The cestodes (tapeworms) are a large group of endoparasitic worms infecting various vertebrates. Most species are included in the Eucestoda (“genuine” tapeworms), characterised (with few exceptions) by a number of “segments” (proglottids). Examples areTaenia(the pig and cattle tapeworms) andDiphyllobothrium(the broad fish tapeworm) infecting man. Two groups, the Gyrocotylidea and Amphilinidea, do not possess proglottids. The Amphilinidea are discussed here. Only eight species included in three genera are known. They are large (several cm long), dorso-ventrally flattened worms infecting the body cavity of freshwater and marine teleost (bony) fishes and freshwater turtles. Larvae are ciliated and possess 10 posterior hooks, which are retained in the adult. By far the best known species is the Australian speciesAustramphilina(=Gigantolina)elongatawith freshwater crustaceans as intermediate and freshwater turtles as final (definitive) hosts. A considerable number of studies deal with its morphology, electron-microscopy and life cycle (). Brief overviews of the Amphilinidea are by Rohde (2005, 2007) . The Tree of Life webpage by the author contains a detailed account of all aspects of Amphilinidea and an extensive bibliography.Austramphilina elongatais discussed in the following.
Structure of adultAustramphilina elongata
The adult worm reaches a length of about 150 or more mm, with a width of about 14 or more mm (Fig.1). As in all amhilinids, the uterus forms three loops in the body: it extends from the posteriorly located ovary to the anterior end, turns back and forward again, opening through a uterine pore at the anterior end. The vagina opens at the posterior end. Vitellaria extend in the lateral parts of the body from the anterior to the posterior ends of the body. Testes are scattered throughout the body, and the male gonopore is located near the female one at the posterior end (Fig.2).
Figure 1.Austramphilina elongata. Several worms in the body cavity of the freshwater turtleChelodina longicollis. Original Klaus Rohde. © Klaus Rohde
Figure 2.Austramphilina elongata, whole mount. X= bodies of unkown function. Original Klaus Rohde. © Klaus Rohde
Structure of larvalAustramphilina elongata
Larvae are ciliated and possess 10 posterior hooks of three different kinds. Two pairs are serrate, the others are sickle-shaped (Fig.3). A large number of transverse muscle bands extends below the tegument (surface layer) of the larva. There are several clusters of sensilla (sensory receptors) (Figs.3 and 4).
Figure 3. Posterior end of larva. Note cluster of sensilla, transverse muscle bands, ciliated epidermis and five pairs of hooks of three types. Original Klaus Rohde. © Klaus Rohde
Figure 4. Larva ofAustramphilina elongataimpregnated with silver. Note transverse muscle bands and receptors. Original Klaus Rohde. © Klaus Rohde
Life cycle ofAustramphilina elongata
Eggs have to get into freshwater for further development (Fig.5). The escape route from the host is unknown. Larvae hatch in freshwater. They swim around in water until they get into contact with a crayfish. On the crayfish the larva bends in such a way that both the anterior and posterior ends are located close together on the cuticle of the host. The sickle-shaped hooks pierce into the cuticle, the serrate ones perform sawing movements, cutting through the cuticle. The three types of anterior glands apparently produce a secretion (which, however, has not been identified) dissolving the surface layer. The larva penetrates into the host’s tissue, shedding the ciliated epidermis in the process. Penetration was observed to occur through the gills, and through the thin junctions between the crayfish’s segments within 30 minutes since first contact. Larvae infective to turtles are several mm long and were observed in the abdomen of crayfish. Turtles become infected by eating crayfish. Here, juvenile worms penetrate through the wall of the oesophagus (Fig.6), migrate along the trachea (Fig.7) and through the septum into the body cavity, where they grow up. Adult worms were seen mainly in the body cavity, but occasionally also in the lungs. This suggests that eggs may perhaps leave the host via the trachea and mouth cavity, from where they are spit out into water. Once, an adult was also seen in the urinary bladder, and once in the oviduct of a turtle, suggesting that eggs may be shed through the cloaca.Freshwater shrimps could also be infected experimentally, but larvae did not reach a size infective to turtles in them.
Figure 5. Life cycle ofAustramphilina elongata. Note: escape route of egg from turtle unknown. Original Klaus Rohde. © Klaus Rohde
Figure 6. Section though the oesophageal wall of a turtle,Chelodina longicollis,showing a penetratingAustramphilinajuvenile (arrow). Original Klaus Rohde. © Klaus Rohde
Figure 7. Two juvenileAustramphilina(arrows) migrating along the trachea towards the body cavity of a turtle. original Klaus Rohde. © Klaus Rohde
Life cycles of other species
Amphilina foliaceauses freshwater amphipods as intermediate andAccipenser(sturgeon) as final hosts. It inhabits the body cavity of the final host and eggs escape through the coelomic pore which connects the body cavity to the outside (it is not present in turtles!). Eggs containing infective larvae are ingested by the amphipods, whose mouthparts break the egg shell, allowing the larva to escape and penetrate into the host.
AdultNesolecithus africanusinfects African freshwater fish. Juveniles were recovered from freshwater prawns.
All figures are originals by the author, who owns copyright. They were published in a somewhat changed form in the author’s Tree of Life webpage on the Amphilinidea.