A comparative study of the egg morphology in four species of Eubothrium (Cestoda: Pseudophyllidea) with comments on their early development

Abstract. Freshly released eggs from four species of the cestode Eubothrium ( Eubothrium crassum, Eubothrium fragile, Eubothrium rugosum , and Eubothrium salvelini ) were subjected to morphological and morphometric analysis. The eggs of the two freshwater species, E. rugosum and E. salvelini , were ovoid with a lobed embryophore whereas the eggs of the two marine species, E. crassum and E. fragile , were more circular with a smooth embryophore. However, the morphological differences between species were not readily evident to permit their clear distinction from one another. To discriminate species, a forward stepwise linear discriminant analysis, using six of the seven measured metric characters made on the eggs, was used, which gave 100% correct classification of two species, E. rugosum and E. salvelini , and a high proportion of correct classification for E. crassum (98%) and E. fragile (83%). Of the latter two species, one specimen of E . crassum and five specimens of E. fragile were misclassified between the respective groups. The principal characters used in the classification of the species were the width of the egg, the length of the mediolateral hooks, and the width of the oncosphere. To provide more information on the life cycle of each species, the eggs were used in a series of infection trials to identify appropriate intermediate hosts. Experimental infections with freshwater copepods were successful when exposed to the eggs of E . salvelini , partially successful when exposed to the eggs of marine E. crassum with 10% of the copepods becoming infected, but no infections were obtained when the eggs of E. fragile were used.     

Host specificity and geographical distribution of Eubothrium in European salmonid fish

The host specificity and distribution of Eubothrium crassum (Bloch, 1779) and Eubothrium salvelini (Schrank, 1790), morphologically fairly similar pseudophyllidean tapeworms parasitizing salmonid fish, were critically assessed on the basis of morphological and genetic evaluation of extensive material collected from different definitive hosts and geographical regions in Europe. Eubothrium crassum occurs in fish of the genera Salmo, i.e. salmon (S. salar - both freshwater and marine), sea trout (S. trutta trutta), brown trout (S. trutta fario), and lake trout (S. trutta lacustris), and also in Danubian salmon (Hucho hucho) and vendace (Coregonus albula). Eubothrium salvelini parasitizes Arctic char (Salvelinus alpinus) and brook trout (Salvelinus fontinalis) in Europe, and also whitefish (Coregonus wartmanni). Rainbow trout (Oncorhynchus mykiss), which is not a native European fish species, was found to be a suitable definitive host for both Eubothrium species, which may occur simultaneously in the same fish. Previous records of E. crassum in Arctic char and brook trout, and those of E. salvelini in fish of the genus Salmo were most probably misidentifications. Most studies of Eubothrium have involved salmonids from the northern part of Europe, with few records from southern and south-eastern Europe. This study also confirmed the reliability of the morphology of the apical disc for the discrimination of E. crassum and E. salvelini.     

A comparison of the internal transcribed spacer of the ribosomal DNA for Eubothrium crassum and Eubothrium salvelini (Cestoda: Pseudophyllidea), parasites of salmonid fish

The internal transcribed spacers (ITS-1 and ITS-2) and 5.8S subunit of the ribosomal RNA gene of Eubothrium crassum and Eubothrium salvelini, cestode parasites of salmonid fish, were sequenced and compared. The lengths and GC contents of the regions sequenced were 1119 bp and 50.1% for E. crassum, and 1117 bp and 50.5% for E. salvelini, respectively. Sequence alignment and comparison of both taxa showed 97.9% similarity. Within the ITS-1 and ITS-2 of both species, the restriction enzymes MspI and Bsh1236I possessed different cleavage sites and could be employed for rapid differentiation of both species of Eubothrium by PCR-RFLP.     

Studies on the biology of Eubothrium salvelini and E. crassum in resident and migratory Salvelinus alpinus and Salmo trutta and in S. salar in North Norway and the islands of Spitsbergen and Jan Mayen

Samples of Eubothrium salvelini and E. crassum were obtained from resident and migratory Salvelinus alpinus and Salmo trutta and from S. salar at sea from North Norway and the islands of Spitsbergen and Jan Mayen. Scolex dimensions proved an unsuitable character for identifying these two parasite species, and use of alternative characters confirmed that E. salvelini is specific to Salvelinus and E. crassum to Salmo. Distributions of both species are co-extensive with their hosts, but levels of infection in any locality relate to the abundance of zooplankton. E. salvelini exhibits seasonal maturation and incidence cycles in char. It is recognised as being a freshwater parasite, although some individuals can survive the period of marine migration of sea char. Returning migrant char may contain such individuals together with small plerocerciform parasites acquired almost immediately upon their re-entry to freshwater. No evidence for marine infections with E. salvelini was found. The presence of plerocerciform and gravid adult E. crassum in salmon caught at sea confirms the existence of a marine life cycle and a marine race in this species. In salmon moving from one medium to the other the marine and freshwater races replace each other, but in sea trout, which spend shorter periods at sea, complete replacement may not occur and at any one time fish may harbour individuals of both races. Recognition of the existence of two races clarifies much of the confusion surrounding the biology of this species.     

A Comparative Study of Eubothrium Salvelini and E. crassum (Cestoda: Pseudophyllidea) Parasites of Arctic Charr and Brown Trout in Alpine Lakes

We compared the morphology, morphometry, and some aspects of the biology and ecology of Eubothrium crassum and E. salvelini, two pseudophyllidean tapeworms in brown trout, Salmo trutta m. lacustris, and Arctic charr, Salvelinus alpinus, in lakes of the western part of the Alps. The taxonomic importance of the apical disc for the discrimination of both taxa was confirmed. The apical disc of E. crassum is rectangular, possessing two deep grooves above the bothriae and 2–16 additional indentations. In E. salvelini, the apical disc is bilobed and bilaterally symmetric. We found new and significant interspecific differences in the length and width of the scolex, the diameter of the apical disc, and the width of the neck, with all measurements being larger in E. crassum. The two species differ in the number and size of the testes (fewer and larger in E. salvelini), the length of the cirrus sac (longer in E. crassum), and the size of vitelline follicles (larger in E. salvelini). Vitelline follicles of E. crassum are always cortical and sometimes enter into the outermost muscle fibres, whereas follicles of E. salvelini are situated largely medullary with few follicles entering between the innermost bundles of muscles. The eggs and the oncospheres of E. salvelini are larger. The embryonic hooks of E. crassum measure 14–18 m whilst those of E. salvelini are 18–26 m. For both taxa, the intraspecific variability in morphometric characters was fairly low. The prevalence and intensity of infection of E. crassum and E. salvelini in their respective fish hosts were very high (prevalence 90–94% mean intensity 36.3 and 6.6, maximums of 172 and 63 tapeworms per fish, respectively). E. salvelini eggs were spontaneously released throughout the year except for winter months; E. crassum laid eggs only in summer (June–August). Natural infection of copepods with Eubothrium procercoids was very low (prevalence 0.002%). The morphogenesis of Eubothrium procercoids was studied in an intermediate host, the copepod Cyclops prealpinus, that had been simultaneously infected with both species.     

Re-description of Eubothrium vittevitellatus Mamaev, 1968 (Cestoda: Pseudophyllidea) from sand fish (Trichodon trichodon Tilesius) in Kamchatka Bay with an assessment of its distribution

Eubothrium vittevitellatus is re-described with particular attention being paid to scolex characters seen by scanning electron microscopy in material obtained from the intestines of the sand fish (Trichodon trichodon) from Kamchatka Bay. It is confirmed as a valid species distinguishable from others in the genus by the combined characters of scolex shape, muscle arrangement and position of vitelline follicles. Peculiarities in the distribution and specificity of the species of Eubothrium highlighted by E. vittevitellatus are discussed.     

Helminths in an intensively stocked population of lake trout, Salvelinus namaycush, from Lake Huron

Eighty stocked lake trout Salvelinus namaycush (Salmonidae), collected from 2 locations in Lake Huron in May 1995, were examined for parasites. The parasite fauna of this top predator in Lake Huron was characterized by only 6 helminth species. Echinorhynchus salmonis infected all lake trout with a mean intensity of 163.9. The intensity of this acanthocephalan species significantly increased with host length and weight. Eubothrium salvelini infected 78 lake trout with a maximum number of 81 scoleces counted. Diplostomum sp., Cyathocephalus truncatus, Capillaria salvelini, and Neoechinorhynchus sp. infrequently infected lake trout. The low parasite species richness in these lake trout is believed to be due to their large size at stocking and to the loss of historical enzootic host-parasite relationships that followed the absence of this fish species in Lake Huron for 26 yr.     

Differences in pyloric caeca morphology between Arctic charr Salvelinus alpinus ecotypes: adaptation to trophic specialization or parasite‐induced phenotypic modifications?

Gut-tract morphology differed between benthivorous and zooplanktivorous Arctic charr Salvelinus alpinus caught in the littoral and pelagic zones of a sub-Arctic lake. The differences were related to trophic niche and infection with the cestodes Cyathocephalus truncatus and Eubothrium salvelini . Measurements that were unrelated to cestode infection, including intestinal length, numbers of pyloric caeca and posterior pyloric caeca morphology, differed between S. alpinus that had consumed different prey types, suggesting an adaptive response to divergent trophic niches. The anterior pyloric caeca of S. alpinus with high numbers of cestodes were significantly wider and shorter than those in fish with lower levels of infection. The differences were strongly associated with infection by C. truncatus , which was site-selective and attached to the anterior caeca with a large scolex. Differences in caecal measurements were probably the result of mechanical damage caused by scolex attachment and host responses to infection with C. truncatus . The differences in anterior caeca were also an indirect indicator of trophic specialization. Benthivorous S. alpinus were more exposed to, and more heavily infected with C. truncatus than were pelagic zooplanktivorous fish, because this cestode had Gammarus lacustris as its intermediate host. Nevertheless, care should be exercized when using gut measurements for making inferences about the trophic ecology of fish because a history of infection with cestodes may have profound effects on caecal morphology.     

The metazoan parasites of freshwater fishes in the Corrib catchment area, Ireland

A survey of the parasites of 243 fish belonging to 13 species, from the Corrib catchment area, western Ireland, was undertaken. At least 32 species of metazoan parasites were shown to be present. Diplostomum gasterostei, Tylodelphys clavata, Eubothrium salvelini, Cucullanus truttae, Raphidascaris acus, Ergasilus gibbus and Salmincola gordoni appear to be first records for Ireland and Phyllodistomum conostomum has not previously been reported from the British Isles. New host records involving Lamptera planeri, Salvelinus alpinus, Abramis brama, Noemacheilus barbatulus, Anguilla anguilla and Pungitius pungitius for the British Isles are given.     

Galaxitaenia toloi n. gen., n. sp. (Eucestoda: Pseudophyllidea) from Galaxias platei (Pisces: Osmeriformes, Galaxiidae), in the Patagonian region of Argentina

Galaxitaenia n. gen. (Eucestoda: Pseudophyllidea) is proposed to accommodate Galaxitaenia toloi n. gen., n. sp. from Galaxias platei, a freshwater fish inhabiting Andean lakes in Argentine Patagonia. Galaxitaenia belongs to the Triaenophoridae because it possesses a marginal genital pore, a ventral uterine pore, and a follicular vitelline gland. The new genus can be distinguished from other triaenophorids by the following combination of characters: a scolex with a prominent rectangular apical disc without hooks, grooves, or indentations; neck present; segments wider than long to quadrangular; testes in 2 lateral fields, often connected anteriorly; cirrus unarmed; an ovary situated posteriorly; a vagina posterior to the cirrus sac; vitelline follicles medullary, in 2 ventral fields forming 2 wings interrupted medially; a uterus saccate to branched; and eggs without operculum. The types, distribution, and density of microtriches were analyzed on the surface of the tegument of scolex, neck, and immature segments. The presence of tumuli were observed on the apical disc of the scolex. Galaxitaenia toloi n. sp. is the first triaenophorid species recorded in a South American freshwater fish and the fourth helminth parasitizing the intestine of G. platei.     

An unusual blood sequestering tapeworm (Sanguilevator yearsleyi n. gen., n. sp.) from Borneo with description of Cathetocephalus resendezi n. sp. from Mexico and molecular support for the recognition of the order Cathetocephalidea (Platyhelminthes: Eucestoda)

Sanguilevator yearsleyi n. gen., n. sp. and Cathetocephalus resendezi n. sp. are described from the Broadfin shark, Lamiopsis temmincki, in Malaysian Borneo and Carcharhinus leucas in Mexico, respectively. The new genus is unusual in its possession of internal chambers and channels in its scolex that appear to house extensive quantities of host white and red blood cells, respectively. Histology reveals an extremely intimate association between host tissue and the surface of the apical pad of the scolex. Positive staining with periodic acid-Schiff of the surface of the pad of the scolex and the linings of the chambers and channels suggests that an adhesive substance may be produced in these regions. However, explanations for how and why host blood cells come to reside within the scolex remain elusive. Cathetocephalus resendezi n. sp. is distinctive in the form of the papillae in the papillate band of the scolex and also in the inconspicuous nature of the rugose base of the scolex. Scanning electron microscopy of both new taxa as well as Cathetocephalus thatcheri, Cathetocephalus australis and an undescribed species of Cathetocephalus collected from Carcharhinus amboinensis in Australia, suggests that the papillae surrounding the pad of the scolex are of significant taxonomic utility in distinguishing among species in these groups. Parsimony and Bayesian analyses of sequence data (766 bases of 18S rDNA and 405 bases of 28S rDNA) generated from ethanol preserved specimens of C. thatcheri and S. yearsleyi, when compared with equivalent data available for 40 cestode species in GenBank, resulted in trees that support previous propositions that Cathetocephalus should be placed in the order Cathetocephalidea. The results suggest that Sanguilevator should also be considered to belong to this order.     

Parasites of arctic charr Salvelinus alpinus (Linnaeus) and their use in separating sea-run and non-migrating charr

Metazoan parasites of arctic charr, Salvelinus alpinus , from the Nettilling Lake–Koukdjuak River system, Baffin Island, were studied. Non-migrating charr hosted 9 species of parasites while sea-run charr hosted 15 species of parasites. Diphyllobothrium spp., Eubothrium salvelini , and Proteocephalus longicollis were found to be good indicators of non-migrating charr and Brachyphallus crenatus, Bothrimonus sturionis and Prosorhynchus squamatus were good indicators of sea-run charr. Using these parasites as biological tags, the proportion of charr caught by the commercial fishery at the outlet of the Koukdjuak River were determined to be approximately 80% sea-run and 20% non-migrating charr. Step-wise discriminant statistical analysis on morphic characters of charr, designated as sea-run and non-migrating by their parasites, showed these charr to differ morphologically.     

Final stages of carbohydrate metabolism in a cestode Eubothrium rugosum (Cestoda: Pseudophyllidea)

Characteristics of final stages of carbohydrate metabolism in a cestode Eubothrium rugosum from the burbot intestine have been studied. It is found out that in the course of in vitro incubation the worms produce lactic acid to the environment with a concomitant acidification of the medium. The rate of pH changes and lactate production eventually fell during the incubation. The medium pH being the result of helminth's metabolism varies within the normal physiological limits of the host intestine pH and pH optima for its basic digestive enzymes.     

Electron microscope investigation of bacteria associated with the tegument of the tapeworm species Eubothrium rugosum, a parasite of the intestine of burbot

Bacteria associated with the tegument of the tapeworm species Eubothrium rugosum (Cestoda, Pseudophyllidea) parasitizing the intestine of burbot (Lota lota) were studied using transmission and scanning electron microscopy. Three morphological types of the bacteria were revealed. Bacteria of the first type are localized between microtrichia and fit to them closely. Bacteria of the second (gram-positive) and third (gram-negative) types are localized over microtrichia of the tegument and do not attach to the surface. Physiological functions of the bacteria are discussed.     

Ecological parasitology of polymorphic Arctic charr, Salvelinus alpinus (L.), in Thingvallavatn, Iceland

The helminth endoparasite fauna in four Arctic charr morphs, Salvelinus alpinus (L.), small benthivorous (SB), large benthivorous (LB), planktivorous (PL) and piscivorous (PI) charr, from Thingvallavatn, Iceland consisted of: Crepidostomum farionis (Trematoda: Allocreadiidae); Diplosttomum sp. (Trematoda: Diplostomatidae); Eubothrium salvelini; Diphyllobothrium dendriticum; D. ditremum (Cestoda: Pseudophyllidae); Proteocephalus longicollis (Cestoda: Proteocepha-lidae): and Philonema oncorhynchi (Nematoda: Filariidae). The morphs exhibited distinctive patterns in prevalences and parasite burdens (mean intensity and mean relative density of parasites). SB charr had high prevalence and parasite burden of the eye fluke Diplostomum sp. and none to very light infections of the other parasite species. LB charr had relatively high prevalence and parasite burden of the intestinal fluke C. farionis , whereas infections of the remaining parasite species were light to moderate. PL and PI charr had high prevalences and worm burdens of Diphyllobothrium spp. and P. longicollis . PL charr differed from PI charr in higher worm burden off P. longicollis and lighter burden of £. salvelini . Prevalences of P. oncorhynchi were high in PL and PI charr. Association of parasite intensities and age and length offish were investigated. The different infection patterns among the morphs agree well with their partitioning in food and habitat utilization, and confirm that there is a high degree of ecological segregation between the morphs. The results demonstrate the importance of ecological factors influencing transmission efficiency of parasites to the fish host.     

Fractionation of δ15N and δ13C for Atlantic salmon and its intestinal cestode Eubothrium crassum

Stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) were measured for Atlantic salmon Salmo salar and their intestinal cestode, Eubothrium crassum , sharing the same diet. Atlantic salmon muscle tissues were enriched in ¹⁵N and depleted in ¹³C compared to their prey (sprat Sprattus sprattus sprattus ) and their intestinal cestode. There was no significant difference in δ¹⁵N or δ¹³C between E. crassum and the sprat. Differences in nutrient uptake and intestine physiology between Atlantic salmon and E. crassum are discussed, as well as how these may give rise to different fractionations of stable isotopes between a host and its parasites. Furthermore, Atlantic salmon contained a significantly higher lipid content than their prey, which may partly explain differences in δ¹³C values between the host and its cestode. In addition, cestodes inhabiting lipid-rich hosts were also lipid rich. Larger Atlantic salmon were enriched in ¹⁵N compared to smaller fish. Cestodes inhabiting large hosts were also enriched in ¹⁵N compared to parasites living in smaller hosts. The last two results were explained by larger fish possibly feeding from a higher trophic level, or from larger and older prey, that resulted in both a higher lipid content and an enrichment in ¹⁵N.     

Metazoan parasite community structure of sea trout on the west coast of Ireland

A total of 129 sea trout smolts from 10 locations on the west coast of Ireland were examined for metazoan parasites. Of 11 species identified only four had a prevalence >20%. Two species Hysterothylacium aduncum and Hemiuris communis have a confirmed marine origin; Eubothrium crassum has been shown to exist in both the sea and fresh water. The remaining species were fresh water. Overall the parasite community was dominated by autogenic species with only Diphyllobothrium dendriticum having allogenic transmission. Infracommunity values for mean species richness, and Brillouins index of diversity were low, indicating the presence of an impoverished helminth community with little potential for interspecific interaction. The relationship between marine helminths and sea lice was also examined.     

Intestinal helminths in rainbow trout, Salmo gairdneri (Richardson): Absence of effect on nutrient absorption and fish growth

Rainbow trout. Salmo gairdneri (Richardson), from a fish farm, were found to be infected with two cestodes, Eubothrium crassum and Proteocephalus sp. Observations are described which suggest that these parasites have no effect on fish growth or on the intestinal absorption of L-leucine and D-glucose.