Hans-Peter Fagerholm

The morphogenesis of Ascaris suum to the infective third-stage larvae within the egg

Studies of the morphology of Ascaris suum larvae developing in the egg during embryonation in vitro at room temperature showed that 2 molts take place within the egg. The first larval stage (L1) appeared in the egg after 17-22 days of cultivation, the first molt to the second larval stage (L2) took place from day 22 to day 27, and the second molt to the third larval stage (L3) started on day 27 and continued during the 60-day observation period. Infectivity of the eggs was studied by oral egg inoculation in mice and showed that the L3 are the infective stage for mice. Molting to the L3 stage occurs gradually over a period of 2-6 wk, and it is recommended to have an additional maturation period so the infectivity of an egg batch may reach maximum level.

The life cycle ofContracaecum osculatum (Rudolphi, 1802) sensu stricto (Nematoda, Ascaridoidea, Anisakidae) in view of experimental infections

Hatched, ensheathed third-stage larvae ofContracaecum osculatum, 300–320 μm long, were shown to be infective to copepods, to nauplius larvae of {jtBalanus} and to small specimens of fishes (sticklebacks, 0-group eelpout). Other fishes such as gobies and small flatfishes became infected by ingesting infected crustaceans. Cod were infected by being given infected small fishes. In the crustacean paratenic hosts, little growth of the larvae occurred, if any. In the liver sinusoids of sticklebacks and gobies the length of most of the unencapsulated third-stage larvae had not even doubled within 6 months of infection. The fate of larvae (max. 2 mm long) given to cod via infected intermediate fish hosts was apparently decided by the size of the larvae only. Small larvae became encapsulated and eventually died in the liver and wall of the gastrointestinal tract. Larger larvae migrated to the liver parenchyma, where some grew to a length of as much as 10 mm. The growth of the larvae in sticklebacks was shown not to be affected by an increase in temperature (infected fish being transferred from 8° to 14° and 20°C), by the intensity of infection and, partly, by the age of infection (e.g some 2-week-old and 6-month-old larvea were of identical size). In the liver and mesentery of plaice the third-stage larvae developed via copepod paratenic hosts to infectivity (i.e. to more than 4 mm in length), showing that the life cycle may be completed with an optional paratenic invertebrate host and only one intermediate fish host. In combination with earlier results showing that the ensheathed third-stage larva (not the second stage) emerges from the egg and with literature data on the occurrence of larvae in fishes and the presence of fourth-stage larvae and adults predominantly in the stomach of grey seals, the life cycle ofC. osculatum is shown experimentally for the first time.

Intra‐specific variability of the morphology in a single population of the seal parasite Contracaecum osculatum (Rudolphi) (Nematoda, Ascaridoidea), with a redescription of the species

Contracaecum spp. are common parasites of seals and piscivorous birds. The world‐wide distribution which has been attributed to the species C. osculatum needs verification. In the present study thc intra‐specific variability of some taxonomically significant characters was studied in this species. Features analysed include the length and morphology of the distal end of the spicules, the number of proximal caudal papillae and the number of papillae in the region of the first 25 cuticular transverse striae anterior to the cloaca, delined as the Pts zone (precloacal transverse striae zone). The spicules reach their maximum length only in fully grown worms. while the morphology of the distal end of the spicule is determined early in the life of adult male worms. Included is a detailed analysis of the observed morphometric variability in the distal end of the spicule. The number of cuticular transverse striae in relation to papillae in the Pts zone remains constant in different length classes of the parasite. A redescription of the parasite is provided, bascd on material obtained from grey seals in the Baltic Sea, to serve as a basis for discussions on the taxonomy of the genus.

Spermiogenesis in the pseudophyllid cestode Eubothrium crassum (Bloch, 1779).

Abstract. Spermiogenesis of the pseudophyllidean tapeworm Eubothrium crassum has been described by the aid of transmission electron microscopy for the first time. Initially, early spermatids form a distal cytoplasmic protrusion, a differentiation zone containing a small electron-dense, apically oriented region. Out of this region, two centrioles with rootlets develop. The centrioles become orientated in the same plane with the appearance of an intercentriolar body. Now, the long axes of the rootlets are parallel with each other and with the long axis of the nucleus. Two flagella of subsequently unequal length are formed very rapidly. Simultaneously, a median cytoplasmic process (MCP) develops distal to the flagella. Two arching membranes appear at the base of the differentiation zone. Each flagellum, still being in contact with an intercentriolar body, rotates to a position parallel with the MCP. The nucleus migrates very rapidly into the MCP at this stage. Subsequently, the two flagella fuse with the MCP. Finally, the basal bodies with the rootlets detach from the flagella, the intercentriolar body changes its structure and spermatids are pinched off from a condensing residual cytoplasm at the level of the arching membranes.

Phylogenetic relationships among species of Contracaecum Railliet & Henry, 1912 and Phocascaris Høst, 1932 (Nematoda: Ascaridoidea) based on nuclear rDNA sequence data.

Nuclear-encoded large-subunit ribosomal DNA sequences were used to infer a phylogenetic hypothesis for 17 taxa (16 nominal species) of the genera Contracaecum and Phocascaris. Phylogenetic trees based on these data have been used to assess the validity of the taxonomic distinction between these genera, which was based on the presence or absence of certain structural features, rather than on explicit hypotheses of evolutionary history. Phylogenetic hypotheses based on parsimony, likelihood, and neighbor-joining analyses of these sequence data strongly support the hypothesis that species of Phocascaris are nested within the clade of Contracaecum species hosted by phocid seals, and are more closely related to species of the Contracaecum osculatum complex than to other Contracaecum species. Alternative tree topologies representing Phocascaris as not nested within the C. osculatum complex were significantly worse interpretations of these sequence data. Phylogenetic analysis also provides strong support for the monophyly of all taxa (Contracaecum and Phocascaris) from phocid seals, which is consistent with Berlands (1964) proposal that such species form a natural group; however, his proposal to recognize all species in phocid seals as Phocascaris, with all species from birds as Contracaecum would result in a paraphyletic Contracaecum, according to the molecular phylogenetic hypothesis.

Incubation in rats of a nematodal larva from cod to establish its specific identity:Contracaecum osculatum, (Rudolphi)

Infective, third-stage larvae of the genusContracaecum from fish were cultured in vitro and fed to laboratory animals in order to obtain later developmental stages to permit their specific identification. The in vitro incubation was unsuccessful. In orally infected laboratory rats and hamsters, fourth-stage larvae were obtained from the stomach at 2–5 days postinfection (p.i.), but no adult worms developed. Larvae introduced surgically into the body cavity of laboratory rats yielded some adult worms from day 42 onwards. Adult males were identified asContracaecum osculatum.

Contracaecum osculatum (Nematoda: Anisakidae) in fish and seals in Bothnian Bay (northeastern Baltic Sea)

Abstract The occurrence of C. osculatum in 7400 fish specimens of 30 species and in two seal species is reported from Bothnian Bay, Baltic Sea. Seven fish species were infected with C. osculatum, the highest prevalences occurring in the salmon, bull-rout, burbot and cod (20%, 20%, 16% and 15%, respectively). Prevalences tended to increase in the larger fish, but no seasonal variations were found in either the prevalence or the intensity of infection. C. osculatum occur and mature only accidentally in the resident seal species, the ringed seal (Phoca hispida botnica), but a mean of 640 worms were found in the grey seal (Halichoerus grypus) with a maximum of 1244 worms. Grey seals regularly visit the present study area for a few weeks in late spring and it is thought that these serve to maintain the C. osculatum infection in the fish of the area. The nematodes were found in the stomachs of the grey seals, where they tended to occur in aggregations of both third- and fourth-stage larvae and adult worms simultaneously. The presence of third-stage larvae demonstrates that the origin of the C. osculatum infection in these seals may also be in Bothnian Bay. in addition to the more southerly areas where grey seals are to be found for most of the year.

Molecular Identification of Two Strains of Third-Stage Larvae of Contracaecum rudolphii Sensu Lato (Nematoda: Anisakidae) From Fish in Poland

Contracaecum sp. larvae (L3) from fish were identified using nucleotide sequences of the internal transcribed spacers ITS-1 and ITS-2 of the ribosomal DNA. The nematode larvae originated from fish in a freshwater situation (crucian carp Carassius carassius, from Selment Wielki Lake in Mazury, northeastern Poland) and a brackish-water region (Caspian round goby Neogobius melanostomus from the Baltic Sea, Gdańsk Bay at the Polish coast). Two strains (Contracaecum rudolphii A and B) of Contracaecum rudolphii senso lato, a parasite common at the adult stage in fish-eating birds, were identified. In fish from the freshwater site, only the strain temporarily designated C. rudolphii B was identified; in the brackish-water region, both strains were found, suggesting that fish serve as paratenic host for both genotypes. Contracaecum rudolphii sensu lato has been recorded in several species of fish-eating birds in Poland, particularly in the great cormorant, Phalacrocorax carbo, in which the abundance is highest. The results, although based on a restricted number of larvae, suggest that the life cycles of both genotypes can be completed in the Polish region and that at least one of them, C. rudolphii B, can develop both in fresh and brackish water.

DIFFERENTIATION OF CUTICULAR STRUCTURES DURING THE GROWTH OF THE THIRD-STAGE LARVA OF ASCARIS SUUM (NEMATODA, ASCARIDOIDEA) AFTER EMERGING FROM THE EGG

In order to monitor the early phases of the development of Ascaris suum from domestic pigs, third-stage larvae, retrieved from the liver and the lungs, were studied by analyzing worm growth and length increase of individual transverse annuli in the cuticle. Material for study using light and scanning electron microscopy was obtained from experimental infections. The results show that the third-stage larva (not the second-stage) after emergence from the egg grows continuously, without an ecdysis in the liver. During growth, each annulus is split into a complex of 2 subannuli, each of which attains a bimodal appearance and is a prominent feature during a late phase of the third-stage larva. The results suggest that the first 2 molts occur inside the egg, a synapomorphic feature of the Ascaridoidea. The third-stage larvae of ascaridoids, with some functional similarities of the dauer-larva stage of Caenorhabditis sp., facilitate transmission of these parasitic worms to the digestive tract of the vertebrate final host (utilizing the tracheal route in A. suum), where the third and the fourth molts take place.

Third-stage larvae emerge from eggs of Contracaecum osculatum (nematoda, anisakidae)

In Contracaecum osculatum Rudolphi, 1802 (Nematoda, Anisakidae) the first and the second ecdyses occur inside the egg. Larvae forced out from the eggs by coverslip pressure are surrounded by 2 cuticles, revealing 2 previous ecdyses. It is thus the third-stage larva that emerges from the hatched egg and not the second-stage larva as previously believed. The cuticle of the first-stage larva is thin, smooth, and remains in the egg, whereas the cuticle of the second-stage larva is thick, striated, and, serving as a cover, loosely ensheaths the released larva.