One of the most complex life-cycles among trematodes: a description of Parvatrema margaritense (Ching, 1982) n. comb. (Gymnophallidae) possessing parthenogenetic metacercariae.

This study used light and electron microscopy to provide observations and morphometric details of the life-cycle of the gymnophallids (Trematoda, Digenea), Parvatrema margaritense (Ching, 1982) n. comb., the parthenogenetic metacercariae ('germinal sacs') of which were previously described by Ching (1982) as Cercaria margaritensis. The research was instigated by the discovery, on the Barents Sea coast, of a high prevalence of gymnophallid sporocysts and cercariae in the bivalve Turtonia minuta and an equivalent presence of distinctive gymnophallid metacercariae in the gastropod Margarites helicinus. Experiments and data obtained from naturally infected M. helicinus demonstrated that cercariae released from the bivalves invaded the gastropods to give rise to the metacercariae. Two generations (M1 and M2) of these parthenogenetic metacercariae were formed in the extrapallial cavities of their bivalve hosts and they, in turn, gave rise to a third generation (M3) which was shown to infect marine ducks such as the eider (Somateria mollissima). As only small numbers of cercariae are released from T. minuta, it was concluded that the inclusion of parthenogenetic metacercariae in the life-cycle is particularly significant. It allows each cercaria that infects M. helicinus to give rise to over 2000 invasive metacercariae. Evidence suggests that the parthenogenetic metacercariae are commensal rather that parasitic in the pallial cavities of their hosts. Implications of this for theories of early digenean evolution are discussed.

A study of the mechanisms by which the cercariae of Microphallus primas (Jag, 1909) Stunkard, 1957 penetrate the shore crab, Carcinus maenas (L).

This study established the mechanisms by which Microphallus primas cercariae penetrate the crab Carcinus maenas in which they form metacercarial cysts. Light and electron microscopy were used to investigate cercarial features and to follow the fate of cercariae released in close proximity to crabs. It was shown that cercariae were carried in respiratory currents into crabs' branchial chambers where each enveloped itself in a transparent penetration cyst on the gill lamellae. When cercariae were present the number of respiratory current reversals performed by crabs increased. Using an 'artificial branchial chamber' it was possible to observe how cercariae attached to crab gills during breaks in current flow that preceded each current reversal. Inside the penetration cysts the now tail-less larvae used their stylets to pierce holes through which they levered themselves into underlying haemolymph channels in the gills. Histochemical tests demonstrated that the penetration cysts were products of glands in the cercariae and that penetration of the crabs was achieved by mechanical means. The importance of crab respiratory current reversals to the success of cercarial penetration is discussed as it represents the exploitation by a parasite of a host behavioural response to an unrelated stimulus.

Effects of host habitat quality on the viability of Cryptocotyle lingua (Trematoda: Digenea) cercariae.

In an earlier study it was shown that Cryptocotyle lingua cercariae, matured in Littorina littorea from a polluted marine lagoon, displayed slower horizontal swimming rates, and reduced longevity compared to cercariae released by periwinkles from a cleaner environment. This work investigated whether the pollution-induced reduction in swimming rates was due to an inefficient swimming action or the adoption of a less direct swimming path. In addition, cercariae from L. littorea that had been transferred from an 'unpolluted' to a 'polluted' site for 1 month provided information on the speed with which pollutants affect cercariae through their intermediate hosts. Results indicated that, in general, horizontal swimming rates were reduced due to slower swimming rather than disorientation and longer swimming pathways. Effects of host transplantation to a polluted site were clearly evident after 1 month. Evidence suggested that the pollutants accumulated by the cercariae via their first intermediate host affected the neuromusculature associated with swimming performance rather than sensory structures. Bearing in mind the reduced viability of C. lingua cercariae in polluted sites it is assumed that high prevalence of this digenean in gastropods (at such sites) must be due to their continual introduction by infected birds attracted to these habitats from other areas.

Evolutionary relationships within 'pygmaeus' group microphallids using genetic analysis and scanning electron microscopy.

There are four species of 'pygmaeus' microphallids, namely Microphallus pygmaeus, M. piriformes, M. pseudopygmaeus and M. triangulatus (Trematoda: Microphallidae) which are parasites of marine birds and their sporocysts give rise to transmissible metacercariae inside littoral gastropods (mostly littorines). Universally primed polymerase chain reaction (UP-PCR) showed no apparent pattern between genetic diversity of the metacercariae as estimated by genomic banding profiles and their geographic region or molluscan host species. At the same time UP-PCR product cross-hybridization showed that M. pseudopygmaeus and M. triangulatus are genetically very similar, indicating that these taxa represent one species complex. In contrast, M. pygmaeus and M. piriformes are genetically well separated from each other and also from the pseudopygmaeus-triangulatus complex. Scanning electron microscopy of ventral spines, and analyses of spine angles and the number of teeth per spine, showed that all species differed significantly from one another. It was concluded that M. piriformes represents the original western member of the 'pygmaeus' group. Microphallus pygmaeus probably diverged from M. piriformes as it progressively specialized for sea duck final hosts. Microphallus pseudopygmaeus and M. triangulatus diverged from each other and the piriformes-pygmaeus ancestral line relatively recently. Microphallus pseudopygmaeus specialized for adoption of a wide range of gastropod host species and M. triangulatus developed morpho-functional specialization associated with final host exploitation.

An ultrastructural study of excretory system development in the cercariae of Prosorhynchoides gracilescens (Rudolphi, 1819) and Prosorhynchus squamatus Odhner, 1905 (Digenea, Bucephalidae).

The ultrastructure of the developing excretory system of Prosorhynchoides gracilescens and Prosorhynchus squamatus cercariae is described. The development pattern was similar in both species. In early embryos the two main collecting tubes were composed of a layer of cells which were wrapped around the lumen. Later, the tubes fused and the excretory epithelium of the fusion zone and that of the lateral caudal ducts became a syncytium. The collecting tubes in the cercarial body retained their cellular organization. As the tails grew, additional excretory pores were formed in the tail stem where thickened portions of the caudal duct epithelium contacted the surface tegument. Following this, the distal portions of the lateral caudal ducts lost contact with the primary excretory pores and progressively degenerated. Excretory atrium development started with differentiation of secretory active cytons peripheral to the fusion zone. These cells gave rise to cytoplasmic extensions that penetrated the fusion zone wall to eventually form a continuous cytoplasmic layer. This layer eventually replaced some of the fusion zone excretory epithelium and became the lining of the excretory atrium. The anterior end of the fusion zone differentiated into an excretory bladder and a short posterior portion gave rise to the caudal vesicle.

Parasite alteration of host shape: a quantitative approach to gigantism helps elucidate evolutionary advantages.

This investigation quantifies some aspects of the parasite-host relationship between the digenean Microphallus piriformes and its intermediate host Littorina saxatilis, the rough periwinkle. M. piriformes has an abridged life-cycle with no free-living stages, metacercariae remain within host viscera. Noticeable differences in shell shape of parasitized and uninfected periwinkles were investigated. These differences in shell shape were defined by growth parameters of height, diameter and beta angle. The relationship between these parameters was examined together with their impact on parasite reproduction. All 3 shape parameters were altered in periwinkles infected by M. piriformes. The alteration in beta angle and height increased the available volume for parasites in the shell spire by about 12%. As metacercarial production per sporocyst has been shown to depend on host size, the increased volume enables considerable additional life-time reproduction by the parasite, of approximately 550-850 additional metacercariae in hosts of the usual size range. The form of gigantism found in this study is discussed in relation to previous concepts. It is suggested that gigantism in permanently castrated hosts is adaptive parasite manipulation of host physiology, favoured in parasites with abbreviated life-cycles, when host viability increases parasite transmission, and when an initially small host individual is infected.

Pollution toxicity to the transmission of larval digeneans through their molluscan hosts.

The increased occurrence of pollutants in ecosystems is a continuing area of concern. It is known that numerous diseases of wild aquatic animals can occur with decreased or increased prevalences in areas associated with high or chronic levels of pollution. This may have serious implications for environmental health. There has consequently been an increasing number of laboratory and field studies on disease transmission under polluted conditions, especially focusing on digeneans of medical or economic importance. The effect of pollutants to the transmission of larval digeneans (miracidia, cercariae, metacercariae) and snail-digenean interactions is therefore considered. An overview and interpretation of the published literature on laboratory and field studies is provided. It is apparent from these studies that the influence of pollutants on digenean transmission is highly complex with much of the observed effects in the laboratory often masked by a complexity of other factors in the field. Future studies would benefit from a standardisation of experimental procedures, increasing the number of combined laboratory and field studies, and increasing the complexity of the experiments undertaken.

Tributyltin and copper effects on encystment and in vitro excystment of Parorchis acanthus larvae.

Effects of tributyltin (TBT) and copper (Cu) on cercariae and metacercariae of the trematode Parorchis acanthus (Digenea: Philophthalmidae) were investigated. Cercariae released by the dogwhelk, Nucella lapillus were maintained in natural seawater (SW) or solutions of TBT or Cu ranging from 0.001-100 microg l(-1) and 1-6 mg l(-1) respectively before they encysted. Over 79% of the cercariae encysted in control and test solutions. Low concentrations of TBT reduced encystment success more than low concentrations of Cu. The percentage of cercariae that formed cysts in the highest concentrations of both pollutants was higher than in the controls, perhaps representing an 'emergency response' to the pollutants. Before being induced to excyst in vitro, metacercariae were left in the heavy metal solutions for 3 weeks. Metacercariae exposed as cercariae to TBT and Cu achieved lower percentage excystment success than those that had encysted in SW. Cyst walls provided greater protection against Cu than TBT. It was concluded that TBT and Cu had a detrimental effect on the larval stages of P. acanthus at the higher concentrations used but the cyst wall afforded an element of protection if formed in unpolluted seawater before the larval stages were subjected to the pollutants.

An ultrastructural study of reproduction in the parthenogenetic metacercariae of Cercaria margaritensis Ching, 1982 (Digenea: Gymnophallidae).

The parthenogenetic metacercarial stages of the gymnophallid trematode Cercaria margaritensis are found in the extrapallial cavity of the subtidal prosobranch mollusc Margarites helicinus. The primary metacercariae (M1) produce second-generation metacercariae (M2) which become independent and give rise to M3 metacercariae which are infective to the definitive host, the common eider (Somateria mollissima). This study used transmission electron microscopy to follow the development of M2 inside M1 organisms and M3 inside M2 organisms. The process is similar in both cases with embryos developing from individual cells from the parent body walls. In each case the brood sac was divided into brood chambers by multilaminated cells and both M2 and M3 embryos developed inside embryonic membranes that originated from specialized blastomeres. The tegument of M2 and M3 embryos developed in a similar manner underneath the embryonic membrane. Both the multilaminated cells and the embryonic membranes possessed features that indicated that they are involved in transport of nutrients. It is suggested that the continuous nature of M2 and M3 embryo development may well be similar to that postulated for ancestral digeneans.

Life history and life cycles: production and behavior of trematode cercariae in relation to host exploitation and next-host characteristics.

Life history trade-offs affect trematode parasites reproducing inside their 1st intermediate hosts. Within the constraint of the effect on host survival, parasite production of cercariae is subject to a size-numbers trade-off. Within each cercaria, resources must be partitioned between host-seeking and subsequent developmental functions. Three species of microphallid trematodes with the same 1st intermediate host (the gastropod Littorina saxatilis) were investigated. Maritrema arenaria periodically released many small cercariae. Microphallus similis released fewer, 15% larger, cercariae without periodicity. Microphallus similis cercariae were strong swimmers, moving toward the dark and downward in turbulent water, whereas Ma. arenaria cercariae remained suspended. Maritrema arenaria cercariae, although smaller in body and tail size, were produced at an average daily volume nearly twice that of M. similis. These differences are interpreted as transmission adaptations related to mobility and predictability of the 2nd intermediate host. Microphallus similis, with a mobile and less predictable crab host, adopted a 'bethedging' prolonged production of fewer cercariae by less intensive host exploitation, each cercaria having a high allocation to host-seeking behavior. Maritrema arenaria, with predictable sessile barnacle hosts, produced less mobile but potentially longer-lived cercariae in larger numbers. Microphallus piriformes metacercariae remain in the gastropod host. The number of M. piriformes metacercariae increased in larger hosts. The 3 species differed in the number of sporocysts and (meta)cercariae per sporocyst within the gastropod but not in the within-host volume of parasites. Variation in host exploitation and life history appeared adaptive for transmission to the next host.