yet to be examined. Acknowsledgements-This study is part of an MSc. project by A.W. at The University of Queensland. We are grateful to the Director and staff of the Fisheries Research Institute for the opportunity and the resources to do the project. Part of the project was supported by a grant to F.R. from the Australian Research Council.

References [l] Thoney DA, Ha@ W Jr. Monogenea (Platyhelminthes) as hazards for fish in confinement. Ann Rev Fish Dis 1991;1991:133-153. [2] Roubal FR, Quartararo N, West A. Infection of captive Pagrus auratus @loch & Schneider) by the monogenean,

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577

Anoplodiscus cirrusspiralis Roubal, Arm&age & R&de (Anoplodiscidae) in Australia. J Fish I% 1992;15:409-415. 131 Paulin CD. Pagrus auratus: a new combination for the species known as “snapper” in Austmlaaian waters (Pisces: Sparidae). N Z J Mar Freshwat Res 1990$4:259-265 tai sp. nav. (Monogma: AnoI41 Ogawa K. Anoplodiscus plodiscidae) from cultured red sea bream Pagrus major. Fish Path01 1994;29:5-10. PI Roubal FR, Quartararo N, West A. Spatial and temporal variation in populations and community of ectoparasites on young snapper, Pagrus auratw (B&h & Schneider) (Sparidae), from the wild and captivity at Port Hacking Sydney, Australia. Mar Fresbwat Res 1996;47:585-593. PI Madhavi R, Anderson RM. Variability in the susceptibility of the fish host, Poecilia reticulata, to infection with C+rodactylus bullatarudis (Monogenea). Parasitology 1985; 91:531-544. [71 Scott ME, Robinson MA. Challenge infection of C;.rrodactylus bullatarudis (Monogenea) on guppies, Poecilia reticulata (Peters), following treatment. J Fish Biol 1984;24:581-586. PI Dogiel VA. General parasitofogy. London: Oliver and Boyd, 1964. [91 Roubal FR, Quartararo N. Observations on the pigmentation of the monogeneans, Anoplodkw spp. (Family Anoplodiscidae) in different microhabitats on their sparid teleost hosts. Int J Parasitol 1992:22:459-&X [lOI Roubal FR. Seasonal changes in ectoparasite infection of juvenile yellowfin bream, Acwthopagru.r australis (Gunther) (Pisces: Sparidae), from a small estuary in northern New South Wales. Aust J Mar Freshwat Res 1990;41:41 l-427. Pll Buchmann K, Uldal A. Gyrodactylus derjavini infections in four salmonids: comparative host susceptibihty and site selection of parasites. Dis Aquat Org 1997;28:201-209. WI Kearn CC. Experiments on host-Iinding and host-specificity in the monogenean skin parasite Entohdellu soleae. Parasitology 1967;57:585605. [131 Whittington ID, Keam CC. Effects of urea analogues on hatching and movement of unhatched larvae of monogenean parasite Acanthocotyle lobianchi from skin of Rqja montagui. J Chem Ecol 1990;16:2523-3529. P41 Forward GM, Ferguson MM, Woo PKT. Susceptibility of brook char, Salvelinus fontinalis to the pathogenic haemoflagellate, Cryptobia salmositica, and the inheritance of innate resistance by progenies of resistant fish. Parasitology 1995;111:337-345. El51 Nigrelli RF, Breder CM Jr. The susceptibility and immunity of certain marine fishes to Epibdella melleni, a monogenetic trematode. Zoologica 1934;20:259-269. P61Roubal FR, Whittington ID. Observations of the attachment by the monogenean. Anoplodiscus austruii,s. to the caudal fin of Acanthopagrus uustralis lnt 5 Parasitol 1990;20:307- 314.