Merging species? Evidence for hybridization between the eel parasites Anguillicola crassus and A. novaezelandiae (Nematoda, Anguillicolidea)

Hybridization between Anguillicola crassus and A. novaezelandiae, and viability of the F1 generation

For decades, it has remained unclear how the Asian swim bladder nematode Anguillicola crassus was able to supplant the previously stable population of its relative from New Zealand Anguillicola novaezelandiae in the Lake Bracciano, Italy. Previously, researchers have hypothesized that A. crassus possesses an ecological advantage due to a more efficient life cycle in combination with a pattern of unidirectional hybridization between A. novaezelandiae females and A. crassus males. The present study focuses on the viability of hybrid offspring and their allelic pattern, particularly in developed adult stages of the hybrid F1 generation. While the percentages of hybrid individuals from A. novaezelandiae mothers and A. crassus fathers increased from egg to adult stages, it was more distinct in egg stages of A. crassus females and A. novaezelandiae males, but did not occur in adult F1 individuals at all. Therefore, we corroborate the hypothesis of unidirectional hybridization by differentiating between egg and adult stages, and suggest this as another explanatory factor for the extinction of A. novaezelandiae in Lake Bracciano in Italy and the predominance of A. crassus.

Size does matter-a closer look on Anguillicola morphology

The present study deals with morphological differences between two closely related parasitic nematode species (Anguillicola crassus Kuwahara et al., 1974 and Anguillicola novaezelandiae Moravec & Taraschewski, 1988) in two different experimentally infected eel species (Anguilla anguilla Linnaeus, 1758 and Anguilla japonica Temminck & Schlegel, 1847). Furthermore, it considers the question whether size differences between those two species are ontogenetically determined or host species-dependent. In order to analyse these questions, experimental infections with the four possible host-parasite systems have been performed, followed by precise morphometric measurements related to body size and head structures of all resulting nematodes 120 days post infection. Body size measurements (length and width) of A. crassus generally exceeded those of A. novaezelandiae, while both Anguillicola species turned out to be smaller in Japanese eels than in European eels. Comparative measurements of neck width, maximum oesophagus width, and posterior head end width were found to be highly significant with regard to the different host-parasite systems. Shape and width of neck have been identified as reliable discriminating factors for species distinction. Generally, the relation of anterior head end width and neck width proved to be distinctly species-specific and can thus serve as a decisive and easily measureable distinguishing feature.

Competing invaders: Performance of two Anguillicola species in Lake Bracciano

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Two Anguillicola spp. occurring as nonindigenous species are compared in the review.

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Both species differ in their life cycles.

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Anguillicola crassus replaced Anguillicola novaezelandiae in Lake Bracciano.

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Possible reasons for the disappearance of A. novaezelandiae are discussed.

Anguillicola crassus is one of the most successful parasitic invasive species as it has spread from its original habitat in East Asia throughout the world and has acquired five new eel host species in the course of its invasion within the last three decades. Records from an Italian lake indicate that this species has even displaced an established population of its close relative A. novaezelandiae originating from New Zealand. In order to analyze the reasons for its high invasive potential, this review highlights recent studies, which substantiate the selective advantages of A. crassus over A. novaezelandiae.

Laboratory infection experiments revealed that A. crassus features a less synchronized development compared to A. novaezelandiae in the European eel, which enables this species to emit eggs over a longer period of time. Differences in the time period required for first egg output and in the maturation process of second stage larvae in intermediate hosts could also be detected, which may lead to differences in infection potential. Finally, microsatellite analyses have shown that hybridization processes are possible, but might only occur between A. crassus males and A. novaezelandiae females. Taken as a whole, the sum of minor selective advantages and differences in life cycle traits could have considerably contributed to a replacement of one species by the other.

Comparison of infection success, development and swim bladder pathogenicity of two congeneric Anguillicola species in experimentally infected Anguilla anguilla and A. japonica

Two closely related parasites, Anguillicola crassus and Anguillicola novaezelandiae, originally parasitizing swim bladders of the Japanese eel (Anguilla japonica and the Short-finned eel (Anguilla australis), respectively, were used for analyzing the infection success of each parasite species on either long-known, recently acquired or new definitive host species and the associated effects on the eels' swim bladders. On that account, European eels (Anguilla anguilla) and Japanese eels were experimentally infected with both Anguillicola species in the laboratory. Susceptibility of the two eel species to both parasite species was determined by analyses of infection data. Subsequently, histopathological effects of the nematodes on the hosts' swim bladders were characterized according to already established indices.The present study revealed significant differences between the four different host-parasite systems regarding recovery rates, infrapopulations, and damage levels. Both nematode species achieved significantly lower recovery rates in Japanese eels than in European eels, since the examined swim bladders of Japanese eels contained a high amount of dead encapsulated larvae, whereas those of European eels contained only living nematodes. Encapsulation of larvae in Japanese eels was associated with a distinct thickening of the swim bladder walls. The swim bladders of uninfected Japanese eels turned out to be generally thicker than those of European eels. Infection with both Anguillicola species resulted in a further thickening process of the swim bladder walls in Japanese eels, whereas those of European eels showed only minor changes. The two established classification systems turned out to be inapplicable, since the measurements and the macroscopic evaluations of the swim bladders of the two infected eel species did not entirely correspond to the underlying criteria.

Can differences in life cycle explain differences in invasiveness? - A study on Anguillicola novaezelandiae in the European eel

Anguillicola crassus is the most invasive species of its genus and it is a successful colonizer of different eel species worldwide. It is so far the only species of the genus Anguillicola whose life cycle has been studied completely. To analyse whether differences in life cycle may explain differences in invasiveness, we infected European eels with Anguillicola novaezelandiae under laboratory conditions. Anguillicola novaezelandiae shows a synchronized development in the European eel. Eggs with second-stage larvae appeared 120 days after infection. No density-dependent effect in parasite development could be found for A. novaezelandiae. The present study shows that the life cycle of A. novaezelandiae differs on final host level compared with A. crassus in ways which result in a less successful invasion of new host species.

Natural Anguillicola novaezelandiae infection--is there seasonality in New Zealand?

Knowledge of natural Anguillicola infections of Short-finned eels (Anguilla australis) in New Zealand is very limited. So far, no study contains data on all life cycle stages of Anguillicola novaezelandiae in naturally infected eels. In order to study the frequency of A. novaezelandiae in New Zealand Short-finned eels, we examined eels of the North and the South Island for the presence of the swim bladder parasite. The results show that A. novaezelandiae is a common parasite of the Short-finned eel. The parasite was present in both regions. Eels from both localities show differences in their infection status with respect to prevalence, abundance and intensity. While eels of the South Island were only infected with larval stages, adult and preadult stages could be detected in eels of the North Island. Nevertheless, infrapopulations at both sites were dominated by larval parasite stages. This unique composition of infrapopulations has never been described for any Anguillicola species before and suggests a seasonal occurrence as a possible reason. Export of live eels should be handled cautiously to prevent the spread of A. novaezelandiae throughout other eel populations.

Hybridization between two cestode species and its consequences for intermediate host range

Background

Many parasites show an extraordinary degree of host specificity, even though a narrow range of host species reduces the likelihood of successful transmission. In this study, we evaluate the genetic basis of host specificity and transmission success of experimental F₁ hybrids from two closely related tapeworm species (Schistocephalus solidus and S. pungitii), both highly specific to their respective vertebrate second intermediate hosts (three- and nine-spined sticklebacks, respectively).

Methods

We used an in vitro breeding system to hybridize Schistocephalus solidus and S. pungitii; hybridization rate was quantified using microsatellite markers. We measured several fitness relevant traits in pure lines of the parental parasite species as well as in their hybrids: hatching rates, infection rates in the copepod first host, and infection rates and growth in the two species of stickleback second hosts.

Results

We show that the parasites can hybridize in the in vitro system, although the proportion of self-fertilized offspring was higher in the heterospecific breeding pairs than in the control pure parental species. Hybrids have a lower hatching rate, but do not show any disadvantages in infection of copepods. In fish, hybrids were able to infect both stickleback species with equal frequency, whereas the pure lines were only able to infect their normal host species.

Conclusions

Although not yet documented in nature, our study shows that hybridization in Schistocephalus spp. is in principle possible and that, in respect to their expanded host range, the hybrids are fitter. Further studies are needed to find the reason for the maintenance of the species boundaries in wild populations.