The microhabitat distribution of two Dactylogyrus species parasitizing the gills of the barbel, Barbus barbus

The microhabitat distribution of two congeneric species Dactylogyrus carpathicus and D. malleus (Monogenea) parasitizing the gills of the barbel (Barbus barbus L.) was investigated. We tested whether congeneric species exhibited microhabitat preference and whether interspecific interactions could be attributed to the microhabitat segregation of congeners. The outlying mean index method was used to evaluate species microhabitats. Gill variables (different microhabitats within gills) were used as environmental factors characterizing the gills. When abundances of both species were highest, and no significant difference was found between the abundance of the two species, the gill segments and gill areas were the most important factors segregating the Dactylogyrus species on the gills. Niche overlap was low within each of the four gill arches, and parasites were segregated in the same microhabitats within each gill arch. When abundances of both species were low, each monogenean species was segregated at the level of the gill arches. When abundances of both species increased, the niche and overlap between species increased. The distribution of both congeneric species confirmed microhabitat preference within the gills. The results suggest that microhabitat preference is dependent on species abundances, species being segregated in the case of low abundance, possibly to increasing mating opportunities. Both niche and overlap between species increased with species abundance. In the case of the high abundance of both species, microhabitat preference seems to be related to interspecific interactions between monogenean species, as previously found for endoparasitic species.     

Spatial distributions of two species of monogeneans on the gills of Siganus fuscescens (Houttuyn) and their seasonal dynamics in caged versus wild-caught hosts

Two monogeneans species, Polylabris mamaevi Ogawa & Egusa, 1980 and Tetrancistrum nebulosi Young, 1967, are sympatric on the gills of Siganus fuscescens (Houttugn, 1782) in the South China Sea. An investigation into their distribution on the host's gills based on monthly samples from both wild and caged populations revealed that these 2 species co-occurred on the same gill arches, but they were segregated. P. mamaevi attached to the distal half of the gill filaments, whereas T. nebulosi attached to the proximal half. This difference may partly be attributed to microhabitat specialization. Microhabitat distributions were intensity dependent for both species. As intensity increased, monogeneans were increasingly found on the second, third, and even fourth gill arches, and niche breadths increased. The changes in distribution and niche breadths at the host gill arch level in relation to differences in host size and sampling period were considered to represent a functional response to increased intensity. Differences in host resources, in either net-caged or wild populations, did not significantly influence the parasite distributions at the host gill arch level. Simultaneous infections of these 2 species were common, but there was no evidence of negative interspecific interactions.     

Microhabitat distributions and species interactions of ectoparasites on the gills of cichlid fish in Lake Victoria,Tanzania

Heterogeneous exposure to parasites may contribute to host species differentiation. Hosts often harbour multiple parasite species which may interact and thus modify each other’s effects on host fitness. Antagonistic or synergistic interactions between parasites may be detectable as niche segregation within hosts. Consequently, the within-host distribution of different parasite taxa may constitute an important axis of infection variation among host populations and species. We investigated the microhabitat distributions and species interactions of gill parasites (four genera) infecting 14 sympatric cichlid species in Lake Victoria, Tanzania. We found that the two most abundant ectoparasite genera (the monogenean Cichlidogyrus spp. and the copepod Lamproglena monodi) were non-randomly distributed across the host gills and their spatial distribution differed between host species. This may indicate microhabitat selection by the parasites and cryptic differences in the host–parasite interaction among host species. Relationships among ectoparasite genera were synergistic: the abundances of Cichlidogyrus spp. and the copepods L. monodi and Ergasilus lamellifer tended to be positively correlated. In contrast, relationships among morphospecies of Cichlidogyrus were antagonistic: the abundances of morphospecies were negatively correlated. Together with niche overlap, this suggests competition among morphospecies of Cichlidogyrus. We also assessed the reproductive activity of the copepod species (the proportion of individuals carrying egg clutches), as it may be affected by the presence of other parasites and provide another indicator of the species specificity of the host–parasite relationship. Copepod reproductive activity did not differ between host species and was not associated with the presence or abundance of other parasites, suggesting that these are generalist parasites, thriving in all cichlid species examined from Lake Victoria.     

Is there evidence of niche restriction in the spatial distribution of Kroyeria dispar Wilson, 1935, K.?papillipes Wilson, 1932 and Eudactylina pusilla Cressey, 1967 (Copepoda: Siphonostomatoida) on the gill filaments of tiger sharks Galeocerdo cuvier off KwaZulu-Natal, South Africa?

Host and microhabitat or site selection is universal among parasites, although to varying degrees between species and groups. The selection of a specific site by a copepod parasite is determined by a set of mostly unknown factors. The spatial distribution of Kroyeria dispar, Kroyeria papillipes and Eudactylina pusilla on the gill filaments of Galeocerdo cuvier was investigated to determine whether niche restriction occurs in the different planes on the gills. In order to do this, the complete sets of left gills of 14 tiger shark hosts were examined and the species, location (hemibranch, horizontal distribution, longitudinal distribution), orientation and gender of each copepod noted. Kroyeria dispar was dominant on most hosts and exhibited a prevalence of 100% and a mean intensity of 73 individuals per shark. Kroyeria papillipes was dominant on the remaining hosts and displayed a prevalence of 78.6% and a mean intensity of 33 individuals per shark while E. pusilla had a prevalence of 85.7% and a mean intensity of 20 individuals per shark on the examined hosts. No evidence of intra- or interspecific competition or microniche restriction was found even though all three species occupy the same fundamental niche. However, distributional preference was observed, and the compound populations of all three species on tiger sharks were aggregated, most likely as a result of their need to reproduce. Handling editor: S. I. Dodson     

Spatial distribution and coexistence of monogenean gill parasites inhabiting two damselfishes from Moorea island in French Polynesia

The ectoparasite fauna of two damselfishes, Stegastes nigricans and Dascyllus aruanus, from Moorea Island in French Polynesia was investigated. Gills of these damselfishes were infected with congeneric Monopisthocotylea Monogenea belonging to the genus Haliotrema. Stegastes nigricans were found to harbour a guild of three Haliotrema species whereas only one species inhabited D. aruanus. Microhabitat distribution, inter- and intraspecific competition and interspecific associations on the gill were studied. Observations on site preference revealed no spatial segregation between the three congeneric species inhabiting the gills of S. nigricans. Juvenile and adult monogeneans of that guild occurred on the same microhabitat. The dominant species Haliotrema sp. 1 did not expand on the microhabitat when the intensity of infection increased. Interspecific association tests revealed positive and negative associations. Haliotrema sp. 4 expanded its distribution on the gills of Dascyllus aruanus when the intensity of infection increased suggesting the likelihood of intraspecific competition. Juvenile and adult monogeneans of Haliotrema sp. 4 appeared to segregate as a result of intraspecific competition. This competition may exist to enhance resource availability when the gill habitat is limited. Overlaps between niche breadth and species microhabitat were revealed for monogenean species inhabiting S. nigricans. Interspecific competition did not appear to play an important role in the distribution of S. nigricans congeneric ectoparasites. Reinforcement of reproductive barriers may have led to the avoidance of hybridization.     

Invasion of Ceratomyxa shasta (Myxozoa) and comparison of migration to the intestine between susceptible and resistant fish hosts

The myxozoan parasite Ceratomyxa shasta infects salmonids causing ceratomyxosis, a disease elicited by proliferation of the parasite in the intestine. This parasite is endemic to the Pacific Northwest of North America and salmon and trout strains from endemic river basins show increased resistance to the parasite. It has been suggested that these resistant fish (i) exclude the parasite at the site of invasion and/or (ii) prevent establishment in the intestine. Using parasites pre-labeled with a fluorescent stain, carboxyfluorescein succinimidyl diacetate (CFSE), the gills were identified as the site of attachment of C. shasta in a susceptible fish strain. In situ hybridization (ISH) of histological sections was then used to describe the invasion of the parasites in the gill filaments. To investigate differences in the progress of infection between resistant and susceptible fish, a C. shasta-susceptible strain of rainbow trout (Oncorhynchus mykiss) and a C. shasta-resistant strain of Chinook salmon (Oncorhynchus tshawytscha) were sampled at consecutive time points following exposure at an endemic site. Using ISH in both species, the parasite was observed to migrate from the gill epithelium into the gill blood vessels where replication and release of parasite stages occurred. Quantitative PCR verified entry of the parasite into the blood. Parasite levels in blood increased 4 days p.i. and remained at a consistent level until the second week when parasite abundance increased further and coincided with host mortality. The timing of parasite replication and migration to the intestine were similar for both fish species. The field exposure dose was unexpectedly high and apparently overwhelmed the Chinook salmon’s defenses, as no evidence of resistance to parasite penetration into the gills or prevention of parasite establishment in the intestine was observed.     

对鲇鱼鳃部寄生的固着鳋的生态研究

作者研究了江西江口水库鲇鳃部寄生固着鳋的种群生态学及其对寄生部位的选择性。固着鳋的感染率和丰盛度表现出明显的季节变化,以夏初最高;其繁殖也具有季节性,怀卵个体的比例同样有夏初最高。固着鳋对寄生部位的选择性似乎与种群的数量有关,当种群数量高时更多的固着鳋分布于中间的鳃片上,对多种寄生虫同时寄生条件下固着鳋寄生部位选择性的研究,将有助于进一步了解固着鳋和寄生鲇鳃部其它种类的寄生虫之间的关系。     

Phylogeny,ultrastructure and histopathology of Myxobolus lomi sp. nov., a parasite of Prochilodus lineatus (Valenciennes, 1836) (Characiformes: Prochilodontidae) from the Peixes River,São Paulo State,Brazil

This paper presents the morphological, histological, molecular and ultrastructural data on Myxobolus lomi sp. nov., a parasite of the gill filaments of Prochilodus lineatus from the Peixes River (48º06′38″W; 22º 49′53.1″S), São Paulo State, Brazil. From 20 P. lineatus specimens examined, 90.0% (n = 18) were infected. The plasmodia were white and round, measuring 250 to 300 μm in diameter and the development occurred in the base of the gill filament. The spores showed symmetrical and smooth valves, with the polar filament having 8 to 11 coils. A thorough comparison with all the Myxobolus species described so far is provided. A partial sequencing of the 18S rDNA gene revealed approximately 1600-bp. The Myxobolus species parasite of P. lineatus did not match any of the Myxozoa available in GenBank. In the phylogenetic analysis, M. lomi sp. nov. is clustered with ten other species and only four of these parasites were from gills. Histological analysis of P. lineatus gills infected by M. lomi sp. nov. revealed numerous well-delimited cysts at the base of the primary lamella, between connective tissue and bone, next to the gill arteries. However no pronounced inflammatory response was found at the infection site.     

Tissue damage and leukocytic infiltration following attachment of the mite Unionicola intermedia to the gills of the bivalve mollusc Anodonta anatina

Unionicola intermedia lives as a parasite in the gill region of Anodonta anatina. It attaches to the gills of its host by means of the pedipalps which cause displacement, rupture, and erosion of the gill epithelium. The pedipalps are sunk deeply into the underlying connective tissue of the gills and produce a leukocytic infiltration into the damaged area and a consequent edema of the gill filaments.     

Morphology and pathology of the ectoparasitic copepod, Nicothoë astaci ('lobster louse') in the European lobster, Homarus gammarus

Ectoparasitic copepods have been reported in a wide range of aquatic animals, including crustacean shellfish. However, with the exception of the salmon louse, Lepeophtheirus salmonis, our knowledge of such parasites in commercial species is rudimentary. The current study examines the morphology and pathology of the parasitic copepod, Nicotho? astaci (the 'lobster louse') in its host, the European lobster, Homarus gammarus. Lobsters were sampled from waters surrounding Lundy Island (Bristol Channel, UK) and all individuals collected were found to harbour female adult N. astaci in their gills, with a mean of 47·3 parasites/lobster. The majority of N. astaci were found in the basal region of pleurobranch gills. The parasite was found to attach to gill filaments via its oral sucker, maxillae and maxillipeds, and to feed on host haemolymph (blood) through a funnel-like feeding channel. It caused varying degrees of damage to the host gill, including occlusion of gill filaments and disruption to the vascular system in the central axis. Although there was evidence of extensive host response (haemocytic infiltration) to the parasite, it was displaced from the parasite attachment site and thus was observed in the central gill axis below. The region of gill filament immediately underlying the parasite feeding channel was devoid of such activity suggesting that the parasite interferes with the cellular defence and haemostatic mechanisms of the lobster in order to maintain invasion of the host.     

Co-existence of nine gill ectoparasites (Dactylogyrus: monogenea) parasitising the roach (Rutilus rutilus l.): history and present ecology

Co-existence among potentially competing species can be favoured by niche specialisation and/or by reducing the overall intensity of competition via aggregated utilisation of fragmented resources. We investigated the respective roles of niche specialisation and aggregation in the case of nine congeneric monogenean parasites on the gills of Roach (Rutilus rutilus L.) belonging to the genus Dactylogyrus. The position of each individual parasite of the nine Dactylogyrus species was recorded. Niche breadth and niche overlap of parasite species were estimated. Comparative methods, which take into account phylogenetic information of the analysed species, were used. We reconstructed a phylogeny of the nine Dactylogyrus species based on morphological characters. We used the 'aggregation model of co-existence' in the model to test if species co-existence is facilitated when intraspecific aggregation exceeds interspecific aggregation. We observed a lack of negative correlation in abundance between pairs of parasites, and a negative correlation between niche size and parasite aggregation, for both intraspecific and interspecific aggregation. Our comparative analysis showed that parasite abundance is positively correlated with niche breadth. Then parasite abundance, and not interactions between Dactylogyrus species, seems to be the most important factor determining niche size This result gives some support to niche segregation by specialisation. Niche size was negatively correlated with both intraspecific and interspecific aggregation. No relationship was found between an increase of interspecific aggregation with an increase of niche overlapping, which suggests that competition may play little role. A lack of competition could be also confirmed by the lack of negative correlation in abundance between species pairs. A parsimony analysis of the evolution of gill distribution indicates a change in one parameter of the niche (arch, segment and/or area) at each branching event.     

Genomic regions of pufferfishes responsible for host specificity of a monogenean parasite, Heterobothrium okamotoi

The genetic mechanisms underlying host specificity of parasitic infections are largely unknown. After hatching, the larvae of the monogenean parasite, Heterobothrium okamotoi, attach to the gill filaments of hosts and the post-larval worms develop there by consuming nutrients from the host. The susceptibility to H. okamotoi infection differs markedly among fish species. While this parasite can grow on tiger pufferfish (also called fugu), Takifugu rubripes, it appears to be rejected by a close congener, grass pufferfish, Takifugu niphobles, after initial attachment to the gills. To determine the genetic architecture of the pufferfish responsible for this host specificity, we performed genome-wide quantitative trait loci analysis. We raised second generation (F₂) hybrids of the two pufferfish species and experimentally infected them with the monogenean in vivo. To assess possible differences in host mechanisms between early and later periods of infection, we sampled fish three h and 21 days after exposure. Genome scanning of fish from the 3 h infection trial revealed suggestive quantitative trait loci on linkage groups 2 and 14, which affected the number of parasites on the gill. However, analysis of fish 21 days p.i. detected a significant quantitative trait locus on linkage group 9 and three other suggestive quantitative trait loci on linkage groups 7, 18 and 22. These results indicated the polygenic nature of the host mechanisms involved in the infection/rejection of H. okamotoi. Moreover the analyses suggested that host factors may play a more important role during the growth period of the parasite than during initial host recognition at the time of attachment. Within the 95% confidence interval of the linkage group 9 quantitative trait locus in the fugu genome, there were 214 annotated protein-coding genes, including immunity-related genes such as Irak4, Muc2 and Muc5ac.     

The attachment of the monogenean parasite Callorhynchicola multitesticulatus to the gills of its holocephalan host Callorhynchus milii

Llewellyn J. and Simmons J. E. 1984. The attachment of the monogenean parasite Callorhynchicola multitesticulatus to the gills of its holocephalan host Callorhynchus milii. International Journal for Parasitology14: 191–196. C. multitesticulatus when juvenile (up to about 10 mm long) attaches itself by four pairs of clamps to the secondary gill lamellae of its host the elephant fish Callorhynchus milii. However, in adult parasites the haptor invades the host tissue and the stalk connecting the haptor with the body-proper becomes surrounded by a sleeve of fibrous host inflammatory reaction tissue. The stalk is provided with annuli with anteriorly-directed flanges which are pressed into the walls of the sleeve of host tissue when the diameter of the stalk increases as a result of the contraction of well-developed sub-tegumentary longitudinal muscles.     

Phylogeny, ultrastructure, histopathology and prevalence of Myxobolus oliveirai sp. nov., a parasite of Brycon hilarii (Characidae) in the Pantanal wetland, Brazil

This paper presents the morphological, histological and ultrastructural characteristics of Myxobolus oliveirai sp. nov., a parasite of the gill filaments in Brycon hilarii from the Brazilian Pantanal. Out of 216 B. hilariispecimens examined (126 wild and 90 cultivated), 38.1% of wild specimens (n = 48) were infected. The parasites form elongated plasmodia primarily in the tip of gill filaments, reaching about 3 mm in length. A thorough comparison with all the Myxobolus species described from South American hosts, as well as nearly all the Myxobolus species described so far is provided. Partial sequencing of the 18S rDNA gene revealed a total of 1,527 bp. The Myxobolus species parasite of B. hilarii did not match any of the Myxozoa available in GenBank. In the phylogenetic analysis, M. oliveirai sp. nov. composed a monophyletic group with eight other species: five species of Myxobolus parasites of mugilid fishes, two parasites of pangasiid and one of centrarchid. Infection prevalence values of the parasite revealed no significant differences between wet and dry seasons or between males and females. The importance of the infection to the farming of the host species is emphasized.     

Parasite fauna of introduced pumpkinseed fish Lepomis gibbosus: first British record of Onchocleidus dispar (Monogenea)

A survey of pumpkinseed Lepomis gibbosus from a stillwater fishery in southern England revealed infections by the ancyrocephalid monogenean Onchocleidus dispar at 100% prevalence. Host specificity of O. dispar to North American centrarchid fish suggests that it is a non-native parasite, introduced to Britain with L. gibbosus. Mean intensity of O. dispar was significantly higher in male (25.5 parasites) compared to female (15) and immature (7) hosts, but was not influenced by host standard length or sampling time. This sex-biased parasitism is likely to be related to both ecological and physiological factors, such as differential exposure to infective stages during nest building, or higher susceptibility to infection due to enhanced host stress levels. O. dispar significantly dominated the external surfaces and the outer gill arches. Analysis of the spatial distribution of the parasite between the 9 regions of gill surface determined that as density increased, higher numbers of parasites were found on the anterior and central gill regions compared to posterior (distal) regions. Apparent microhabitat selection is discussed in terms O. dispar life cycle, maximising oxygen availability, avoiding unstable areas and increasing intraspecific contact. The absence of O. dispar in the gills of roach, rudd and gudgeon sampled from the same fishery supports the assumption that this parasite is currently of little threat to native fish populations.     

Host, macrohabitat, and microhabitat specificity in the gill parasite Afrodiplozoon polycotyleus (Monogenea)

Studies on species of Monogenea have shown that these parasites often infect only a specific host species, genus, or family, and that they attach only to specific sites within hosts. Few studies, however, examine habitat specificity across host and habitat scales. In this study, we focused on host, macrohabitat, and microhabitat specificity in the monogenean diplozoon Afrodiplozoon polycotyleus, a gill parasite of African cyprinid fishes, Barbus spp. We first compared the occurrence of A. polycotyleus among 4 species of Barbus from a single location in the Mpanga River of western Uganda; Barbus neumayeri was the only species infected with the parasite. We then quantified parasite prevalence and mean abundance in B. neumayeri from a series of river and swamp sites in the same drainage, looking for environmental predictors of diplozoon prevalence and abundance over a broad habitat scale. The prevalence and mean abundance of A. polycotyleus on gills of B. neumayeri was highest in the hypoxic swamp habitat, followed by the intermittent stream sites, and faster flowing river sites. Parasite prevalence and mean abundance across habitats were negatively related to both water current and dissolved oxygen concentration. Within hosts, A. polycotyleus was strongly specific among hemibranchs in poorly oxygenated water and was found on arch 2, hemibranch 4 most frequently.     

Muddied waters: suspended sediment impacts on gill structure and aerobic scope in an endangered native and an invasive freshwater crayfish

Suspended sediment (SS) loadings in freshwater habitats have increased over the past century and SS is now a significant environmental stressor. Greater tolerance to environmental stressors has been proposed as a factor in the success of aquatic invasive species. Further, parasites may interact with environmental stressors to increase host susceptibility to loss of fitness and mortality. We compared the effects of SS exposure on the gill structure and aerobic scope of the endangered white-clawed crayfish (Austropotamobius pallipes), and the invasive signal crayfish (Pacifastacus leniusculus), and assessed impacts in relation to parasite burden. SS caused gill fouling and reduction in aerobic scope in both species, though A. pallipes was more susceptible than invasive P. leniusculus. The parasite Branchiobdella astaci, a crayfish worm that infests the gills, interacted with the sediment to affect gill structure whereas infection with the microsporidian parasite Thelohania contejeani had no effect on crayfish response to SS. Juvenile P. lenisuculus had a higher standard metabolic rate than A. pallipes, which may be linked to competitive advantages such as higher growth rate and behavioural dominance. Conservation of A. pallipes often involves relocation of threatened populations to isolated stillwaters; our findings suggest that SS levels should be assessed before relocation.     

Does timing matter? How priority effects influence the outcome of parasite interactions within hosts

In nature, hosts are exposed to an assemblage of parasite species that collectively form a complex community within the host. To date, however, our understanding of how within-host–parasite communities assemble and interact remains limited. Using a larval amphibian host (Pacific chorus frog, Pseudacris regilla) and two common trematode parasites (Ribeiroia ondatrae and Echinostoma trivolvis), we experimentally examined how the sequence of host exposure influenced parasite interactions within hosts. While there was no evidence that the parasites interacted when hosts were exposed to both parasites simultaneously, we detected evidence of both intraspecific and interspecific competition when exposures were temporally staggered. However, the strength and outcome of these priority effects depended on the sequence of addition, even after accounting for the fact that parasites added early in host development were more likely to encyst compared to parasites added later. Ribeiroia infection success was reduced by 14 % when Echinostoma was added prior to Ribeiroia, whereas no such effect was noted for Echinostoma when Ribeiroia was added first. Using a novel fluorescent-labeling technique that allowed us to track Ribeiroia infections from different exposure events, we also discovered that, similar to the interspecific interactions, early encysting parasites reduced the encystment success of later arriving parasites by 41 %, which could be mediated by host immune responses and/or competition for space. These results suggest that parasite identity interacts with host immune responses to mediate parasite interactions within the host, such that priority effects may play an important role in structuring parasite communities within hosts. This knowledge can be used to assess host–parasite interactions within natural communities in which environmental conditions can lead to heterogeneity in the timing and composition of host exposure to parasites.     

Gyrodactylus unicopula Glukhova, 1955, from young plaice Pleuronectes platessa L. with notes on the ecology of the parasite

A population of Gyrodactylus unicopula Glukhova, 1955, is described from the gill filaments of young plaice from a nursery ground on the north-west coast of Scotland. The seasonal incidence of this parasite was followed by sampling the 1965 year class of plaice at intervals of from one to three months from April 1965 to April 1967. The level of intensity is usually low and shows no progressive increase, although the fish are shown to be capable of supporting much higher populations of the parasite, which suggests the existence of some mechanism controlling the level of intensity in the natural population. The distribution of G. unicopula on the filamentous surface of the gills is shown in detail. χ² analyses indicated no significant association or antagonism between this parasite and two ciliates commonly found on the gills of the same population of plaice. Gyrodactyliasis, the disease caused by parasites of the genus Gyrodactylus, is discussed with special reference to marine fish farming.     

Involvement of host recognition by oncomiracidia and post-larval survivability in the host specificity of Heterobothrium okamotoi (Monogenea: Diclidophoridae)

Heterobothrium okamotoi, a monogenean gill parasite, shows high host specificity for tiger puffer, Takifugu rubripes. In the present study, in vivo and in vitro experimental infections were conducted using various fish species, including T. rubripes, to understand the mechanisms of specificity. In in vivo experiments, T. rubripes, grass puffer, Takifugu niphobles, olive flounder, Paralichthys olivaceus, and red sea bream, Pagrus major, were exposed to oncomiracidia of H. okamotoi labelled with a fluorescent dye, 5- (and -6) carboxyfluorescein diacetate succinimidyl ester, and the numbers of parasites on the gills and skin were recorded at intervals. Oncomiracidia were attached to gills and skin of all the experimental fish species immediately after exposure, and the infection intensity on T. rubripes was higher than that on T. niphobles and much higher than those on the other two species. After 2 days, the attached parasites remained on the gills of T. rubripes, but disappeared from the other hosts. During in vitro experiments, gill filaments excised from seven different fish species (four fish species used in the in vivo experiments and panther puffer, Takifugu pardalis, southern flounder, Paralichthys lethostigma and spotted halibut, Verasper variegates) were exposed to oncomiracidia and the attachment to each fish species and subsequent larval behaviour was observed. The percentage of post-larvae that attached to T. rubripes was slightly higher than those which attached to congeneric fish species and much higher than those of non-tetraodontid fish species. In vivo and in vitro experiments demonstrated that oncomiracidia of H. okamotoi have an affinity for their natural host, T. rubripes, and congeneric fish species. The disappearance of attached post-larvae from 'alien' hosts within 2 days during in vivo experiments suggested that host recognition by oncomiracidia and subsequent post-larval survivability are involved in the host specificity of H. okamotoi.