Argentophilic structures as a diagnostic criterion for the discrimination of species of the genus Gyrodactylus von Nordmann (Monogenea)

Parasite Dispersal From the Ornamental Goldfish Trade

Goldfish, Carassius auratus Linnaeus, 1758, are immensely popular ornamental cyprinid fish, traded in more than 100 countries. For more than 500 years, human translocation has facilitated the spread of goldfish globally, which has enabled numerous and repeated introductions of parasite taxa that infect them. The parasite fauna assemblage of goldfish is generally well documented, but few studies provide evidence of parasite coinvasion following the release of goldfish. This review provides a comprehensive synopsis of parasites that infect goldfish in farmed, aquarium-held, native, and invasive populations globally and summarises evidence for the cointroduction and coinvasion of goldfish parasites. More than 113 species infect goldfish in their native range, of which 26 species have probably coinvaded with the international trade of goldfish. Of these, Schyzocotyle acheilognathi (Cestoda: Bothriocephalidae), Ichthyophthirius multifiliis (Ciliophora: Ichthyophthiriidae), Argulus japonicus (Crustacea: Argulidae), Lernaea cyprinacea (Crustacea: Ergasilidae), Dactylogyrus anchoratus, Dactylogyrus vastator and Dactylogyrus formosus (Monogenea: Dactylogyridae) are common to invasive goldfish populations in more than four countries and are considered a high risk of continued spread. Coinvasive parasites include species with direct and complex life cycles, which have successfully colonised new environments through utilisation of either new native hosts or suitable invasive hosts. Specifically, I. multifiliis, A. japonicus and L. cyprinacea can cause harm to farmed freshwater fish species and are important parasites to consider for biosecurity. These species may threaten other aquatic animal industries given their low host specificity and adaptable life histories. Future attention to biosecurity, management and border detection methods could limit the continued spread of exotic parasites from the ornamental trade of goldfish.

Ultrastructure of the external sensory apparatus of Gyrodactylus gasterostei Gläser, 1974

The aim of this study, therefore, was to investigate the ultrastructure of gyrodactylid sensilla and to ascertain how these may be employed in the colonisation of new hosts using the Gyrodactylus gasterostei Gläser, 1974--Gasterosteus aculeatus L. model. As Gyrodactylus has no specific transmission stage in its life-cycle, movement between hosts must be achieved by strategies employed by the adult. This study suggests that certain sensilla, presented for the first time in Gyrodactylus gasterostei, may serve as chemoreceptors and mechanoreceptors and possible photoreceptors. The results of this study provide photographic data using scanning electron microscope (SEM) and transmission electron microscopy (TEM) concerning the sensory structures that are found on the tegument and in the sub-tegumental zone of G. gasterostei that infect 3-spine sticklebacks (Gasterosteus aculeatus L.). For this reason, it is vital to understand the factors underlying transmission to a new host, and a detailed, ultrastructural examination of the sensory structures that are used may improve current understanding of the receptors that Gyrodactylus species employ to interpret both their host and ambient environments. Such information may assist in the interpretation of transmission behaviors, particularly their responses to chemical or physical cues which gyrodactylids employ in host location during the transmission process.

The biology of gyrodactylid monogeneans: the "Russian-doll killers"

This article reviews the history of gyrodactylid research focussing on the unique anatomy, behaviour, ecology and evolution of the viviparous forms while identifying gaps in our knowledge and directions for future research. We provide the first summary of research on the oviparous gyrodactylids from South American catfish, and highlight the plesiomorphic characters shared by gyrodactylids and other primitive monogeneans. Of these, the most important are the crawling, unciliated larva and the spike sensilla of the cephalic lobes. These characters allow gyrodactylids to transfer between hosts at any stage of the life cycle, without a specific transmission stage. We emphasise the importance of progenesis in shaping the evolution of the viviparous genera and discuss the relative extent of progenesis in the different genera. The validity of the familial classification is discussed and we conclude that the most significant division within the family is between the oviparous and the viviparous genera. The older divisions into Isancistrinae and Polyclithrinae should be allowed to lapse. We discuss approaches to the taxonomy of gyrodactylids, and we emphasise the importance of adequate morphological and molecular data in new descriptions. Host specificity patterns in gyrodactylids are discussed extensively and we note the importance of host shifts, revealed by molecular data, in the evolution of gyrodactylids. To date, the most closely related gyrodactylids have not been found on closely related hosts, demonstrating the importance of host shifts in their evolution. The most closely related species pair is that of G. salaris and G. thymalli, and we provide an account of the patterns of evolution taking place in different mitochondrial clades of this species complex. The host specificity of these clades is reviewed, demonstrating that, although each clade has its preferred host, there is a range of specificity to different salmonids, providing opportunities for complex patterns of survival and interbreeding in Scandinavia. At the same time, we identify trends in systematics and phylogeny relevant to the G. salaris epidemics on Atlantic salmon in Norway, which can be applied more generally to parasite epidemiology and evolution. Although much of gyrodactylid research in the last 30 years has been directed towards salmonid parasites, there is great potential in using other experimental systems, such as the gyrodactylids of poeciliids and sticklebacks. We also highlight the role of glacial lakes and modified river systems during the ice ages in gyrodactylid speciation, and suggest that salmon infecting clades of G. salaris first arose from G. thymalli in such lakes, but failed to spread fully across Scandinavia before further dispersal was ended by rising sea levels. This dispersal has been continued by human activity, leading to the appearance of G. salaris as a pathogen in Norway. We review the history and current status of the epidemic, and current strategies for elimination of the parasite from Norway. Finally, we consider opportunities for further spread of the parasite within and beyond Europe.

Chaetotaxy applied to Norwegian Gyrodactylus salaris Malmberg, 1957 (Monogenea) clades and related species from salmonids

Gyrodactylus salaris Malmberg, 1957 is a major pathogen of wild Salmo salar L. parr populations in Norway, and its delimitation from non-pathogenic species is important. The present study was undertaken to test the power of chaetotaxy to differentiate between three populations belonging to both the same and different clades (as stated by mtDNA) of G. salaris, in addition to three different species of gyrodactylids (G. salaris, G. thymalli and G. caledoniensis). The gyrodactylids were processed for chaetotaxy in situ and a maximum of 50 specimens per collection site were used to construct a generalised map over the sensilla. The sensilla were found in all populations to be symmetrically distributed around the median longitudinal axis, according to a formula of 7 dorsal (34 sensilla) and 8 ventral (44 sensilla) clusters on each side of the median line. The three Norwegian populations of G. salaris were found identical, as were the population of G. thymalli. The specimens of G. caledoniensis from Scotland, however, were found to differ from the Norwegian species G. salaris and G. thymalli by the position of one sensillum in two of the clusters. A comparison of the sensillum pattern of laboratory maintained G. salaris (River Lierelva) with results obtained ten years earlier, questions the temporal stability of the chaetotaxy pattern. The present results indicate that chaetotaxy can be used to discriminate between certain Gyrodactylus spp. but not generally.

Gyrodactylus quadratidigitus n. sp. (Monogenea: Gyrodactylidae), a parasite of the leopard-spotted goby Thorogobius ephippiatus (Lowe) from the south-western coast of the UK

Gyrodactylus quadratidigitus n. sp. (Monogenea, Gyrodactylidae) is described from the gills, fins and skin of Thorogobius ephippiatus (Lowe) (leopard-spotted goby) from Portland Castle Bay, Dorset, UK. G. quadratidigitus n. sp. most closely resembles the species of Gyrodactylus previously described by Geets from Pomatoschistus microps (Krøyer) in the approximate shape and size of the attachment hooks but can be readily distinguished from other species of the genus by the distinctive square shape of the toe region of the marginal hook sickle, the anteriorly positioned cirrus bulb and the unusually short intestinal crura which extend only as far as the level of the testes.

Chaetotaxy of the monogeneans Macrogyrodactylus clarii and m. congolensis from the gills and skin of the catfish clarias gariepinus in Egypt, with a note on argentophilic elements in the nervous system

A comparison has been made between the chaetotaxy of the gyrodactylid monogeneans Macrogyrodactylus clarii Gussev, 1961 and M. congolensis (Prudhoe, 1957) Yamaguti, 1963 from the gills and skin, respectively, of the catfish Clarias gariepinus (Burchell) from the river Nile in Egypt. Bilaterally arranged argentophilic structures on the surface of these parasites are presumed to be sensilla and are more abundant in M. clarii than in M. congolensis especially on the ventral surface (124 vs. 66). In both species these sensilla are concentrated on the head lobes and in the pharyngeal region, but there are features of the sensilla patterns that can be used to distinguish the two species. Comparison is made with sensilla patterns of other gyrodactylids. A system of cells and dendritic processes, most probably part of the nervous system, also has an affinity for silver in the two species. There are no previous records of extensive argentophilic elements in the nervous systems of monogeneans.

Life history specializations of monogenean flatworms: a review of experimental and microscopical studies

This review illustrates the use of experimental approaches combined with microscopy to study the biology of monogenean parasites. Studies of feeding, development, reproduction, and systematics have been based on gyrodactylids, flatworms infecting teleost fishes. In a contrasting system involving an amphibian host in a desert environment, analysis of adaptations to extreme conditions has focused on Pseudodiplorchis americanus. The unusual reproductive strategies, particularly the interactions between mother and offspring, are highlighted for both monogeneans. Species of Gyrodactylus are viviparous, maintaining up to three generations of embryos simultaneously in utero, and many of their reproductive specializations are related to progenesis. Embryo nutrition takes place via a metabolically-active syncytial uterine lining that has close association with the parental gut. Microscopy has also proved an essential adjunct to molecular studies of speciation and host specificity. P. americanus is ovoviviparous and the adaptations for embryo maintenance are unique. The primary keratin-type eggshell is replaced by a flexible secondary elastin capsule produced by the uterus; parental nutrients are transferred through cytoplasmic connections to the developing embryo. TEM has demonstrated unique adaptations of P. americanus to its micro-environments, including secretion of tegumental vesicles that provide protection from digestive enzymes during migration through the host gut. This paper highlights the potential of monogeneans for studies of fundamental biological principles.

The application of chaetotaxy in the discrimination of Gyrodactylus salaris Malmberg, 1957 (Gyrodactylidae: Monogenea) from species of the genus parasitising British salmonids

The chaetotaxy of argentophilic structures on three species of the monogenean genus Gyrodactylus was investigated in an attempt to distinguish species of this genus. Maps were prepared for Gyrodactylus salaris from Scandinavia and compared with two native species of Gyrodactylus parasitising salmonids in Britain, namely Gyrodactylus derjavini and Gyrodactylus truttae. The maps were subsequently refined and analysed for zones of homology and differentiation. The results demonstrate that G. salaris can be readily distinguished by this technique, which is, therefore, of great potential value in the identification of this notifiable pathogen. The key aggregations of sensilla discriminating G. salaris are, ventrally, the antero-ventral set, the medio-lateral set and the postero-lateral set, and, dorsally, the postero-dorsal set.