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The effect of internal and external X-cell lesions on common dab, Limanda limanda L.
Aquaculture, 67 (1987) 127-133 Elsevier Science Publishers B.V., Amsterdam -
127 Printed in The Netherlands
The Effect of Internal and External X-cell Lesions on Common Dab, Limandu Zimandu L. A. DIAMANT’
and A.H. McVICAR
DAFS Marine Laboratory, P.O. Box 101, Victoria Road, Aberdeen AB9 8DB (Great Britain) ‘Present address: National Center for Mariculture, Israel Oceanographic and Limnological Research Ltd., P.O. Box 1212, Eilat 88112 (Israel) (Accepted 22 January 1987)
ABSTRACT Diamant, A. and McVicar, A.H., 1987. The effect of internal and external X-cell lesions on common dab, Limanda limanda L. Aquaculture, 67: 127-133. X-Cell lesions were found for the first time in internal organs of fish. Studies on X-cell condition in dab from the North Sea suggest that even severely affected fish may fully recover from the disease. X-Cell disease has a potentially serious impact on wild and cultivated fish populations.
INTRODUCTION
X-Cell lesions were named after the polygonal, voluminous so-called X-cells they contain by Brooks- et al. (1969)) who described the general structure of the cells and associated lesions in some detail. To date, these lesions have been reported from over 20 species of marine teleosts, all from temperate or cold regions of both hemispheres. The disease is known primarily among pleuronectids (about 10 species) and gadoids (about five species), but occurs in at least two more families, Gobiidae and Lycodidae (Ito et al., 1976; Stich et al., 1976; Alpers et al., 1977; Desser and Khan, 1982). The greatest significance of X-cell disease is in gadoids in which massive X-cell tumours have been associated with pseudobranchs of Pacific and Atlantic cod ( AIpers et al., 1977; Morrison et al., 1982 1, and Pacific pleuronectids, in which a range of cutaneous and branchial hyperplasias, angioepithelial nodules and papillomas has been reported ( Wellings et al., 1976). The disease has been a source of continuing controversy over the last 20 years, with no firm conclusions being reached as to its definite origin. The lesions are now commonly referred to as ‘pseudoneoplasms’, and believed to represent xenomas produced by an amoeba (Dawe, 1981; Morrison et al., 1982; Watermann and Dethlefsen, 1982; Harshbarger, 1984).
0044-8~6/87/$03,50
0 1987 Elsevier Science Publishers B.V.
128
Various studies have dealt with the histology and fine structure of the X-cell lesions, and occasionally, details of the histopathology involved have also been described (Morrison et al., 1982; McVicar et al., 1987). Relatively little attention, however, has been directed towards understanding the effect the disease has on the individual, or its impact at the population level. The present study investigates the effect of the disease on the survival of a flatfish species which is of commercial importance. ~ATE~AIS
AND ~THODS
Common dab (Limanda Zimanda L. ) were caught by DAFS Research Vessel RV Goldseeker in the Moray Firth (north-east Scotland) during May 1986. The results are based on a total of 266 infected dabs examined, of which 111 were female. Tissue processing for light and electron microscopy was carried out by standard techniques as described by h&Vicar et al. ( 1987). RESULTS
Clinical signs Diseased dabs were ~stin~ishable grossly by their creamy-white coloured gills, and were noticeably lethargic and often emaciated. The survival rate fallowing capture was low in spite of immediate transfer to tanks of flowing sea water, with only about 30% surviving the first 24 h in captivity, and less than 5% the first week. In comparison, over 95% of non-affected dabs survived under the same conditions. Respiration of the diseased fish appeared rapid and shallow, and their survival was limited to tanks which were supplied with strong aeration. Microscopically, a proliferation of X-cells could be seen on the branehial lamella, as described by McVicar et al. ( 1987). Pse~d~branc~iul lesions Affected pseudobranch had a pale, swollen appearance, similar to affected gills. In sections, X-cells infiltrated the pseudobranchial tissue, displacing or replacing normal cells (Fig. 1) . The X-cells usually occurred in the proximal part of the pseudobranch among the chloride cells, but occasionally reached the distal portion of the organ and were found alongside ‘pseudobranch-type cells (see Mattey et al., 1978). No involvement of tissue underlying the pseudobranch was detected.
129
Fig. 1. Section through proximal portion of dab pseudobranch, showing X-cells (X) alongside chloride cells (CC) I Erythrocytes (Efand a lymphocyte (arrowed) are also present. Uranyl acetate and lead citrate f bar = 10 pm). Fig. 2. Kidney X-cell lesion in dab. Masses of X-cells (X) replace hemopoietic pulp adjacent to the renal tubule (T) . Uranyl acetate and lead citrate (bar = 10 p) . Fig. 3. Amoeba-like X-cell (X) attached to the connective theca of an oocyte (0) in dab. A granulocyte ( WBC) seen near the collagen-rich ovarian stroma (arrowed). Uranyl acetate and lead citrate (bar = 5 Grn) . Fig. 4. Intense macrophage activity at the fringes of a dab X-cell gill lesion (M=macrophage; X=X-cell; EC = envelope cells). The electron-dense debris in the phago-lysosomes (arrowed) are probably remnants of collapsed X-cells. Uranyl acetate and lead citrate (bar= 5 m) .
130
Spleen and kidney lesions Spleen and kidney lesions were observed in two cases. In gross examination, the kidney was swollen, pale and ‘spongy’ in texture, and while normal kidney bled profusely when incised, X-cell affected kidney did not. Diseased spleen did not show any gross changes. Microscopically, the lesions were strikingly similar in both organs, appearing as packed clumps of. X-cells in the hemopoietic tissue (Fig. 2 ) . Individual, free floating amoeba-like X-cells could also be observed. In the spleen, reticular connective cells containing bundles of microfil~ents and interconnected by desmosomal tight junctions were frequently observed to envelope the X-cells. X-Celb associated with the gonad All 111 female dab examined with branchial X-cell lesions showed retardation in development of the ovary. The organ size was reduced in length to about l/3-1/4 that of normal dabs of comparable length. Examination of the ovaries indicated all oocytes were immature. When examined with electron microscopy, amoeba-like X-cells were seen in one case attached to the fibrous sheath enveloping oocytes ( Fig. 3 ) . Regression of X-cell lesions Two dabs with moderate to heavy branchial X-cell lesions which survived in captivity showed complete recovery and loss of lesions when examined after 57 days. Fig. 4 shows X-cells in the process of being phagocytised by a massive in~ltration of macroph~ges, probably an important stage in the regression of the X-cell lesions. DISCUSSION
X-Cells were previously known from skin and gills of common dab (Watermann, 1982; McVicar et al., 1987). The present report extends X-cells also to the pseudobranch, kidney, spleen and ovary. Internal X-cell lesions have never before been reported from fish, ~though samples of visceral organs were often examined for X-cells in fish displaying external lesions (Alpers et al., 1977; Desser and Khan, 1982). Kidney and spleen lesions, although present in dab, do not appear to be commonplace, and could possibly represent secondary infection sites. It seems unlikely that X-cells stem from haemopoietic cells, as in such a case we would expect to find X-cells in kidney and spleen at least as frequently as in the gills. The replacement of functional haemopoietic tissue with nests of X-cells and fibroblasts could adversely affect formation of both red and white blood cells, thus being detrimental not only to the efficiency of
131
respiration, but also to the cellular immune response. As in Atlantic cod (Morrison et al., 1982)) protozoan and bacterial infections in dab could be facilitated by the poor clinical condition of the fish. For example, the bodonid flagellate lchthyobodo sp. was observed to invade crypts left behind by disintegrating Xcells in dab gill (Diamant, 1987). Pseudobranch X-cell lesions in dab were mo~hologically more similar to the diffuse X-cell gill lesions of eelpout (Desser and Khan, 1982 ) than to pseudobranchial lesions described from Pacific cod ( Alpers et al., 1977)) Atlantic cod (Lange and Johannessen, 1977; Morrison et al., 1982) or pollack ( McCain et al., 1979). No formation of massive tumours was evident, nor was there any indication of involvement of layers underlying the organ as often observed in cod. The pseudobranchial integrity was as a rule disrupted by invasion of Xcells, a condition rarely found in cod ( Morrison et al., 1982 ) . It is not clear whether diffuse X-cell lesions such as those found in eelpout (Desser and Khan, 1982)) Chilean hake (Gorgollon et al., 1982) or common dab ( McVicar et al., 1987) develop as rapidly as massive pseudobranchial tumours in cod (Morrison et al., 1982). However, in all these cases functional branchial tissue is replaced by masses of X-cells, a situation which was thought to affect respiratory function (Desser and Khan, 1982; McVicar et al., 1987). The present results provide clear evidence that branchial X-cell lesions severely reduce the capacity of dab to withstand the stress of capture and transfer to holding tanks. This can clearly relate to respiratory constraints, since survival of the affected fish depended on strong aeration, The reproductive capacity of individual dabs (and consequently of heavily affected populations) was significantly affected by the occurrence of X-cell lesions as indicated by the retardation of ovary development. This result agrees with the observations of Knust and Dethlefsen ( 1987), working with the same species in the southern North Sea. Similarly, the stress caused by rapidly developing pseudobranchial lesions was assumed to restrict gonadal development in Atlantic cod (Morrison et al., 1982). Possible gonadal retardation was also associated with X-cell disease in Pacific cod ( Wellings et al., 1976). In all these cases, stress may indirectly influence gonadal development. However, the association of amoeba-like X-cells with the ovarian follicle in dab suggests that a more direct link between X-cell disease and the gonad may exist in all these cases. To the best of our knowledg,e, there have been no reports as yet on the occurrence of X-cell disease in cultured fish. It seems clear that if introduced, Xcell disease could present a serious threat to young cod and flatfish cultures. Cod is a traditional favourite with British consumers, but its supplies in Britain have continually fallen over the past two decades, and future stocks may depend at least in part on farming (Jones, 1984).Recent cod culture experiments in Norway and the United Kingdom have produced good results (Kvenseth and Oiestad, 1984).Similarly, flatfish stocks are steadily declining, and
132
rearing of flatfish such as turbot, Dover sole, plaice (in Europe) and winter and summer flounder (in North America) have been carried out successfully (Danielsson et al., 1981; Jones et al., 1981). The epidemiological data collected in the field studies present strong evidence that we are dealing with an infectious disease, and as with Pacific flatfish and cod, that young dab are predominantly affected by the condition (Diamant and Mc’dicar, in press). The present study showed that spontaneous regression of X-cell lesions can occur. Recovery from an infectious disease often results in acquired immunity, and this could explain the absence of X-cell lesions in larger fish. It seems likely that mortality due to the condition would also be a contributing factor (see Stich et al., 1976; Morrison et al., 1982). ACKNOWLEDGEMENTS
We thank Miss Carey Fraser for her skillful assistance in the field and laboratory work. This study was partly supported by British Council and G. Meyerbaum Oceanography Foundation post-doctoral fellowships (to A.D. ) .
REFERENCES Alpers, C.E., McCain, B.B., Myers, M.S., Wellings, S.R., Poore, M., Bagshaw, J. and Dawe, C.J., 1977. Pathologic anatomy of pseudobranch tumors in Pacific cod, Gadus mtacrocephatus.J. Natl. Cancer Inst., 59: 377-398. Brooks, R.E., McAm, G.E. and Wellings, S.R., 1969. Ultrastructural observations on an unidentified cell type found in epidermal tumors of flounders. J. Natl. Cancer Inst., 43: 97-110. Danielsson, D.S., Moksness, E. and Oiestad, V., 1981. Duoculture of plaice (Pleuronectesplatessa L.) and lobster (Homarw gammarus L.) fry in two concrete enclosures based on natural production. Rapp. P.-v. Reun., Cons. Int. Explor. Mer, 178: 511-513. Dawe, C.J., 1981. Poiyoma tumors in mice and X-cell tumors in fish viewed through telescope and microscope. In: C.J. Dawe, J.C. Harshbarger, S. Kondo, T. Sugimura and S. Takayama (Editom), Phyletic Approaches to Cancer. Japan Sci. Sot. Press, Tokyo, pp. 19-49. Desser, S.S. and Khan, R.A., 1982. Light and electron microscope observations on patholo~c~ changes in the gills of the marine fish Lycodes Zava~ae~ Vladykov and Tremblay associated with the proliferation of an unidentified cell. J. Fish Dis., 5: 351-364. Diamant, A., 1987. Ultrastructure and pathogenesis of Ichthyobodo sp. from wild common dab, Limanda limanda L., in the North Sea. J. Fish Dis., 10: 241-247. Diamant, A. and McVicar, A.H., in press. Distribution of X-cell disease in common dab, Limanda Zimanda L., in the North Sea, and ultrastructural observations of previously undescribed developmental stages. J. Fish Dis. Gorgollon, P., Alfaro, A. and Kunzar, J., 1982. Caracterizacion morfologica de un tumor branquial en la Merluza (Merluccius guyi gayi). Rev. Biol. Mar., 18: 159-181. Harshbarger, J.C., 1984. Pseudoneoplasms in ectothermic animals. Natl. Cancer Inst., Monogr., 65: 251-273. Ito, Y., Kimura, I. and Miyake, T., 1976. Histopsthological and virological investigations of papillomas in soles and gobies in coastal waters of Japan. Prog. Exp. Tumor Res., 20: 86-93. Jones, A., 1984. The future of cod farming and the responsib~ity of restocking and possibly of
133 restocking local coastal populations: panel discussion. The propagation of cod Gadus morhua 1;. An International Symposium, Arendal, Norway. Flodev. Rapp., 1: 773-785. Jones, A., Prickett, R.A. and Douglas, M.T., 1981. Recent developments in techniques for rearing marine flatfish larvae, particularly turbot (Scophthalus maximus L.) , on a pilot commercial scale. Rapp. P.-V. Reun., Cons. Int. Explor. Mer, 178: 522-526. Knust, R. and Dethlefsen, V., 1987. X-Cells in gills of North Sea dab (Limanda Zimanda L.) epizootiolo~ and impact on condition. Arch. Fischereiwiss, 37: 11-24. Kvenseth, P.G. and Oiestad, V., 1984. Large scale rearing of cod fry on the natural food production in an enclosed pond. Flodev. Rapp., 1: 645-655. Lange, E. and Johannessen, J.V., 1977. Hi&chemical and ultrastructural studies of chemodectoma-like tumors in the cod (Gadw morrhua L.). Lab. Invest., 37: 96-103. Mattey, D.L., Moate, R. and Morgan, M., 1978. Comparison of ‘pseudobranch’type and ‘chloride’ type cells in the pseudobranch of marine, freshwater and euryhaline teleosts. J. Fish Biol., 13: 535-542. McCain, B.B., Gronlund, W.D., Myers, MS. and Wellings, S.R., 1979. Tumours and microbial diseases of marine fishes in Alaskan waters. J. Fish Dis;, 2: 111-130. McVicar, A.H., Bucke, D., Watermann, B. and Dethlefsen, V., 1987. Gill X-cell lesions of the dab Limanda l~rnand~ L. in the southern North Sea. Dis. Aquat. Org., 2: 197-204. Morrison, C.A., Shum, G., Appy, R.G., Odense, P. and Annand, C., 1982. Histology and prevalence of X-cell lesions in Atlantic cod (Gadus mo~hua). Can. J. Fish. Aquat. Sci., 39: 1519-1530. Stich, H.F., Acton, A.B. and Forrester, C.R., 1976. Fish tumors and sublethal effects of pollutants. J. Fish. Res. Board Can., 33: 1993-2001. Watermann, B., 1982. An unidentified cell type associated with an inflammatory condition of the subcutaneous connective tissue in dab (Limanda limanda) . J. Fish Dis., 2: 257-261. Watermann, B. and Dethlefsen, V., 1982. Histology of pseudobranchial tumors in Atlantic cod (Gadus morhua) from the North Sea andBaltic Sea. Helgol. Wiss. Meeresunters., 35: 231-242. Wellings, S.R., McCain, B.B. and Miller, R.S., 1976. Epidermal papillomas in Pleuronectidae of Puget Sound, Washington. Review of the current status of the probelm. Prog. Exp. Tumor. Res., 20: 55-74,
127 Printed in The Netherlands
The Effect of Internal and External X-cell Lesions on Common Dab, Limandu Zimandu L. A. DIAMANT’
and A.H. McVICAR
DAFS Marine Laboratory, P.O. Box 101, Victoria Road, Aberdeen AB9 8DB (Great Britain) ‘Present address: National Center for Mariculture, Israel Oceanographic and Limnological Research Ltd., P.O. Box 1212, Eilat 88112 (Israel) (Accepted 22 January 1987)
ABSTRACT Diamant, A. and McVicar, A.H., 1987. The effect of internal and external X-cell lesions on common dab, Limanda limanda L. Aquaculture, 67: 127-133. X-Cell lesions were found for the first time in internal organs of fish. Studies on X-cell condition in dab from the North Sea suggest that even severely affected fish may fully recover from the disease. X-Cell disease has a potentially serious impact on wild and cultivated fish populations.
INTRODUCTION
X-Cell lesions were named after the polygonal, voluminous so-called X-cells they contain by Brooks- et al. (1969)) who described the general structure of the cells and associated lesions in some detail. To date, these lesions have been reported from over 20 species of marine teleosts, all from temperate or cold regions of both hemispheres. The disease is known primarily among pleuronectids (about 10 species) and gadoids (about five species), but occurs in at least two more families, Gobiidae and Lycodidae (Ito et al., 1976; Stich et al., 1976; Alpers et al., 1977; Desser and Khan, 1982). The greatest significance of X-cell disease is in gadoids in which massive X-cell tumours have been associated with pseudobranchs of Pacific and Atlantic cod ( AIpers et al., 1977; Morrison et al., 1982 1, and Pacific pleuronectids, in which a range of cutaneous and branchial hyperplasias, angioepithelial nodules and papillomas has been reported ( Wellings et al., 1976). The disease has been a source of continuing controversy over the last 20 years, with no firm conclusions being reached as to its definite origin. The lesions are now commonly referred to as ‘pseudoneoplasms’, and believed to represent xenomas produced by an amoeba (Dawe, 1981; Morrison et al., 1982; Watermann and Dethlefsen, 1982; Harshbarger, 1984).
0044-8~6/87/$03,50
0 1987 Elsevier Science Publishers B.V.
128
Various studies have dealt with the histology and fine structure of the X-cell lesions, and occasionally, details of the histopathology involved have also been described (Morrison et al., 1982; McVicar et al., 1987). Relatively little attention, however, has been directed towards understanding the effect the disease has on the individual, or its impact at the population level. The present study investigates the effect of the disease on the survival of a flatfish species which is of commercial importance. ~ATE~AIS
AND ~THODS
Common dab (Limanda Zimanda L. ) were caught by DAFS Research Vessel RV Goldseeker in the Moray Firth (north-east Scotland) during May 1986. The results are based on a total of 266 infected dabs examined, of which 111 were female. Tissue processing for light and electron microscopy was carried out by standard techniques as described by h&Vicar et al. ( 1987). RESULTS
Clinical signs Diseased dabs were ~stin~ishable grossly by their creamy-white coloured gills, and were noticeably lethargic and often emaciated. The survival rate fallowing capture was low in spite of immediate transfer to tanks of flowing sea water, with only about 30% surviving the first 24 h in captivity, and less than 5% the first week. In comparison, over 95% of non-affected dabs survived under the same conditions. Respiration of the diseased fish appeared rapid and shallow, and their survival was limited to tanks which were supplied with strong aeration. Microscopically, a proliferation of X-cells could be seen on the branehial lamella, as described by McVicar et al. ( 1987). Pse~d~branc~iul lesions Affected pseudobranch had a pale, swollen appearance, similar to affected gills. In sections, X-cells infiltrated the pseudobranchial tissue, displacing or replacing normal cells (Fig. 1) . The X-cells usually occurred in the proximal part of the pseudobranch among the chloride cells, but occasionally reached the distal portion of the organ and were found alongside ‘pseudobranch-type cells (see Mattey et al., 1978). No involvement of tissue underlying the pseudobranch was detected.
129
Fig. 1. Section through proximal portion of dab pseudobranch, showing X-cells (X) alongside chloride cells (CC) I Erythrocytes (Efand a lymphocyte (arrowed) are also present. Uranyl acetate and lead citrate f bar = 10 pm). Fig. 2. Kidney X-cell lesion in dab. Masses of X-cells (X) replace hemopoietic pulp adjacent to the renal tubule (T) . Uranyl acetate and lead citrate (bar = 10 p) . Fig. 3. Amoeba-like X-cell (X) attached to the connective theca of an oocyte (0) in dab. A granulocyte ( WBC) seen near the collagen-rich ovarian stroma (arrowed). Uranyl acetate and lead citrate (bar = 5 Grn) . Fig. 4. Intense macrophage activity at the fringes of a dab X-cell gill lesion (M=macrophage; X=X-cell; EC = envelope cells). The electron-dense debris in the phago-lysosomes (arrowed) are probably remnants of collapsed X-cells. Uranyl acetate and lead citrate (bar= 5 m) .
130
Spleen and kidney lesions Spleen and kidney lesions were observed in two cases. In gross examination, the kidney was swollen, pale and ‘spongy’ in texture, and while normal kidney bled profusely when incised, X-cell affected kidney did not. Diseased spleen did not show any gross changes. Microscopically, the lesions were strikingly similar in both organs, appearing as packed clumps of. X-cells in the hemopoietic tissue (Fig. 2 ) . Individual, free floating amoeba-like X-cells could also be observed. In the spleen, reticular connective cells containing bundles of microfil~ents and interconnected by desmosomal tight junctions were frequently observed to envelope the X-cells. X-Celb associated with the gonad All 111 female dab examined with branchial X-cell lesions showed retardation in development of the ovary. The organ size was reduced in length to about l/3-1/4 that of normal dabs of comparable length. Examination of the ovaries indicated all oocytes were immature. When examined with electron microscopy, amoeba-like X-cells were seen in one case attached to the fibrous sheath enveloping oocytes ( Fig. 3 ) . Regression of X-cell lesions Two dabs with moderate to heavy branchial X-cell lesions which survived in captivity showed complete recovery and loss of lesions when examined after 57 days. Fig. 4 shows X-cells in the process of being phagocytised by a massive in~ltration of macroph~ges, probably an important stage in the regression of the X-cell lesions. DISCUSSION
X-Cells were previously known from skin and gills of common dab (Watermann, 1982; McVicar et al., 1987). The present report extends X-cells also to the pseudobranch, kidney, spleen and ovary. Internal X-cell lesions have never before been reported from fish, ~though samples of visceral organs were often examined for X-cells in fish displaying external lesions (Alpers et al., 1977; Desser and Khan, 1982). Kidney and spleen lesions, although present in dab, do not appear to be commonplace, and could possibly represent secondary infection sites. It seems unlikely that X-cells stem from haemopoietic cells, as in such a case we would expect to find X-cells in kidney and spleen at least as frequently as in the gills. The replacement of functional haemopoietic tissue with nests of X-cells and fibroblasts could adversely affect formation of both red and white blood cells, thus being detrimental not only to the efficiency of
131
respiration, but also to the cellular immune response. As in Atlantic cod (Morrison et al., 1982)) protozoan and bacterial infections in dab could be facilitated by the poor clinical condition of the fish. For example, the bodonid flagellate lchthyobodo sp. was observed to invade crypts left behind by disintegrating Xcells in dab gill (Diamant, 1987). Pseudobranch X-cell lesions in dab were mo~hologically more similar to the diffuse X-cell gill lesions of eelpout (Desser and Khan, 1982 ) than to pseudobranchial lesions described from Pacific cod ( Alpers et al., 1977)) Atlantic cod (Lange and Johannessen, 1977; Morrison et al., 1982) or pollack ( McCain et al., 1979). No formation of massive tumours was evident, nor was there any indication of involvement of layers underlying the organ as often observed in cod. The pseudobranchial integrity was as a rule disrupted by invasion of Xcells, a condition rarely found in cod ( Morrison et al., 1982 ) . It is not clear whether diffuse X-cell lesions such as those found in eelpout (Desser and Khan, 1982)) Chilean hake (Gorgollon et al., 1982) or common dab ( McVicar et al., 1987) develop as rapidly as massive pseudobranchial tumours in cod (Morrison et al., 1982). However, in all these cases functional branchial tissue is replaced by masses of X-cells, a situation which was thought to affect respiratory function (Desser and Khan, 1982; McVicar et al., 1987). The present results provide clear evidence that branchial X-cell lesions severely reduce the capacity of dab to withstand the stress of capture and transfer to holding tanks. This can clearly relate to respiratory constraints, since survival of the affected fish depended on strong aeration, The reproductive capacity of individual dabs (and consequently of heavily affected populations) was significantly affected by the occurrence of X-cell lesions as indicated by the retardation of ovary development. This result agrees with the observations of Knust and Dethlefsen ( 1987), working with the same species in the southern North Sea. Similarly, the stress caused by rapidly developing pseudobranchial lesions was assumed to restrict gonadal development in Atlantic cod (Morrison et al., 1982). Possible gonadal retardation was also associated with X-cell disease in Pacific cod ( Wellings et al., 1976). In all these cases, stress may indirectly influence gonadal development. However, the association of amoeba-like X-cells with the ovarian follicle in dab suggests that a more direct link between X-cell disease and the gonad may exist in all these cases. To the best of our knowledg,e, there have been no reports as yet on the occurrence of X-cell disease in cultured fish. It seems clear that if introduced, Xcell disease could present a serious threat to young cod and flatfish cultures. Cod is a traditional favourite with British consumers, but its supplies in Britain have continually fallen over the past two decades, and future stocks may depend at least in part on farming (Jones, 1984).Recent cod culture experiments in Norway and the United Kingdom have produced good results (Kvenseth and Oiestad, 1984).Similarly, flatfish stocks are steadily declining, and
132
rearing of flatfish such as turbot, Dover sole, plaice (in Europe) and winter and summer flounder (in North America) have been carried out successfully (Danielsson et al., 1981; Jones et al., 1981). The epidemiological data collected in the field studies present strong evidence that we are dealing with an infectious disease, and as with Pacific flatfish and cod, that young dab are predominantly affected by the condition (Diamant and Mc’dicar, in press). The present study showed that spontaneous regression of X-cell lesions can occur. Recovery from an infectious disease often results in acquired immunity, and this could explain the absence of X-cell lesions in larger fish. It seems likely that mortality due to the condition would also be a contributing factor (see Stich et al., 1976; Morrison et al., 1982). ACKNOWLEDGEMENTS
We thank Miss Carey Fraser for her skillful assistance in the field and laboratory work. This study was partly supported by British Council and G. Meyerbaum Oceanography Foundation post-doctoral fellowships (to A.D. ) .
REFERENCES Alpers, C.E., McCain, B.B., Myers, M.S., Wellings, S.R., Poore, M., Bagshaw, J. and Dawe, C.J., 1977. Pathologic anatomy of pseudobranch tumors in Pacific cod, Gadus mtacrocephatus.J. Natl. Cancer Inst., 59: 377-398. Brooks, R.E., McAm, G.E. and Wellings, S.R., 1969. Ultrastructural observations on an unidentified cell type found in epidermal tumors of flounders. J. Natl. Cancer Inst., 43: 97-110. Danielsson, D.S., Moksness, E. and Oiestad, V., 1981. Duoculture of plaice (Pleuronectesplatessa L.) and lobster (Homarw gammarus L.) fry in two concrete enclosures based on natural production. Rapp. P.-v. Reun., Cons. Int. Explor. Mer, 178: 511-513. Dawe, C.J., 1981. Poiyoma tumors in mice and X-cell tumors in fish viewed through telescope and microscope. In: C.J. Dawe, J.C. Harshbarger, S. Kondo, T. Sugimura and S. Takayama (Editom), Phyletic Approaches to Cancer. Japan Sci. Sot. Press, Tokyo, pp. 19-49. Desser, S.S. and Khan, R.A., 1982. Light and electron microscope observations on patholo~c~ changes in the gills of the marine fish Lycodes Zava~ae~ Vladykov and Tremblay associated with the proliferation of an unidentified cell. J. Fish Dis., 5: 351-364. Diamant, A., 1987. Ultrastructure and pathogenesis of Ichthyobodo sp. from wild common dab, Limanda limanda L., in the North Sea. J. Fish Dis., 10: 241-247. Diamant, A. and McVicar, A.H., in press. Distribution of X-cell disease in common dab, Limanda Zimanda L., in the North Sea, and ultrastructural observations of previously undescribed developmental stages. J. Fish Dis. Gorgollon, P., Alfaro, A. and Kunzar, J., 1982. Caracterizacion morfologica de un tumor branquial en la Merluza (Merluccius guyi gayi). Rev. Biol. Mar., 18: 159-181. Harshbarger, J.C., 1984. Pseudoneoplasms in ectothermic animals. Natl. Cancer Inst., Monogr., 65: 251-273. Ito, Y., Kimura, I. and Miyake, T., 1976. Histopsthological and virological investigations of papillomas in soles and gobies in coastal waters of Japan. Prog. Exp. Tumor Res., 20: 86-93. Jones, A., 1984. The future of cod farming and the responsib~ity of restocking and possibly of
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