PARASITES OF SOUTH AFRICAN FRESHWATER FISH. I. SOME NEMATODES OF THE CATFISH [CLARIAS GARIEPINUS(BURCHELL, 1822)] FROM THE HARTBEES POORT DAM

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1 Onderstepoon J. vet. Res., 49, 4 5 (982) PARASITES O SOUTH ARICAN RESHWATER ISH. I. SOE NEATODES O THE CATISH [CLARIAS GARIEPINUS(BURCHELL, 822)] RO THE HARTBEES POORT DA J. BOOKER, Department of Parasitology, aculty of Veterinary Science, University of Pretoria, P.O. Box 258, Onderstepoort, ABSTRACf BOOK.ER, J., 982. Parasites of South African freshater fish. I. Some nematodes of the catfish [Clarias gariepinus (Burchell, 822)] from the Hartbeespoort Dam. Onderstepoort Journal of Veterinary Research, 49, 45 (982). A seasonal study of the parasites of fish in the Hartbeespoort Dam as undertaken in 979. This paper deals ith 4 nematode species recovered from catfish, namely, Paracamallanus cyathoplu!rynx (Baylis, 92), Procamallanus Laeviconchus (Wedl, 862), Contracaecum sp. and Skrjabinocara sp. Total numbers of parasites recovered are tabulated and their seasonal variation illustrated diagrammatically. Paracamallanus cyathoplu! rynx as recovered from 2 out of 4 catfish examined and Procamallanus Laeviconchus from, hile Contracaecum sp. larvae ere present in all the catfish. Skrjabinocara sp. as recovered from catfish only, but it is not regarded as being parasitic in fish, as it as also recovered from out of 4 cormorant examined. Paracamallanus cyathoplu!rynx and Procamallanus laeviconchus are illustrated and the measurements of the Hartbeespoort Dam material compared ith those given by various authors ho recovered the same parasites from other fish species elsehere in Africa. INTRODUCTION Although the parasites of freshater fish in Africa have already received considerable attention (Khalil, 969, 97), those of South African freshater fish have not been studied in any great detail. Khalil ( 97) lists Contracaecum sp. larvae as the only nematodes occurring in catfish. Ortlepp (95), Price, cclellan, Druckenmiller & Jacobs (969) and Lombard (968) mention some parasites they found in fish, and Prudhoe & Hussey ( 977) list a fe species of parasites found in fish in the Transvaal, South Africa. ashego (977) recorded Paracamallanus cyathophilrynx (Baylis, 92), Procamallanus Laeviconchus (Wedl, 862) and Contracaecum sp. larvae in catfish in Leboa, South Africa, for the first time. This paper deals ith some nematodes found in catfish from the Hartbeespoort Dam, as ell as their seasonal variation during 979. ATERIALS AND ETHODS The site All the catfish utilized in this study ere collected from the Hartbeespoort Dam, hich is situated about 4 km to the est of Pretoria, Transvaal (S25 4', E27 5'). The dam is 67 m above sea level and has a total surface area of 2 ha hen full (Dept. of Water Affairs, 964, cited by Steijn, Toerien & Visser, 975). The dam is highly eutrophic (Steijn et al., 975), nitrogen and phosphorus entering the dam mostly through the Crocodile River (ig. ). ish ere collected from areas hich, by a number of trial nettings, they ere knon to frequent (ig. ). Site No. 2 yielded most of the catfish, but some ere also caught at sites Nos. and. Collection of fish Usually fish ere caught ith seine nets, except in those cases hen the quota could not be met. In such cases additional fish ere caught ith handlines. An attempt as made to collect 5 specimens on each collecting trip over consecutive months. During the coldest months, June and July, hoever, no fish ere caught, because they moved to ater that as too deep to be netted, and attempts to catch them ith handlines also failed. Received 9 November 98Editor 4 Collection of parasites Immediately after being landed, fish ere examined macroscopically for ectoparasites. All visible parasites ere then collected in 7% ethyl alcohol and their sites of attachment noted. Large fish ere transported alive to the laboratory, here they ere killed and smears ere taken from the blood, gills and body. The fish ere then scrubbed ith a bristle brush under running ater, and the ashings sieved onto a sieve ith apertures of 5 p.m. The residue as collected and preserved in % formalin. Small fish ere killed at the collection sites and, after smears of the blood, gills and body ere taken, they ere placed individually in 5% ethyl alcohol. At the laboratory they ere transferred to another container, scrubbed ith ater, and this ater, together ith the 5% alcohol in hich they ere transported, as sieved. The residue as preserved in % formalin. The fish ere opened ventrally ith scissors, and the entire digestive tract, together ith the liver and spleen, as removed. After the mesenterium, liver and spleen had been removed, the stomach and intestines ere opened separately and thoroughly ashed in normal saline. The ashings ere heated to 6 oc in a aterbath, after hich they ere sieved (8 p.,m aperture) and the residue fixed in % formalin. The mucosae of the stomach and intestines of the large fish ere removed by scraping ith a knife or a glass slide and digested as described by Reinecke (97). In the case of small fish, or those ith thinalled stomachs and intestines, the entire organs ere digested in pepsin and HCl (Reinecke, 97). As large numbers of Contracaecum larvae ere found in all the catfish, the entire mesenterium as digested for hour at room temperature. This resulted in the liberation of live larvae, hich ere then fixed in boiling alcoholglycerine (approximately 6 C) and preserved in alcoholglycerine. Blood smears ere made according to standard techniques (Wintrobe, 947). Gill smears ere made by scraping the surface of the gills and smearing the resulting epithelium onto precleaned glass slides. Body smears ere made in the same ay. Impression smears of the spleen and kidney ere made according to the technique described by Ashley & Smith (964). The various smears ere fixed and stained, as described in an earlier paper (Boomker, 98).

2 PARASITES O SOUTH ARICAN RESHWATER ISH. I. The livers of all the fish ere examined macroscopically for parasites and thereafter cut into,5 em cubes. These ere kept in normal saline at 4 C for 2 hours, after hich the tissue as thoroughly ashed and discarded. The saline and ashings ere sieved through a sieve ith 8 p,m apertures and the residue preserved in % formalin. In addition to the above procedures, the gills, sim bladders, abdominal cavities and reproductive organs ere examined macroscopically and then ashed. The ashings ere sieved through a sieve ith 8 p,m apertures and fixed in % formalin. Where organs shoed distinct pathological lesions, tissue blocks ere collected in % neutral buffered formalin for histological examination. The various collections and residues ere examined in a counting chamber ith the aid of a stereoscopic microscope. Total counts ere made, except in the case of very large fish or large volumes, hen aliquots ere prepared and counted. In all cases, total counts of the Contracaecum larvae ere done. To determine the monthly incidence, the mean number of a parasite species as calculated by dividing the total number of that parasite collected during a month by the number of hosts caught during the same month. Additional nematodes of the genera Paracamallanus and Procamallanus from Looss' collection from Egypt and ashego' s collection from Leboa ere loaned from the British useum (Natural History) and compared ith the Hartbeespoort Dam material. or identification purposes, nematodes ere cleared and examined in lactophenol. Draings ere made ith the aid of a Nikon Optiphot microscope ith Nomarski differential interference contrast illumination and a Sankei draing tube. RESULTS Only 4 species of nematodes ere recovered from the 4 catfish examined. They ere Paracamallanus cyathopharyn.x (Baylis, 92) and Procamallanus laeviconchus (Wedl, 862) (Camallamidae), Contracaecum sp. larvae (Anisakidae) and Skrjabinocara sp. (Acuariidae). Of the lastnamed genus, only female and larvae ere recovered. The numbers of parasites recovered from each catfish are given in Table, and in ig. 25 variation in their prevalence is graphically illustrated. Paracamallanus cyathopharyn.x as found in the intestines, especially near the rectum of 5,5% (2) of the fish examined. Procamallanus laeviconchus occurred in the stomachs of only,2% () of the catfish hile Contracaecum sp. larvae ere found in the mesenterium of %. Some Contracaecum sp. larvae ere also recovered from the stomach mucosa. Skrjabinocara sp. as found in the stomach of catfish only. The configuration of the buccal capsule and pharynx of Paracamallanus cyathopharyn.x (ig. 6 & 7) sets it apart from other nematodes of fish. The spicules and tail of the male of the Hartbeespoort Dam material are illustrated in ig. 8, and the female tail and vulvar region in ig. & 2, hile the morphology of Procamallanus laeviconchus of catfish from the Hartbeespoort Dam is illustrated in ig. 8. The 4th larval moults of both species are illustrated in ig. 92. ashego (977), ho as the first to record Paracamallanus cyathopharyn.x and Procamallanus laeviconchus from catfish in South Africa, does not provide any measurements of the nematodes he collected. In Table 2 the measurements of 7 male and 8 female Paracamallanus cyathopharyn.x from the Hartbeespoort Dam catfish are compared ith those of specimens identified by Baylis ( male, female ex coil. Looss, on loan from British useum), as ell as ith measurements given by Baylis (92) for material from Clarias anguillaris from Egypt, and that given by oravec (974) from material from Clarias lazera and C. anguillaris, also from Egypt. The principal measurements of 6 male and 5 female Procamallanus laeviconchus of the Hartbeespoort Dam material are compared ith those given by Baylis (92) in Table, as ell as those of 2 females from Bagrus bayad hich Baylis identified as Procamallanus laeviconchus. DISCUSSION Yorke & aplestone (926) created the genus Paracamallanus for nematodes found in the clariid fish Clarias anguillaris (syn. Heterobranchus anguillaris) from Egypt. These nematodes ere originally described as Camallanus cyathopharyn.x by Baylis (92). Since then the parasites have been recorded from a number of clariid fishes from various countries (Vassiliades, 97; Khalil, 97; oravec, 974; ashego, 977). oravec (974) redescribed Paracamallanus cyathopharyn.x and suggested that Camallanus longitridentatus ernando & urtado, 96 from Clarias batrachus be transferred to the genus Paracamallanus. He also regarded Paracamallanus senegalensis Vassiliades, 97 from Clarias senegalensis as synonymous ith Paracamallanus cyathopharyn.x (oravec, 974). The data in Table 2 indicate that the Hartbeespoort Dam material is considerably larger than the material from Looss' collection. Hoever, the measurements given by Baylis (92) and oravec (974) correspond ell ith those of the Hartbeespoort Dam material. rom Table 2 it is also apparent that there is a considerable variation in the size of this nematode, a fact hich could be attributed to the influence of the host in hich it occurs. Procamallanus laeviconchus (Wedl, 862) as originally recorded from Synodontis schaal from Egypt and described as Cucullanus laeviconchus Wedl, 862, but as subsequently transferred to the genus Camallanus (Railliet & Henry, 95). Baylis (92) compared material from Bagrus bayad from Egypt ith that of Wedl (862) and erected a ne genus, Procamallanus, to hich he assigned his on material as ell as Cucullanus laeviconchus Wedl, 862. oravec (975) stated that Procamallanus laeviconchus is one of the most prevalent nematodes of fish, and that it has been recorded from fish belonging to the families Clariidae, ormyridae, Characidae, Siluriidae, Tetraodontidae and Cichlidae. Until 97, Procamallanus brevis (Kung, 948) and Procamallanus slomei (Southell & Kirschner, 97) ere the only species recorded in South Africa, and both occur in frogs (Ivashkin, Sobolev & Khromova, 97). ashego (977) recorded Procamallanus laeviconchus from the catfish, C. gariepinus from Leboa, South Africa, for the first time. The Procamallanus sp. recovered from the Hartbeespoort Dam catfish resembles Procamallanus laeviconchus but differs from it in that only spicule could be found, and that there is an additional pair of sublateral papillae on the tail of the male. aterial collected by ashego (977) from the Olifants River as examined and found to be similar to that from Hartbeespoort Dam. Despite the differences mentioned above, the Hartbeespoort Dam material is assigned to Procamallanus laeviconchus. 42

3 oravec (974, 975) studied the life cycle of Paracamallanus cyathopharynx and Procamallanus laeviconchus in Egypt and found that esocyclops leukarti (Copepoda) harbours the first larval stages of both these nematodes. The copepod must then be ingested by the catfish to continue the life cycle. The only immature stages that ere found in this study ere the 4th stage larvae and 4th larval moults. The latter are illustrated in ig. 92. This confirms the observations of oravec (974). The numbers of Paracamallanus cyathopharynx and Procamallanus laeviconchus collected on a montly basis are given in ig 2 &. In the case of Paracamallanus cyathopharynx a seasonal variation in parasite burdens seems to occur. Peak orm burdens ere seen in ebruary and again in November, hich are 2 of the hottest months in this country. Beteen these 2 months the numbers of nematodes in the fish declined but ere not completely absent. The surviving adult nematodes acted as source of infection for the copepods, hich are more abundant during the summer months, thereby creating J. BOOKER favourable conditions for the transmission of the parasites. rom ig. 2 it can be seen that 4th stage larvae of Paracamallanus cyathopharynx ere recovered from catfish that ere caught during the armer months, ith the exception of November 979 and January 98. This indicates that the intermediate host is more abundant during these months and that catfish are seasonally infested. Procamallanus laeviconchus seems to be nonseasonal in its infestation rate, as can be seen from ig.. Although some 4th stage larvae ere recovered (Table ), their numbers ere too small for any conclusion to be made as regards their life cycle and seasonal occurrence. The adult orms, hoever, ere more often recovered during the summer months, and especially late summer (January to arch). During these months more adults ere collected than during all the other months together (Table ). This is in agreement ith the observations of Imam (97) and oravec (975). TABLE The total orm burdens recoverd from Clarias gariepinus from Hartbeespoort Dam for the period January, 979January, 98 ish No. Date collected Sex Contracaecum sp. Length (em) L2 L Paracamallanus Procamallanus cyathopharynx laeviconchus (Baylis, 92) (Wed!, 862) L. L " <!) > u ;;;!< A Jan. '79 B* Jan. '79 I Jan. '79 2 eb. '79 eb. '79 4 ar. '79 5 ar. '79 6 ar. '79 7 ar. '79 8 ar. '79 9 Apr. '79 Apr. '79 Apr. '79 2 Apr. '79 ay '79 4 ay '79 5 ay '79 6 ay '79 7 ay '79 8 Aug. '79 9 Aug. '79 2 Sept. '79 2** Sept. '79 22 Sept. '79 2 Oct. '79 24 Oct. '79 25 Nov. '79 26 Dec. '79 27 Dec. '79 28 Dec. '79 29 Dec. '79 Dec. '79 Dec. '79 2 Dec. '79 Dec. '79 4 Dec. '79 5 Dec. '79 6 Dec. '79 7 Jan. '8 8 Jan. '8 9 Jan. '8 4* Jan. '8 4 Jan. '8???? 4 7? ,5 I 7 88,5 27 8, ,5 I ,5 4 7, I 4 4 I I I 2 2 I I 2 I 6 I I =ale L2=2nd stage larvae L=rd stage larvae L 4 =4th stage larvae =emale *To unidentifiable nematodes found in each catfish **One adult Skrjabinocara female, fourth stage larvae and I unidentifiable nematode found in this catfish 4

4 PARASITES O SOUTH ARICAN RESHWATER ISH. I. TABLE 2 Comparative measurements of Paracamallanus cyathopharynx from different hosts+ Author Baylis, 92 oravec, 974 Specimens ex coli. Looss, this paper This paper Host C. anguillaris C. anguillaris and C. lazera Host not given C. gariepinus Length (mm) 5,9 9,2 2,46,54 5,8,75,76 4,9 4,5,45,422,52 Width ,65,2 6,892,4 Oesophagus, length of muscular part ,85, ,8 length of glandular part , Buccal capsule, length * * ,265 72,88,2 idth ,6 7,288,4 Pharynx, length * * ,8 59,865 idth ,462,4 72,88,2 Distance of nerve ring from anterior end ,668,2 92,428 Distance of cervical papillae from anterior end )(!hind nerve ring ,284,6 87,222,8 Trident, length of lateral part ,657,2 59,87,2 length of median part 2 44,257,2 5262,4 Distance of vulvaanus (mm) 4,254,86 Distance of anustail ,2496,6 Distance of vulvatail (mm) 4, 2,45 7,4 2,6 4,655,2 Right spicule, length ,2,8 Left spicule, length External spicular sheath, length Distance of taildoaca 57 + All measurements given as m unless otherise stated * Combined length given as m by Baylis, 92 =ales =emales TABLE Comparative measurements of Procamalanus laeviconchus from different hosts* 28,6,2 28,6, ,49 Author Baylis, 92 Specimens ex collection Looss, this paper This paper Host B. bayad B. bayad C. gariepinus Length (mm),65 Width Buccal capsule length idth Oesophagus, length of muscular part 4 length of glandular part 6 Distance of nerve ring from anterior end Distance of cervical papillae from anterior end Distance of vulvaanus (mm) Distance of anustail Distance of tail vulva (mm) Right spicule, length 5 Left spicule, length 5 Caudal alae, length Distance of tailcloaca 7 Preanal papillae, No. of pairs 9 (8) Postanal papillae, No. of pairs 45 Adana! papillae, No. of pairs I * All measurements given in m unless otherise stated =ales =emales + No males available During the armer months, the intermediate hosts are also expected to increase, especially in an eutrophic dam such as the Hartbeespoort Dam. This increase usually takes place in the armer, shallo ater hich catfish frequent during their spaning activities (Holl, 968; Van der Waal, 97 4) and here they no doubt also become infested by ingesting infested copepods. The species of copepod that acts as intermediate host for Paracamallanus cyathopharynx and Procamallanus laeviconchus in this country has yet to be determined. Larval Contracaecum spp. ere collected from all of the catfish examined, and the numbers recovered are given in Table I. As it is impossible to assign the larvae to a species, a number of piscivorous birds have been collected and their intestinal parasites identified. + 5,5,75,77 5,8 7,28, ,9 2, , ,672,8 8,296,2 42, ,25 59,87, ,44,4 4,849, , , , , ,8 262,2,84,5,, ,8,94,6,,6 6,67,8 not found 8224,2 5259,8 8 (9) + Thomas (97) described some aspects of the life cycle of Contracaecum spiculigerum, under experimental conditions. He found that eggs ere laid in a morulated stage and that the larvae moulted tice ithin the eggs ithout shedding their sheaths. Upon hatching they attach to the substrate by means of the loose anterior ends of the sheaths (Thomas, 97). The larvae lost the sheaths only hen ingested by a suitable fish host (Thomas, 97). Sprent (954) postulated that the Ascarididoidea evolved through the marine arthropods and vertebrates. Because of a lack of hostspecificity the 2nd stage larvae may enter the tissues of a ide variety of invertebrate and vertebrate hosts and may remain there for an indefinite period ithout any further development (Sprent, 44

5 J. BOOKER N A IG. Schematic representation of the Hartbeespoort Dam and the sites at hich the catfish ere caught 7 6 ADULTS a a LARVAE P::. > 5 u 4 :: z z <: 2 J A H J HONTH J A s N D J IG. 2 ean number of Paracamallanus cyathopharynx recovered each month 45

6 PARASITES O SOUTH ARICAN RESHWATER ISH. I. 2,6 C> ADULTS LARVAE 2 c:. p; :> u ::: z z :. J A J ONTH J s N D J IG. ean number of Procammallanus laeviconchus recovered each month ::.,s a...l C<:: :> (._; cr. ') L "7 "Z; < """"' J A J J ONTH A s N D J IG. 4 ean number of 2nd stage Contracaecum larvae recovered each month 46

7 J. BOOKER 45 z J A T J ONTH IG. 5 ean number of rd stage Contracaecum larvae recovered each month J A s N D E ::l.. II) 6 5 fl m 7 8 IG. 6 Anterior extremity of Paracamalanus cyathopharyn.x, ventral vie IG. 7 Anterior extremity of Paracamallanus cyathopharyn.x, lateral vie IG. 8 Posterior extremity of Paracamallanus cyathopharyn.x male, lateral vie 47

8 PARASITES O SOUTH ARICAN RESHWATER ISH. I. Ul, t'm I 9 2 IG. 9 Posterior extremity of Paracamallanus cyathopharynx male, ventral vie IG. Paracamallanus cyathopharynx, spicules; (A) proximal end of right spicule, (B) distal end of right spicule, (C) left spicule IG. Paracamallanus cyathopharynx, posterior end of female IG. 2 Paracamallanus cyathopharynx, vulvar region of female E II) (II '"'t: 4 5 IG. Anterior extremity of Procamallanus laeviconchus, ventral vie IG. 4 Anterior extremity of Procamallanus laeviconchus, lateral vie IG. 5 Posterior end of Procamallanus laeviconchus male, lateral vie 48

9 J. BOOKER (II ""!: E., A pm IG. 6 Procamallanus laeviconchus, right spicule, (A) proximal and (B) distal ends IG. 7 Procamallanus laeviconchus, female uterus IG. 8 Procamallanus laeviconchus, tail of female, lateral vie E II) (II (II IG. 9 Paracamallanus cyathopharynx, 4th moult; shaded part is the buccal capsule of 4th stage larva, dorsal vie IG. 2 Anterior end of Procamallanus aeviconchus, dorsal vie of 4th moult. Shaded part is the buccal capsule of the 4th stage larva IG. 2 Procamallanus laeviconchus, tail of 4th moult, lateral vie 49

10 PARASITES O SOUTH ARICAN RESHWATER ISH. I. 952). Hoever, Thomas (97) shoed that domestic ducks and fol are not susceptible to infection ith Contracaecum spiculigerum, thereby implying a degree of host specificity here the final host is concerned. A tentative scheme for life cycle patterns in the Ascarididoidea as given by Sprent (954). He stated that larvae after hatching are ingested by an intermediate host, hich in tum has to be eaten by the final host and used the life cycle of Contracaecum spiculigerum as an example of this type. A second type of life cycle involves the ingestion of embryonated eggs by an intermediate host. The larvae that hatch from these eggs remain in the tissues of the intermediate host until they are eaten by a second intermediate host or by the final host. It is only in the final host that the larvae ill develop into 4th stage larvae and adult nematodes (Sprent, 954). A nematode that has this type of life cycle is C ontracaecum microcephalum (Sprent, 954). Hartbeespoort Dam supports a large number of ater birds of hich the hitebreasted cormorant (Phalacrocorax carbo), the reed cormorant (Phalacrocorax africanus) and the darter (Anhinga rufa) are the most prevalent predators of fish. Various species of heron, egrets and occasionally the fish eagle (Haliaetus vocifer) also prey on fish, but these are regarded as being of lesser importance in the transmission of Contracaecum because they do not consume the same quantities of fish as the cormorants and are also not as numerous on Hartbeespoort Dam. A possible exception is the cattle egret (Bubulcus ibis) and the little egret (Egretta garzetta), hich are found in large numbers. clachlan & Liversidge ( 978) record the food of both species as including fish, especially the smaller fish, such as Canary kurper (Chetia flaviventris), from hich Contracaecum spp. larvae have also been recovered (Boomker, unpublished data). Egrets may therefore also play a role in the transmission of Contracaecum. This assumption is strengthened by observations of the feeding habits of catfish. During the summer months, large numbers of both species of cormorants and egrets mentioned above breed in the trees hich stand in shallo ater along the banks of the dam. Catfish move into the shallo ater and avidly consume the bird droppings as ell as bird eggs and chick that fall out of the nests. Sometimes as many as 2 birds nest in the same tree and on numerous occasions up to 8 catfish have been caught beneath these trees. During the months that the birds are not breeding, catfish ill move in at night under the trees here the bird roost and consume their droppings (Boomker, unpublished data). alvestuto & OgamboOngoma (978) are of the opinion that the first intermediate host, usually a crustacean, is not needed in the life cycle of Contracaecum. The abovementioned feeding behaviour of catfish seems to support their opinion, but because the eggs of the nematodes are shed in the faeces of the birds in a morulated stage, they must embryonate and hatch in the stomach of the fish intermediate host. Such a process has as yet not been shon to occur in the case of Contracaecum. One ust also remember that the bird droppings are very fluid and disperse immediately after falling into te ater. Catfish ingest only a small amount, that is sieved by the gill rakers hich hold back only the larger undigested pieces. Therefore, most of the Contracaecum eggs are lost and the majority of the rd stage larvae probably result from the ingestion of either the crustacean intermediate host or the ingestion of smaller fish that harbour the parasites.. Th presence of the large numbers of rd stage larvae IS attnbuted to the constant intake of small numbers of either embryonated nematode eggs or infested interme diate hosts. As all the catfish examined in this study ere large fish, over 6 em long (Table ), they have conceivably been exposed to infestation for a number of years. Hoever, the numbers of larvae are probably not directly proportional to the number of infective stages ingested, as not all the larvae ill develop into dormant rd stage larvae. One must consider that the immune mechanisms of the catfish, albeit slo in developing, may cause inhibition and later destruction of the infective stages in the stomach mucosa in a ay similar to that seen ith Haemonchus spp. in ruminants (Reinecke, personal communication, 98). This mechanism, if present in catfish, ill account for the fe 2nd stage larvae recovered. urthermore, the mortality rate of encapsulated rd stage larvae must also be considered, although very fe dead larvae ere recovered in this study. The method employed to liberate the larvae from their protective capsules orked very ell, and minutes after the infested mesenteria had been placed in the digesting fluid, larvae could already be seen moving about freely inside the container. This method resulted in the liberation of % of the larvae. Ortlepp (98) described Contracaecum carlislei from the oesophagus and stomach of icrocarbo africana africanides (= Phalacrocorax africanus africanus). Prudhoe & Hussey (977) state that species of Contracaecum commonly occur in African fisheating birds, namely, Contracaecum micropapillatum (Stossich, 89) in cormorants and pelicans, and Contracaecum microcephalum (Rudolphi, 89) and Contracaecum spiculigerum (Rudolphi, 89) usually in cormorants, pelicans and herons. The latter 2 parasites have been recorded from hite pelican (Pelecanus onocrotalis) and hitebreasted cormorants from lake St Lucia, Natal, by Whitfield & Heeg (977). In the course of this study, reed cormorants and hitebreasted cormorant ere also examined and numerous adult Contracaecum spp. ere recovered. Preliminary studies indicate that they are C. spiculigerum and C. carlislei. It therefore seems reasonable to assume that the Contracaecum larvae found in catfish belong to the species found in the birds. Three 4th stage larvae and female of a nematode belonging to the genus Skrjabinocara ere found in catfish only. Their occurrence in catfish seems to be erratic, as none of the 7 species of Skrjabinocara listed by Yamaguti (96) occur in fish. Skrjabinocara squamatum (Von Linsto, 88) has the idest distribution and has not only been found in Phalacrocorax carbo in Turkestan, the Volga Delta and Adelaide (Australia), but also in Phalacrocorax auritus from Cuba and Phalacrocorax cristatellus from Indochina (Yamaguti, 96). One species, Skrjabinocara buckleyi Ali, 957, has been found in Phalacrocorax niger from India and the others from various fisheating birds in Russia (Yamaguti, 96). The fact that these nematodes have been found in only of the 4 catfish examined has led to the opinion that they ere ingested by the catfish after accidental regurgitation by hitebreasted cormorant hilst feeding the chicks, and that they are not normally parasitic in fish. This vie has been confirmed by the finding of adult male and female Skrjabinocara sp. from the gizzard and stomach of the hitebreasted cormorant examined. ACKNOWLEGEENTS The author ishes to express his gratitude to the Transvaal Department of Nature Conservation for placing the material at his disposal; to essrs. van der ere and A. oolman, Provincial isheries, Hartbeespoort Dam, for assisting ith the collections, and the Pretoria Spinfishing Society for providing some of 5

11 J. BOOKER the material. A special ord of thanks is due to esdes E. Visser and. R. Bron and r I. L. de Vtlhers for their technical assistance and to Dr D. Gibson, British useum (Natural History), London for the loan of material from their collection. REERENCES ASHLEY, L.. & SITH, C. E., 964. Advantages of tissue imprints over tissue sections in studies of blood cell formation. The Progressive ish Culturist, 25, 996. BAYI:IS, H. A., 92. Report on a collection of parasitic nematodes, mamly from Egypt. Parasitology, 5, 248. BOOKER, J., 98. The haemocytology and histology of the haemopoietic organs of South African freshater fish. II. Erythrocytes and thrombocytes of Clarias gariepinus and Sarotherodon mossambicus. Onderstepoort Journal of Veterinary Research, 47, 95. ERNANDO, C. H.. & URTADO, J. I., 96. A study of some helminth parasites of freshater fishes in Ceylon. Zeitschrift for Parasitenkunde, 2, 46. HOLL, A. E., 968. 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