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The Echinorhynchidae has a long research history, but its mitochondrial genome evolution remains poorly understood, hindering phylogenetic resolution. In this study, we report the first complete mitochondrial genome of the genus Echinorhynchus , obtained from its type species, Echinorhynchus gadi . The circular mitogenome was 17,696 bp in length and contained 39 genes: 12 protein-coding genes (lacking atp8 ), two ribosomal RNA genes, and 25 transfer RNA genes, including two extra copies of trnW and one extra copy of trnV . Five non-coding regions were identified; the major non-coding region contained tandem repeats and pseudogene fragments, consistent with a tandem duplication and random loss mechanism. Phylogenetic analysis based on the concatenated amino acid sequences of the 12 protein-coding genes placed E. gadi and E. truttae in a well-supported monophyletic clade representing the genus Echinorhynchus . This clade was sister to Aspersentis megarhynchus , supporting a close relationship between Echinorhynchidae and Heteracanthocephalidae. Because the published E. truttae mitogenome is incomplete, this study fills a critical genomic gap and provides a valuable molecular resource for future taxonomic, systematic, and evolutionary studies of Acanthocephala.

Amphipods Eogammarus tiuschovi were experimentally infected by the acanthocephalan Echinorhynchus gadi (Acanthocephala: Echinorhynchidae). Within the first four days post-infection, acanthors of the acanthocephalan caused the cellular response of the host, which ended with their complete encapsulation on day 4 post-infection. The acanthors obtained through the experiment were examined ultrastructurally. Two syncytia (frontal and epidermal) and a central nuclear mass are found in the acanthor’s body. The frontal syncytium has 3–4 nuclei and contains secretory granules with homogeneous, electron-dense contents. Since the secretory granules occupy only the anterior one-third of this syncytium, it is suggested that the contents of these granules are involved in the acanthor’s migration through the gut wall of the amphipod. The central nuclear mass consists of an aggregation of fibrillar bodies and a few electron-light nuclei distributed on the periphery. Some of these nuclei, located near the central nuclear mass, are assumed to be a source of the acanthocephalan’s internal organs. The epidermal syncytium surrounds the frontal syncytium and the central nuclear mass. It is represented by a superficial cytoplasmic layer, but the bulk of the cytoplasm is concentrated in the posterior one-third of the acanthorʼs body. Syncytial nuclei are evenly distributed throughout the cytoplasm. The muscular system of the acanthors consists of 10 longitudinal muscle fibers located below the superficial cytoplasmic layer and two muscle retractors crossing the frontal syncytium.

The acanthocephalan genus Echinorhynchus Zoega in Müller, 1776 (sensuYamaguti 1963) is a large and widespread group of parasites of teleost fish and malacostracan crustaceans, distributed from the Arctic to the Antarctic in habitats ranging from freshwaters to the deep-sea. A total of 52 species are currently recognised based on the conventional morphological species concept; however, the true diversity in the genus is masked by cryptic speciation. The considerable diversity within Echinorhynchus is an argument for subdividing the genus if monophyletic groups with supporting morphological characters can be identified. With this objective in mind, partial sequences of two genes with different rates of evolution and patterns of inheritance (nuclear 28S rRNA and mitochondrial cytochrome c oxidase subunit I) were used to infer the phylogenetic relationships among eight taxa of Echinorhynchus. These included representatives of each of three genus group taxa proposed in a controversial revision of the genus based on cement gland pattern, namely Echinorhynchus (sensu stricto), Metechinorhynchus Petrochenko, 1956 and Pseudoechinorhynchus Petrochenko, 1956. These groupings have previously been rejected by some authorities, because the diagnostic character is poorly defined; this study shows that Echinorhynchus (sensu stricto) and Metechinorhynchus are not natural, monophyletic groups. A revision of Echinorhynchus will require tandem molecular phylogenetic and morphological analyses of a larger sample of taxa, but this study has identified two morhological characters that might potentially be used to define new genera. The estimated phylogeny also provides insight into the zoogeographical history of Echinorhynchus spp. We postulate that the ancestral Echinorhynchus had a freshwater origin and the genus subsequently invaded the sea, probably several times. The freshwater taxa of the Echinorhynchusbothniensis Zdzitowiecki & Valtonen, 1987 clade may represent a reinvasion of freshwater by one or more ancestral marine species.

The present study is the first to analyse the parasite fauna of sole Solea solea, dab Limanda limanda, hake Merluccius merluccius, whiting Merlangius merlangus, and plaice Pleuronectes platessa in the Pomeranian Bay, as well as saithe Pollachius virens from the Szczecin Lagoon (Poland). The aim of this study was to determine the occurrence of parasites in migrating and rare fish in the Pomeranian Bay and the Szczecin Lagoon and to determine the composition of the diet of these fish. The fish for analysis were obtained in the years 2010–2019. The typical marine nematode Capillaria (Procapillaria) gracilis, rarely recorded in Poland, was found, in addition to the following parasites: Ichthyophonus hoferi, Trichodina jadranica, Diphyllobothrium sp., Dichelyne (Cucullanellus) minutus, Raphidascaris acus, Anisakis simplex, Contracaecum osculatum, Hysterothylacium aduncum, Pseudoterranova decipiens, and Echinorhynchus gadi. Because the fish species analysed in the study are not typically present in the Baltic (with the exception of plaice), and because we do not know how long they feed while they are in the Baltic, we cannot be certain which parasites they acquired in the water bodies analysed in the study and which were introduced during the migration of fish. Although these fish are outside of their normal geographic range (except for plaice), in the new environment, there were enough suitable intermediate hosts for the parasites of these fish to complete their life cycle and survive.

We suggest helminthological investigations of cod as a supplement to traditional biological and hydrographical methods for elucidation of ecological changes in the Baltic Sea. It is under discussion if oxygen deficit or seal abundance should explain the present critical situation of Baltic cod. A comparative investigation of endoparasitic helminths in Baltic cod ( Gadus morhua ), captured in the same marine habitat with an interval of 35 years (1983/2018) recorded 11 species of helminths comprising trematodes ( Hemiurus luehei , Podocotyle atomon , Lepidapedon elongatum ), nematodes ( Contracaecum osculatum , Hysterothylacium aduncum , Capillaria gracilis , Cucullanus cirratus ), cestodes ( Bothriocephalus sp.) and acanthocephalans ( Echinorhynchus gadi , Pomphorhynchus laevis , Corynosoma semerme ). Significant prevalence and intensity increases were recorded for third-stage larvae of the nematode C. osculatum (liver location) and larvae of C. semerme (encapsulated in viscera). Both parasite species use grey seal as their final host, indicating the recent expansion of the Baltic seal population. A lower E. gadi intensity and an increased prevalence of L. elongatum of small cod (31–40 cm body length) suggest a lowered intake of amphipods (intermediate host) and elevated ingestion of polychaetes, respectively, but no significant changes were seen for other helminths.

Previous studies have shown considerable variability in morphological features and the existence of genetically distinct sibling species in the acanthocephalan Echinorhynchus gadi Zoega in Mueller, 1776. The aim of the present study was to follow up and extend those earlier studies by using a combination of DNA analysis and morphometrics to investigate differences between samples of E. gadi from Atlantic cod Gadus morhua L. caught at five fishing grounds in the Baltic Sea and three in different parts of the North Atlantic. Twelve morphological features were measured in 431 specimens of E. gadi, 99 individuals were studied by polymerase chain reaction-restriction fragment length polymorphosm (PCR-RFLP), and selected PCR products were sequenced. The molecular analyses showed the nucleotide sequences of E. gadi rDNA from cod caught at all the sampling sites to be identical. The comparative morphological study, in contrast, revealed significant differences between samples of E. gadi from different sampling sites and showed the separation of E. gadi into two groups corresponding approximately to the systematic classification of cod into the two subspecies, Atlantic G. morhua morhua and Baltic G. morhua callarias. The E. gadi infrapopulation size had a significant effect on some of the morphological features. The results are discussed in relation to cod population biology, the hydrography of the study area and the history of the Baltic Sea formation.

Harvest quality rainbow trout ( Oncorhynchus mykiss Walbaum, 1792) raised in an integrated multi-trophic aquaculture (IMTA) unit off the German coast in the Baltic Sea were studied for ectoparasites and endoparasites. One hundred five specimens were sampled between November 2013 and 2014. Four endoparasite taxa were found, while no zoonotic or fish pathogenic ectoparasites could be detected. Molecular identified metacercariae of Diplostomum baeri were recorded at a non-critical level (18.1% prevalence, 1–4 intensity), getting introduced into the Baltic Sea through the cultured rainbow trout from the freshwater-based hatchery. The isolated acanthocephalan Echinorhynchus truttae was of freshwater origin, too. Pomphorhynchus laevis (Acanthocephala) and Brachyphallus crenatus (Digenea) seem to be transmitted into the net cage from Baltic Sea intermediate hosts. The parasite fauna was compared with those of wild living sea trout, Salmo trutta trutta studied at the immediate area (Unger and Palm in Parasitol Res 115(1):165–174, 2016 ). Stomach content analyses demonstrated that the farmed pellet-fed fish also utilized natural resources, especially fouling organisms. A single escapee, caught from the wild in the vicinity of the cages, had a massive intestinal infection with Baltic Sea parasites, the acanthocephalans Echinorhynchus gadi , P. laevis and the nematode Hysterothylacium aduncum . The salinity (helminths) as well as the culture conditions (trichodinids) at this pilot facility prevent significant transfer of fish pathogenic and zoonotic parasites as well as ectoparasite establishment and reduce the parasite risk for maricultured rainbow trout. This sampled location is particularly suitable for healthy and environmentally friendly aquaculture farming in the Baltic Sea.

The acanthocephalan Echinorhynchus salmonis Müller, 1784 is a common parasite of salmonid fish, but it has rarely been reported from an intermediate host. Samples of benthic amphipods, Monoporeia affinis (Lindström), were taken from multiple, deep sites (usually below 70 m) in the Gulf of Bothnia over the course of more than a decade and examined for acanthocephalans. Overall, only 0.44% of 23 296 amphipods were infected, all with just a single worm. This prevalence is consistent with several previous reports of acanthocephalans in deep-water, benthic amphipods, but it appears low compared to that often reported for acanthocephalan species infecting littoral amphipods. Parasite occurrence did not exhibit a clear regional pattern (i.e. northern vs southern sites) nor did it have any relationship with site depth. At sites sampled over multiple years, parasite abundance was consistently low (mostly < 0.01), though two spikes in abundance (over 0.06) were also observed, indicating that infection can be substantially higher at particular times or in particular places. The median density of E. salmonis in samples containing the parasite was estimated as 8.4 cystacanths per m(2).

The hooks of the acanthocephalan proboscis exhibit serial variation in size and shape. The Meristogram was developed by Huffman and Bullock (1975) to provide a graphical representation of this positional variation in hook morphology. Initial studies demonstrated the ability of the Meristogram to discriminate species within the genera Echinorhynchus and Pomphorhynchus (Huffman and Bullock 1975, Huffman and Nickol 1978, Gleason and Huffman 1981). However, the reliability of the method for taxonomic work was questioned by Shostak et al. (1986) after they found intra-specific variation in two Echinorhynchus species. Uncertainty about the usefulness of the Meristogram and the absence of a readily available software implementation of the algorithm, might explain why this abstract proboscis character has yet to be adopted by acanthocephalan systematists. The Meristogram algorithm was implemented in the R language and a simple graphical user interface created to facilitate ease of use (the software is freely available from https://github.com/WaylandM/meristogram). The accuracy of the algorithm's formula for calculating hook cross-sectional area was validated by data collected using a digitizing tablet. Meristograms were created from data in public respositories for eight Echinorhynchus taxa: E. bothniensis , E. 'bothniensis' , E. gadi spp. A, B and I, E. brayi , E. salmonis and E. truttae . In this preliminary analysis, the meristogram differentiated E. bothniensis , E. brayi , E. gadi sp. B, E. salmonis and E. truttae from each other, and from the remaining taxa in this study, but independent data will be required for validation. Sample sizes for E. 'bothniensis' and E. gadi spp. A and I were too small to identify diagnostic features with any degree of confidence. Meristogram differences among the sibling species of the E. gadi and E. bothniensis groups suggest that the 'intra-specfic' variation in meristogram previously reported for some Echinorhynchus taxa, may have actually represented morphological divergence between unrecognized cryptic species. Hierarchical clustering of taxa based on Meristogram data yielded dendrograms that were largely concordant with phylogenetic relationships inferred from DNA sequence data, indicating the presence of a strong phylogenetic signal.

The snakeblenny, Lumpenus lampretaeformis, is a post glacial relict from the last ice age in the Baltic Sea. Reliable data on its diet, parasite fauna, distribution, population size, and population trends in the Baltic Sea are lacking. In the Polish zone it has been observed only in ICES subdivisions 25 (Slupsk Furrow) and 26 (Puck Bay, Krynica Morska, Wladyslawowo and Vistula mouth fishing grounds) at depths of 30-70 m; however, in recent decades only one finding of snakeblenny in Polish waters has been reported. The record of one female specimen from the Gulf of Gdansk (Baltic Sea) is reported. The individual's morphological characters, stomach contents, parasitic fauna, age and gonadosomatic index were examined. The parasitological examination, which was undertaken for the first time on a specimen from the Baltic Sea since the 1930s, revealed that L. lampretaeformis was the host of one parasite species, namely a juvenile acanthocephalan Echinorhynchus gadi. Additionally, Pontoporeia fermorata and Halicryptus spinulosus were recorded in the stomach of snakeblenny for the first time in the Baltic Sea.