The life history of Cryptocotyle lingua (Creplin), with notes on the physiology of the metacercariae

Neuronal gene expression in two generations of the marine parasitic worm, Cryptocotyle lingua

Trematodes, or flukes, undergo intricate anatomical and behavioral transformations during their life cycle, yet the functional changes in their nervous system remain poorly understood. We investigated the molecular basis of nervous system function in Cryptocotyle lingua, a species of relevance for fisheries. Transcriptomic analysis revealed a streamlined molecular toolkit with the absence of key signaling pathways and ion channels. Notably, we observed the loss of nitric oxide synthase across the Platyhelminthes. Furthermore, we identified upregulated neuronal genes in dispersal larvae, including those involved in aminergic pathways, synaptic vesicle trafficking, TRPA channels, and surprisingly nitric oxide receptors. Using neuronal markers and in situ hybridization, we hypothesized their functional relevance to larval adaptations and host-finding strategies. Additionally, employing a behavior quantification toolkit, we assessed cercaria motility, facilitating further investigations into the behavior and physiology of parasitic flatworms. This study enhances our understanding of trematode neurobiology and provides insights for targeted antiparasitic strategies.

No Tail No Fail: Life Cycles of the Zoogonidae (Digenea)

The Zoogonidae is the only digenean family where known cercariae lack the tail but actively search for the second intermediate host. However, the data on the zoogonid life cycles are scarce. In the present study, we elucidated and verified life cycles of the Zoogonidae from the White Sea. Using rDNA data, we showed that Pseudozoogonoides subaequiporus utilizes gastropods from the family Buccinidae as the first intermediate host and protobranch bivalves as the second one. This life cycle can be facultatively truncated: some cercariae of P. subaequiporus encyst within the daughter sporocysts. Molecular data also confirmed previous hypotheses on Zoogonoides viviapus life cycle with buccinid gastropods acting as the first intermediate hosts, and annelids and bivalves as the second intermediate hosts. We demonstrated the presence of short tail primordium in the developing cercariae of both species. Based on the reviewed and our own data, we hypothesize that the emergence of tailless cercariae in the evolution of the Zoogonidae is linked to the switch to non-arthropod second intermediate hosts, and that it possibly happened only in the subfamily Zoogoninae. Basally branching zoogonids have retained the ancestral second intermediate host and might have also retained the tail.

New data on pipefishes’ and seahorse’s endohelminths off Crimean coasts of the Black Sea

A total of 357 Syngnathidae fishes (Syngnathus abaster Risso, 1827, S. typhle Linnaeus, 1758 and Hippocampus hippocampus (Linnaeus, 1758) caught in different habitats along Crimean Black Sea shelf were examined for presence of endohelminths and revealed to be hosts of 15 helminth species. The fishes are second intermediate hosts for five “birds’” digenean species and nematodes (larvae of three species and immature adults of one more species) completing life-cycles in waterbirds and fish; for two acanthocephalans and three cestodes larvae ending development in fish. We suggest, basing on data on feeding of the Black Sea predatory fish and waterbirds, that Syngnathid fishes are paratenic hosts in parasitic systems of most cestodes, nematodes and acanthocephalans. All the trematodes found are generalists at metacercarial stage; specialists Timoniella imbutiformis and Aphallus tubarium use Syngnathidae as definitive hosts. Cestodes, nematodes as well as acanthocephalans found are generalists, too. Based on infection indices, S. typhle are main final host and H. hippocampus are main 2nd intermediate hosts for T. imbutiformis; Syngnathids are accidental hosts for other trematodes as well as for all the cestodes, nematodes and acanthocephalans. Cryptocotyle concava and Pygidiopsis genata are important as Syngnathid fishes’ threats, especially in marine protected aquatoria, being potential agents of “black spot disease”.

Cryptocotyle lata (Trematoda: Heterophyidae) Adult from a Korean Raccoon Dog, Nyctereutes procyonoides koreensis

Total 513 heterophyid flukes were collected from a carcass of wild Korean raccoon dog, Nyctereutes procyonoides koreensis, in Korea. With morphological and molecular characteristics, the flukes were identified to Cryptocotyle lata. The adult C. lata were minute, transparent, pentagonal, 522 μm long by 425 μm wide. Ceca extended into post-testicular region. Ventrogenital sac elliptical, 79 μm by 87 μm with genital pore and ventral sucker. Two testes semielliptical and slightly lobed, located in the posterior region, right testis 173 μm by 155 μm, left testis 130 μm by 134 μm. In a phylogenetic tree, the fluke specimen of this study was grouped with C. lata divergent from Cryptocotyle lingua. We report here N. procyonoides koreensis first as a natural definitive host of C. lata.

Morphological and molecular identification of Cryptocotyle lingua metacercariae isolated from Atlantic cod (Gadus morhua) from Danish seas and whiting (Merlangius merlangus) from the English Channel

Trematode larvae (metacercariae) causing black spot disease occur frequently in gills, fins, skin and the superficial muscle layers of marine fish. Species within the genus Cryptocotyle Lühe, 1899 are frequently associated with this disease. Descriptions of the metacercarial stage are relatively limited and none has hitherto been reported from fish from the English Channel. The present study reports the morphological and molecular identifications of encysted black spot-inducing parasites from whiting (Merlangius merlangus) and Atlantic cod (Gadus morhua) caught respectively from the north coast of France (English Channel) and from Danish sea waters. Metacercariae were characterised morphologically based on microscopic observations and molecularly using Sanger sequencing of fragments of the mitochondrial cox1 gene and rDNA ITS region. Morphological data were compared with available data in the literature. Phylogenetic trees including reference sequences were built to confirm morphological and molecular identifications. This survey constitutes the first description of C. lingua metacercariae in the English Channel ecosystems.

A review of molecular identification tools for the opisthorchioidea

The superfamily Opisthorchioidea encompasses the families Cryptogonimidae, Opisthorchiidae and Heterophyidae. These parasites depend on the aquatic environment and include marine and freshwater species. Some species, such as Clonorchis sinensis and Opisthorchis viverrini, have a high impact on public health with millions of infected people worldwide and have thus been the object of many studies and tool developments. However, for many species, tools for identification and detection are scarce. Although morphological descriptions have been used and are still important, they are often not efficient on the immature stages of these parasites. Thus, during the past few decades, molecular approaches for parasite identification have become commonplace. These approaches are efficient, quick and reliable. Nonetheless, for some parasites of the superfamily Opisthorchioidea, reference genomic data are limited. This study reviews available genetic data and molecular tools for the identification and/or the detection of this superfamily. Molecular data on this superfamily are mostly based on mitochondrial and ribosomal gene sequence analyses, especially on the cytochrome c oxidase subunit 1 gene and internal transcribed spacer regions respectively.

Intestinal Flukes Recovered from a Herring Gull, Larus argentatus, in the Republic of Korea

Trematode specimens were collected from the intestine of a herring gull, Larus argentatus, which was found in a critical condition on the shore of a small island (Yubu-do, Seocheon-gun, Chungcheongnam-do) located at the western coast of the Korean peninsula. Total 11 specimens of intestinal flukes, including 3 Cryptocotyle lingua (Heterophyidae), 1 Himasthla alincia (Echinostomatidae), 5 Cardiocephaloides medioconiger (Strigeidae), and 2 Diplostomum spathaceum (Diplostomidae), were recovered. C. lingua was morphologically characterized by the presence of a large ventrogenital apparatus and 2 obliquely tandem testes. H. alincia had an elongated body and a head collar equipped with 31 collar spines. C. medioconiger had a bisegmented body and a voluminous copulatory bursa containing the seminal vesicle and ejaculatory duct. D. spathaceum also had a bisegmented body and its vitellaria extended up to the anterior border of the tribocytic organ. It is of note that C. lingua is potentially zoonotic that can occur in birds and humans. Three of them, i.e., C. lingua, C. medioconiger, and D. spathaceum, are new trematode fauna in Korea. Studies on trematode fauna of migratory birds should be continued in Korea.

Scaphanocephalus-associated dermatitis as the basis for black spot disease in Acanthuridae of St. Kitts, West Indies

Acanthurus spp. of St. Kitts and other Caribbean islands, including ocean surgeonfish A. bahianus, doctorfish A. chirurgus, and blue tang A. coeruleus, frequently show multifocal cutaneous pigmentation. Initial reports from the Leeward Antilles raised suspicion of a parasitic etiology. The aim of this study was to quantify the prevalence of the disease in St. Kitts' Acanthuridae and describe its pathology and etiology. Visual surveys demonstrated consistently high adjusted mean prevalence at 3 shallow reefs in St. Kitts in 2017 (38.9%, 95% CI: 33.8-43.9) and 2018 (51.5%; 95% CI: 46.2-56.9). There were no differences in prevalence across species or reefs, but juvenile fish were less commonly affected than adults. A total of 29 dermatopathy-affected acanthurids were sampled by spearfishing for comprehensive postmortem examination. Digenean metacercariae were dissected from <1 mm cysts within pigmented lesions. Using partial 28S rDNA sequence data they were classified as Family Heterophyidae, members of which are commonly implicated in black spot disease of other fishes. Morphological features of the parasite were most typical of Scaphanocephalus spp. (Creplin, 1842), and 2 genetic profiles were obtained suggesting more than 1 digenean species. Histologically, pigmented lesions had mild chronic perivascular dermatitis and increased melanophores and melanin density, often centered on encysted digenean metacercariae. In 1 affected A. chirurgus, similar metacercariae were histologically identified in skeletal and cardiac muscle. Further research is needed to clarify impact on host fitness, establish the number of heterophyid digenean species that cause black spots on Caribbean fishes and to determine the intermediate and definitive host species.

Mucous cell responses to contaminants and parasites in shorthorn sculpins (Myoxocephalus scorpius) from a former lead‑zinc mine in West Greenland

Previous studies of sculpins from the former lead (Pb) - zinc (Zn) mine near Maarmorilik, West Greenland, have shown that these fish are affected by heavy metal exposure from the mine. In this study, we applied mucosal mapping (a stereological method for mucosal quantification in fish) to uncover interactions between the host, parasites and heavy metal exposure (Pb and Zn) in shorthorn sculpins from the Maarmorilik mining site at a gradient of 3 stations. Skin and gill mucosal epithelia of shorthorn sculpins were significantly affected and reflected the exposure to environmental heavy metals and parasites. Size of skin mucous cells was significantly smallest in the sculpin from the station 3 where heavy metal contamination was lowest and the skin parasite load was highest. Gill filament mucous cells were largest and densest in fish from station 1 which was the most contaminated site. In gill lamellae the density of mucous cell followed a toxicity gradient and was significantly highest at the most contaminated station and significantly lowest at the least contaminated station. The persistent presence of toxic Pb and Zn levels in the sediment at the most contaminated station may have induced a small but measurable reduction in the surface area available for respiration and may have affected diffusion distance. The strong correlation between size of filamentous mucous cells and Pb concentrations in liver suggests that these cells can play an active role in reducing the somatic load of Pb in sculpin. We suggest that mucosal mapping can be used to assess effects of contaminant and parasite exposure in future environmental field studies.

Evolution, phylogenetic distribution and functional ecology of division of labour in trematodes

Division of labour has evolved in many social animals where colonies consist of clones or close kin. It involves the performance of different tasks by morphologically distinct castes, leading to increased colony fitness. Recently, a form of division of labour has been discovered in trematodes: clonal rediae inside the snail intermediate host belong either to a large-bodied reproductive caste, or to a much smaller and morphologically distinct ‘soldier’ caste which defends the colony against co-infecting trematodes. We review recent research on this phenomenon, focusing on its phylogenetic distribution, its possible evolutionary origins, and how division of labour functions to allow trematode colonies within their snail host to adjust to threats and changing conditions. To date, division of labour has been documented in 15 species from three families: Himasthlidae, Philophthalmidae and Heterophyidae. Although this list of species is certainly incomplete, the evidence suggests that division of labour has arisen independently more than once in the evolutionary history of trematodes. We propose a simple scenario for the gradual evolution of division of labour in trematodes facing a high risk of competition in a long-lived snail host. Starting with initial conditions prior to the origin of castes (size variation among rediae within a colony, size-dependent production of cercariae by rediae, and a trade-off between cercarial production and other functions, such as defence), maximising colony fitness (R₀) can lead to caste formation or the age-structured division of labour observed in some trematodes. Finally, we summarise recent research showing that caste ratios, i.e. relative numbers of reproductive and soldier rediae per colony, become more soldier-biased in colonies exposed to competition from another trematode species sharing the same snail, and also respond to other stressors threatening the host’s survival or the colony itself. In addition, there is evidence of asymmetrical phenotypic plasticity among individual caste members: reproductives can assume defensive functions against competitors in the absence of soldiers, whereas soldiers are incapable of growing into reproductives if the latter’s numbers are reduced. We conclude by highlighting future research directions, and the advantages of trematodes as model systems to study social evolution.

Consistency of trematode infection prevalence in host populations across large spatial and temporal scales

Parasites can impart heavy fitness costs on their hosts. Thus, understanding the spatial and temporal consistency in parasite pressure can elucidate the likeliness of parasites' role as agents of directional selection, as well as revealing variable environmental factors associated with infection risk. We examined spatiotemporal variation in digenetic trematode infection in 18 populations of an intertidal host snail (Littorina littorea) over a 300 km range at an 11-yr interval, more than double the generation time of the snail. Despite a complete turnover in the snail host population, the average change in infection prevalence among populations was <1% over the 11-yr span, and all but three populations remained within 5 percentage points. This consistency of prevalence in each population over time suggests remarkable spatiotemporal constancy in parasite delivery vectors in this system, notably gulls that serve as definitive hosts for the parasites. Thus, despite gulls' high mobility, their habitat usage patterns are ostensibly relatively fixed in space. Importantly, this spatiotemporal consistency also implies that sites where parasites are recruitment limited remain so over time, and likewise, that parasite hotspots stay hot.

The life cycle of a zoonotic parasite reassessed: Experimental infection of Melanoides tuberculata (Mollusca: Thiaridae) with Centrocestus formosanus (Trematoda: Heterophyidae)

Centrocestus formosanus is a foodborne intestinal trematode that is native to Asia and has been introduced into the Americas and Europe. Although there are several studies of C. formosanus in definitive vertebrate hosts (birds and mammals, including humans), and in intermediate vertebrate hosts (fish and amphibians), there is little published information regarding interaction with its transmitting mollusc. In this study we studied the miracidial development of C. formosanus using a mouse as a source of eggs. Adult parasites were maintained in water in order to develop miracidia in intrauterine eggs. Miracidia appeared at 12 days of incubation, with no hatching observed for up to 40 days. Subsequently, we placed dead C. formosanus containing eggs with miracidia individually in contact with 48 specimens of Melanoides tuberculata, and observed the absence of the parasites after 1h of exposure, suggesting that they were ingested by the snails. Of the 33 experimentally-infected snails that were alive after 84–89 days post-infection (DPI), seven (21%) shed cercariae. We detected young C. formosanus rediae in 21/33 (64%) M. tuberculata at 90 DPI. To our knowledge, this report is the first to show that, in the life cycle of C. formosanus, infection of molluscs occurs passively by ingestion of eggs, followed by a long intramolluscan phase. We compare these data with those described for other Heterophyidae, and discuss on the phylogenetic background of the pattern of miracidial development verified in these parasites.

Fishborne zoonotic heterophyid infections: An update

Fishborne heterophyid trematodes infecting humans are at least 29 species worldwide and belong to 13 genera. Its global burden is much more than 7 million infected people. They include Metagonimus (M. yokogawai, M. takahashii, M. miyatai, M. minutus, and M. katsuradai), Heterophyes (H. heterophyes, H. nocens, H. dispar, and H. aequalis), Haplorchis (H. taichui, H. pumilio, H. yokogawai, and H. vanissimus), Pygidiopsis (P. summa and P. genata), Heterophyopsis (H. continua), Stellantchasmus (S. falcatus), Centrocestus (C. formosanus, C. armatus, C. cuspidatus, and C. kurokawai), Stictodora (S. fuscata and S. lari), Procerovum (P. varium and P. calderoni), Acanthotrema (A. felis), Apophallus (A. donicus), Ascocotyle (A. longa), and Cryptocotyle (C. lingua). Human infections are scattered around the world but the major endemic areas are located in Southeast Asia. The source of human infection is ingestion of raw or improperly cooked fish. The pathogenicity, host-parasite relationships, and clinical manifestations in each species infection are poorly understood; these should be elucidated particularly in immunocompromised hosts. Problems exist in the differential diagnosis of these parasitic infections because of close morphological similarity of eggs in feces and unavailability of alternative methods such as serology. Molecular diagnostic techniques are promising but they are still at an infant stage. Praziquantel has been proved to be highly effective against most of the patients infected with heterophyid flukes. Epidemiological surveys and detection of human infections are required for better understanding of the geographical distribution and global burden of each heterophyid species. In this review, the most updated knowledge on the morphology, biology, epidemiology, pathogenesis and pathology, immunology, clinical manifestations, diagnosis and treatment, and prevention and control of fishborne zoonotic heterophyid infections is provided.

Trematodes with a reproductive division of labour: heterophyids also have a soldier caste and early infections reveal how colonies become structured

Recent findings have extended the documentation of complex sociality to the Platyhelminthes, describing the existence of a reproductive division of labour involving a soldier caste among the parthenitae of trematode parasites. However, all species examined to date occupy high positions in trematode interspecific dominance hierarchies and belong to two closely related families, the Echinostomatidae and the Philophthalmidae (Superfamily Echinostomatoidea). Further, the two species documented as lacking soldiers also belong to the Echinostomatidae. Here, we examine four species of intermediate dominance, all belonging to the family Heterophyidae (Superfamily Opisthorchioidea): Euhaplorchis californiensis, Phocitremoides ovale, Pygidiopsoides spindalis and Stictodora hancocki, all of which infect the California horn snail, Cerithideopsis californica (=Cerithidea californica). We quantify morphology, distribution and behaviour of rediae from fully developed colonies. We also provide information on colony structure for three developing heterophyid colonies to better understand colony development. We discuss the implications of our findings, particularly with respect to how they suggest alternatives to the conclusions of other researchers concerning the nature of trematode sociality. Our analyses of morphological, distributional and behavioural patterns of developed colonies indicate that these heterophyid trematodes have a non-reproductive caste whose function is defence of the colony from invading trematodes. Hence, a soldier caste occurs for species lower in dominance hierarchies than previously known, and is present in at least two superfamilies of digenean trematodes, suggesting that selection for a soldier caste may be much more common among the Trematoda than previously recognised.

Molecular Cytogenetics in Digenean Parasites: Linked and Unlinked Major and 5S rDNAs, B Chromosomes and Karyotype Diversification

Digenetic trematodes are the largest group of internal metazoan parasites, but their chromosomes are poorly studied. Although chromosome numbers and/or karyotypes are known for about 300 of the 18,000 described species, molecular cytogenetic knowledge is mostly limited to the mapping of telomeric sequences and/or of major rDNA clusters in 9 species. In this work we mapped major and 5S rDNA clusters and telomeric sequences in chromosomes of Bucephalus minimus, B. australis, Prosorhynchoides carvajali (Bucephaloidea), Monascus filiformis (Gymnophalloidea), Parorchis acanthus (Echinostomatoidea), Cryptocotyle lingua (Opisthorchioidea), Cercaria longicaudata, Monorchis parvus (Monorchioidea), Diphterostomum brusinae, and Bacciger bacciger (Microphalloidea). Whilst single major and minor rDNA clusters were mapped to different chromosome pairs in B. minimus and P. acanthus, overlapping signals were detected on a single chromosome pair in the remaining taxa. FISH experiments using major rDNA and telomeric probes clearly demonstrated the presence of highly stretched NORs in most of the digenean taxa analyzed. B chromosomes were detected in the B. bacciger samples hosted by Ruditapes decussatus. Although the cercariae specimens obtained from Donax trunculus, Tellina tenuis, and R. decussatus were in agreement with B. bacciger, their karyotypes showed striking morphological differences in agreement with the proposed assignation of these cercariae to different species of the genus Bacciger. Results are discussed in comparison with previous data on digenean chromosomes.

Spatial ecology of coastal Atlantic cod Gadus morhua associated with parasite load

Acoustic tags and receivers were used to investigate the spatial ecology of coastal Atlantic cod Gadus morhua (n = 32, mean fork length: 50 cm, range: 33-80 cm) on the Norwegian Skagerrak coast in 2012. Monthly home ranges (HR), swimming activity and depth use varied considerably among individuals and through the months of June, July and August. HR sizes for the period ranged from 0.25 to 5.20 km2 (mean = 2.30 km2. Two thirds of the tagged G. morhua were infected with black spot disease Cryptocotyle lingua parasites; these fish had larger HRs and occupied deeper water compared with non-infected fish. The infected fish also tended to be more active in terms of horizontal swimming. From an ecological and evolutionary perspective, any environmental change that modifies G. morhua behaviour may therefore also alter the parasite load of the population, and its conservation and fishery status.

Consistent pattern of local adaptation during an experimental heat wave in a pipefish-trematode host-parasite system

Extreme climate events such as heat waves are expected to increase in frequency under global change. As one indirect effect, they can alter magnitude and direction of species interactions, for example those between hosts and parasites. We simulated a summer heat wave to investigate how a changing environment affects the interaction between the broad-nosed pipefish (Syngnathus typhle) as a host and its digenean trematode parasite (Cryptocotyle lingua). In a fully reciprocal laboratory infection experiment, pipefish from three different coastal locations were exposed to sympatric and allopatric trematode cercariae. In order to examine whether an extreme climatic event disrupts patterns of locally adapted host-parasite combinations we measured the parasite's transmission success as well as the host's adaptive and innate immune defence under control and heat wave conditions. Independent of temperature, sympatric cercariae were always more successful than allopatric ones, indicating that parasites are locally adapted to their hosts. Hosts suffered from heat stress as suggested by fewer cells of the adaptive immune system (lymphocytes) compared to the same groups that were kept at 18°C. However, the proportion of the innate immune cells (monocytes) was higher in the 18°C water. Contrary to our expectations, no interaction between host immune defence, parasite infectivity and temperature stress were found, nor did the pattern of local adaptation change due to increased water temperature. Thus, in this host-parasite interaction, the sympatric parasite keeps ahead of the coevolutionary dynamics across sites, even under increasing temperatures as expected under marine global warming.

In vitro excystment of the metacercaria of Cryptocotyle concavum from the common goby Pomatoschistus microps

The metacercaria of the heterophyid digenean Cryptocotyle concavum (Creplin, 1825) Lühe 1899, as seen by light and transmission electron microscopy, from the common goby (Pomatoschistus microps) is enclosed within a rigid cyst wall composed of four layers of parasite origin surrounded by a host-derived capsule. The encysted parasite contains anterior glands with secretion-filled ducts opening at the anterior extremity of the ‘head’. Excystment in vitro occurred most readily at 40–42°C on treatment with alkaline bile salts following pretreatment with acid pepsin. After 1 min in pepsin, the metacercaria became intermittently active, periods of rotation lasting 1–3 sec alternating with quiescent periods of 20–40 sec. Metacercariae excysted, some in less than 1 min, after transfer to bile salts. The metacercaria emerged through a hole, which appeared in the cyst wall at the site of application of the ‘head’ of the parasite. Some of the anterior gland ducts of excysted metacercariae were empty, indicating that their secretions are involved in penetrating the cyst wall. Following pretreatment in acid pepsin, no excystment occurred in EBSS (Earle's balanced salt solution) ranging in pH from 2.0 to 4.0, but above pH 4.0, excystment increased, reaching a maximum between pH 7.5 and 8.5. Pepsin and bile salts were not essential for excystment and metacercariae emerged, although less readily, in alkaline EBSS with or without acid EBSS pretreatment. Some emerged in distilled water with no pretreatment. Although no excystment occurred in acid pepsin or in EBSS at pH 2.0 and already-excysted metacercariae were rapidly killed by acid (pH 2.0), acid pepsin pretreatment enhanced excystment in alkaline bile salts.

Some Larval Trematodes from the Coast in the Region of Roscoff, Finistère

Seven species of cercariae and four of metacercariae collected at Roscoff, Finistère, during August, 1931, are described. They have been referred to appropriate systematic groups and wherever possible, probabilities concerning life histories are suggested. Four of the cercariae and two of the metacercariae are recorded as new to science.

Infestation of littorinids by larval Digenea around a small fishing port

Larval Digenea infecting Littorina rudis, L. littorea and L. obtusata around Portavogie harbour, Co. Down, were examined. High levels of infection were found among the littorinids collected from sites on the harbour shore, though levels of infection varied somewhat with position on the shore. There was a direct relationship between size of the snail host and likelihood of infection. There was a negative association between certain pairs of helminth species in the same host, particularly where the rediae of Cryptocotyle lingua or Himasthla sp. were involved. Infection rates in gastropods declined away from the harbour and it is concluded that the high infection rate centres around the harbour were probably due to the increased concentration of definitive hosts, mainly seagulls, attracted to this area by dumping of raw fish offal. The infection of Pholis gunnellus with the metacercariae of C. lingua was also examined. Older fish were more heavily infected than younger fish but no direct relationship was found between high snail infection rates and high P. gunnellus infection rates.

The functional organization and fine structure of the tail musculature of the cercariae of Cryptocotyle lingua and Himasthla secunda

The functional organization of the tail musculature of two species of cercariae has been studied by examination of serial sections in the light and transmission electron microscopes. A three-dimensional reconstruction of the tail has been established. The fine structure of the muscle cells has been investigated and the complexity of organization is related to the requirement for a fast contracting system in the cercarial tail.

Cryptocotyle lingua (Creplin) (Digenea: Heterophyidae): observations on the morphology of the redia, with special reference to the birth papilla and release of cercariae

Rediae of Cryptocotyle lingua were studied using transmission and stereoscan electron microscopy. The mouth is surrounded by numerous ciliated sensory structures. Posteriorally, the body wall is concentrically folded and its surface area is greatly increased by the closely ridged configuration of the outer membrane of the tegument. Attached to the surface, especially at the anterior end, are numerous, small spherical vesicles. At the posterior end there is a retractable papilliform process. The appearance of the surface of the birth papilla is variable. The birth pore is lined by an epithelium which is specialized to withstand extreme distortion. A single cilium is frequently visible in the birth pore lumen. The release of each cercaria is preceded by the emission of a tiny drop of fluid from the birth pore. It is suggested that this fluid acts as a lubricant to the passage of the cercaria through the pore.

The immune respone of plaice (Pleuronectes platessa L.) to the metacercariae of Cryptocotyle lingua and Rhipidocoytle johnstonei

The humoral immune response of plaice to infestations of selected metacercariae was studied. Precipitating antibodies to both Cryptocotyle lingua and Rhipidocotyle johnstonei were demonstrated using agar geldiffusion techniques. Immunochemical investigations of these antibodies indicated they were macroglobulins, resembling immunaglobulin M(IgM) of mammals. The rate and magnitude of antibody production by the host was determined by the ambient temperature

Digenean metacercariae (Timoniellaspp.,Labratrema minimusandCryptocotyle concava) from the flounder,platichthys flesus, in the tidal Thames

A detailed examination of the abundant flatfish speciesPlatichthys flesus, the flounder, in the tidal Thames has revealed the presence of four digenean metacercarial parasites,Cryptocotyle concava(Creplin, 1825),Timoniella imbutiforme(Molin, 1859),T. praeterita(Looss, 1901) andLabratrema minimus(Stossich, 1887). Flounders were recorded as a new second intermediate host forT. praeteritaandL. minimus. They were also recorded as second intermediate hosts for the first time in British waters forT. imbutiforme. The temporal and spatial characteristics of these infections were examined and were believed to provide indirect parasitological evidence of the movement patterns of flounders during their utilization of the Thames Estuary as a nursery ground. From these data it was also surmised that the first intermediate host ofT. imbutiforme,T. praeteritaandC. concavawas probably the molluscan speciesHydrobia ulvaein the lower Thames Estuary, whereasL. minimuswas most likely to occur in the molluscan hostCerastoderma edule, also present in the lower estuary.

Parasites make male pipefish careless

Parasite-mediated sexual selection is expected to favour the avoidance of matings with infected individuals. However, the extent to which the costs and benefits of discriminating against parasitized mates trade off may depend upon numerous factors. I investigated the effects of sex and infection status on choosiness in sex-role reversed deep-snouted pipefish (Syngnathus typhle L.) that were either artificially infected with the trematode parasite Cryptocotyle sp. or sham-infected. Sham-infected males were significantly more likely to associate with a sham-infected female rather than with a Cryptocotyle-infected female. Infected males failed to discriminate against infected potential partners. Males were choosier the larger they were relative to the females available for choice. Females were not discriminatory, regardless of their infection status. Given an inverse relation between female fecundity and parasite load, choosy unparasitized males may gain enhanced reproductive success from their choice decisions. In contrast, more heavily infected wild-caught males gave birth to slightly fewer, but not smaller offspring than did uninfected or lightly infected males, suggesting only a low direct premium on choosy females. The detrimental effects of parasitism on male choosiness, and the lack of female discrimination against infected males likely have profound repercussions on the strength of sexual selection acting on the two sexes and on the dynamics of host-parasite interactions in this system.

Trematode behaviours and the perceptual worlds of parasites

There is a great deal of empirical data and theoretical predictions on the patterns and processes of trematode behaviour, particularly in relation to host-finding activities by the free-living stages and site-finding migrations by the parasitic stages within their hosts. Ecological and evolutionary models of trematode life histories often make explicit assumptions about how these organisms must perceive and respond to signals in their worlds as they move from host to host and as they parasitize each host. Nevertheless, it is unclear how natural selection shapes the parasites' behavioural strategies. In addition, at each stage in their life cycle, trematodes are adorned with elaborate sensory organs and possess sophisticated neuromuscular systems, but it is not clear how they use these complex machinery to perceive their worlds. The purpose of this review is to address this question through insights gathered from a century of research on trematode behaviour. Core theoretical assumptions from modern animal behaviour are used to provide the context for this analysis; a key concept is that all animals have unique perceptual worlds that may be inferred from their behaviours. A critical idea is that all animals possess complex patterns of innate behaviour which can be released by extremely specific signals from the environment. The evidence suggests that trematode parasites live in ecologically predictable aquatic and internal host environments where they perceive only small subsets of the total information available from the environment. A general conclusion is that host finding in miracidia and cercaria, and site-finding by trematodes migrating within their definitive hosts, is accomplished through the release of innate patterns of behaviours which are adaptive within the context of conditions in the worm's environment. Examples from empirical studies are used to support the contention that, despite the apparent complexity of their free-living and parasitic environments, the perceptual worlds of trematodes are impoverished, and complex patterns of behaviour may be released by only a few signals in their environment.

Effects of trematode parasitism on the behaviour and ecology of a common marine snail (Littorina littorea (L.))

Cryptocotyle lingua (Creplin) is a digenean trematode parasite of the littoral prosobranch gastropod Littorina littorea (L.). The literature suggests the snails become infected by grazing guano of the final host, the herring gull, Larus argentatus Pontoppidan. The parasite emerges from the snail as free-swimming cercariae. Interactions between the snail and the parasite at cellular and life-history levels are well established, but little is known of the influences the interaction has on the behaviour and the ecology of the snail. We tested the response of the snail to encounters with cercariae, examined the longevity of the guano on-shore and tested the responses of the snail to encounters with guano. Over half the L. littorea tested were able to detect both cercariae and a filtered homogenate of cercariae in conspecific mucus trails, approximately one-third of animals refusing to cross the treatments. Chemoreception by the mouth or foot is considered the most likely means of detection. Guano samples (mean weight 3.22 g) naturally deposited at approximately mid-tide level were completely washed away by one tidal inundation. We consider this period too brief to allow for ingestion of eggs in guano by the snail. Further, snails would not cross guano placed in conspecific trails. Most snails would not cross guano diluted by 10(3)x(10 mg ml(-1)) and some snails could still detect guano diluted by 10(6)x(10 &mgr;g ml(-1)), though all were prepared to cross it. Detection of guano is again believed to be by chemoreception by the mouth or foot. These results are discussed in terms of the mating and aggregating behaviour of L. littorea. Ingestion of the parasite by L. littorea is likely to take place once the guano has washed away as the eggs are negatively buoyant in seawater and may adhere to rock (biofilm) or algal fronds which may be grazed by the snail.

The effects of age, temperature, light quantity and wavelength on the swimming behaviour of the cercariae of Cryptocotyle lingua (Digenea: Heterophyidae)

Laboratory experiments indicated that the active life-span of the cercariae of Cryptocotyle lingua was temperature-dependent. An increase in water temperature and population age both correlated with a tendency for the organisms to become decaudate. The larvae were strongly photoresponsive to lateral light but with time, horizontal swimming rates (HSRs) from release point to light source progressively declined. When measured over a range of light quantities, HSRs peaked at 30 microM/m2/s. HSRs were also influenced by water temperature. A rapid increase occurred up to 15 degrees C after which there was a precipitous decline. HSRs to coloured light were negatively correlated with increasing wavelength. When offered a choice between colours, cercariae favoured the shorter wavelengths. The implications of these responses for successful transmission are discussed.

Behavioural responses of the cercariae of Cryptocotyle lingua (Digenea: Heterophyidae) to computer-controlled shadow sequences

The cercariae of Cryptocotyle lingua have a brief but active life during which they do not feed. In order to maximize the probability of infection they must respond to a variety of host-related stimuli yet conserve their finite energy reserves by minimizing unnecessary muscular activity. In laboratory experiments, simulated shadows representing the passage of a fish host were found to increase population activity levels. Evidence was also found for a relationship between shadow duration and the duration of subsequent bursts of swimming activity. Adaptation to continuous shadowing was recorded, its onset and magnitude being dependent on both the frequency and duration of shadows delivered. As cercariae aged they became less responsive to shadow stimuli, especially those of short duration.

The morphology and ultrastructure of the female reproductive ducts in the metacercaria and adult of Cryptocotyle lingua (Creplin) (Digenea:Heterophyida)

Encysted metacercariae of C. lingua require 38 days in the fish second intermediate host before they are infective to the bird definitive host. Morphogenesis of the metacercaria is arrested shortly after 38 days. The ovary and testes are recognizable at 10 days. By 38 days the lumen of the oviduct is apparent and cilia form on the epithelial lining. The receptaculum seminis remains rudimentary. The cells of the uterine primordium are vacuolated but no lumen is present. Laurer's canal, vitelline glands, and Mehlis's glands are not recognizable. Morphogenesis is resumed in the bird intestine. During day 1 the oviduct and receptaculum seminis complete their development; Laurer's canal and the uterine lumen are formed. During day 2 the cilia in the female ducts become motile and the vitelline and Mehlis's glands are present. Fertilization occurs on day 2 to day 3 and eggs arrive in the uterus on day 3.