Dolphins are rarely observed in the Baltic Sea, making only sporadic appearances as a result of their migrations. The study included six specimens: four white-beaked dolphins Lagenorhynchus albirostris and two striped dolphins Stenella coeruleoalba. Their gastrointestinal tracts were found to contain the nematodes Anisakis simplex (in both species) and Contracaecum sp. (only in the white-beaked dolphins). This is the first record of Anisakidae nematodes in dolphins found in the Baltic Sea.

The common harbor porpoise is a widely-distributed marine mammal with three known subspecies, including P. phocoena phocoena, with a clearly distinct and critically endangered (CR) subpopulation from the Baltic Sea (Baltic Proper). As part of an assessment of the condition and health threats of these mammals, it is important to conduct parasitological monitoring. The aim of the study was therefore to compare the data on porpoise parasitofauna from this subpopulation with those on porpoises from other areas. The study included 37 individuals from 1995 to 2019; eight species of parasites were found (prevalence 83.8%, mean intensity 724.2, range 2–3940), with a predominance of lung nematodes – Stenurus minor (94.7%), Torynurus convolutus (69.4%), Pseudalius inflexus (63.8%), Halocercus invaginatus (22.2%); the highest intensity was recorded for S. minor (989, 53–2928). Two species of Anisakidae (Anisakis simplex – 33.3%, Contracaecum sp. – 20.0%) were found in the digestive tracts, which were a new record for this population. The fluke Campula oblonga was found in the livers of 31.3% of porpoises. The tapeworm Diphylobothrium stemmacephalum was also recorded in the intestine of one individual; this is typical for these hosts, but previously undetected in the Baltic subpopulation. Parasites coexisted in numerous hosts, constituting a heavy burden for them. The obtained data were compared with those from the P. phocoena parasitofauna from other regions, based on a compiled checklist (1809–2021) including all species of porpoise parasites (55 taxa). Compared to the worldwide porpoise parasitofauna checklist, the number of parasites found in the nominative subspecies (Baltic Proper subpopulation) is small: including only 10 taxa (eight in the current study). These species are typical of porpoises and usually the most common; however, the level of infection of Baltic porpoises (intensity and total parasite load) is very high, which can undoubtedly have a negative impact on their condition and overall health.

The Ascaridoidea are parasites with heteroxenous life cycles. The study shows that fish can be paratenic, intermediate, or final hosts for parasites, and parasitic fauna reflects the feeding behavior of the hosts. Each species of parasites has also different environmental preferences and host specificity. Parasitic nematodes of fish representing Pleuronectidae, Gadidae, Sebastidae, and Macrouridae were studied. Worms were collected separately from different infection sites: stomach, intestine, liver and body cavity. Nematodes were identified using both morphological and molecular methods (PCR-RFLP). Six nematode species were recorded: Anisakis simplex s.s., Contracaecum osculatum A, B, and C. osculatum C (s.s), Hysterothylacium aduncum and Pseudoterranova bulbosa. Anisakis simplex s.s. was the most numerous nematode species of all catches combined. Differences in parasite species composition were related to the depth and location of sampling areas. In the fish from deep waters, the abundance of A. simplex s.s. decreased compared to fish from shallow waters and P. bulbosa was the dominant species. Ascaridoid species have specific preferences regarding the impact on various internal organs of fish, which is reflected in their abundance. The presence of Ascaridoidea in the Barents Sea is associated with the distribution of hosts and varying food preferences related to the age of fish. The abundance of parasites varied between different host species.

This study is a report of lesions associated with the nematode Cystidicoloides ephemeridarum (Linstow, 1972). It is a common parasite of the stomach mucosa of fish, mainly salmonids. In stomachs of 30 graylings were observed strong infection with C. ephemeridarum caught in July 2013 and February 2014 in the Dobrzyca River were examined. Adult nematodes, including females with mature eggs, and L3 and L4 larvae were recorded. Stomachs of graylings with intensive infection (146–261examples) were examined. The histological examination of a sample taken from the tissue surrounding nematodes revealed the presence of more or less confluent focal areas of destruction of gastric laminae. The numerous invasions of C. ephemeridarum are associated with intensive feeding of fish, but the different developmental stages of the parasite do not induce sufficiently strong histopathological changes in the stomach of graylings to be a cause of their death.

Department of Mycology, Faculty of Biology and Biotechnology, University of Warmia and Mazury in Olsztyn, Oczapowskiego 1A, 10-917 Olsztyn, Poland Department of Ecology and Vertebrates Zoology, Faculty of Biology, Gdańsk University, W. Stwosz 59, 80-308 Gdańsk, Poland Department of Zoology, Faculty of Biology and Biotechnology, University of Warmia and Mazury in Olsztyn, Oczapowskiego 5, 10-718 Olsztyn, Poland Department of Invertebrates Zoology and Parasitology, Faculty of Biology, Gdańsk University, W. Stwosz 59, 80-308 Gdańsk, Poland

All the nematodes found in body cavities of the examined endemic Baikal fishes: 88 Baikal yellowfin Cottocomephorus grewingkii (Dybowski, 1874) and 35 longfin Baikal sculpin Cottocomephorus inermis (Yakovlev, 1890) were identified as Contracaecum osculatum baicalensis (Mozgovoi and Ryjikov, 1950) L3 larvae. The prevalence, mean intensity, intensity range and abundance of the nematodes in C. grewingkii were 37.5%, 2.55, 1-31, and 0.96, respectively, the corresponding values in C. inermis were 60.0%, 2.43, 1-10, and 1.46. The infestation level in C. grewingkii was significantly higher than in C. inermis (Mann-Whitney U-test, p<0.02). The number of parasites was found to increase with the fish length. Although in both and C. inermis, the anisakids were more frequent in males (prevalence of 52.17 and 67.76%, respectively) than in females (prevalence of 35.39 and 42.86%, respectively), differences between the sexes in the infestation level in the two species were not significant (Mann-Whitney U-test, P=0.09 and P=0.23, respectively). The molecular method applied (PCF-RFLP) allows to identify all the nematodes in both examined fish species as C. osculatum baicalensis.

A total of 190 nematodes was isolated from the stomachs of 13 beluga whales Delphinapterus leucas from the Arctic part of Hudson Bay and Hudson Strait. Infection intensity ranged from 1 to 57 specimens and prevalence was 84.62%. Morphological examination of the nematodes revealed the presence of 3 species: Pseudoterranova decipiens sensu lato, Contracaecum osculatum s.l., and Anisakis simplex s.l. Molecular analysis by PCR-based restriction fragment length polymorphism (PCR-RFLP) resulted in the identification of 4 species: Pseudoterranova bulbosa, Contracaecum osculatum A and C, and Anisakis simplex sensu stricto. The nematodes were present in 3 developmental stages: L3 (159 specimens), L4 (16 larvae), and adults (15 worms: 11 males and 4 females).

Total, 94 specimens of gudgeon (Gobio ohridanus Karaman, 1924) from the Macedonian part of the Lake Ohrid were examined and 61 fishes (64.89%) were infected with parasites. The presence of 4 parasite species was established: Dactylogyrus cryptomeres f. typica (Monogenea), Cystidicoloides tenuissima and Philometra ovata (Nematoda), and Pomphorhynchus laevis (Acanthocephala). The highest prevalence and intensity of infection was with Dactylogyrus cryptomeres f. typica (59.57%; mean intensity 6.07). The lowest one was with Cystidicoloides tenuissima and Philometra ovata (2.13%; 1.0). Dactylogyrus cryptomeres f. typica is recorded for the first time in the ichthyoparasitofauna of Lake Ohrid and Macedonia.

The activities of trehalose-6-phosphate synthase (TPS) and trehalose-6-phosphate phosphatase (TPP) were determined in third-stage larvae (L3) of Anisakis simplex. The optimum pH of TPS and TPP was 7.0 and optimum temperatures were 55 and 40 degrees C, respectively. The thermostability of TPP was found to be higher than that of TPS. The activity of TPS at 20-50 degrees C was approximately 20% of the maximum activity; at 65 degrees C the enzyme was inactivated. The activity of TPP at 25-30 degrees C was approximately 40% of the maxithum activity; at 60 degrees C the enzyme was inactivated. Effects of chemical compounds on the enzymes were determined. Supernatants used for enzyme preparations were obtained from homogenised worms spun for 15 min at 4 degrees C and 1500 g. TPS activity increased up to 25-fold under the influence of trehalose. In the case of fructose and sorbitol an inverse relationship was shown between concentration and enzyme activity. Proline was demonstrated to be another TPS inhibitor. TPS was activated by 20 mM MgCl2, NaCl and KCl. On the other hand, it was inhibited by CuCl2, CaCl2 and CoCl2. TPP was activated by 10 mM MgCl2, CaCl2, CoCl2, ZnCl2 and NaCl and 20 mM FeCl3, ZnCl, KCl and ethylenediaminetetra-acetic acid.

Histopathological research is very important in diagnosing human and animal diseases. Detection of histopathological changes during certain parasitic invasions is particularly important for differential diagnosis and often confirms the presence of parasitic diseases. Such studies allow also to conclude on the primary cause of the disease.

The availability of aquatic insects (Odonata: Coenagrionidae, Libellulidae and Trichoptera: Integripalpia) as potential intermediate hosts for the nematode Contracaecum rudolphii Hartwich, 1964 sensu lato was studied under laboratory conditions. The infective material consisted of nematode eggs, newly hatched larvae, as well as in vitro infected cyclopoid copepods. High prevalence and intensity of infection associated with a low mortality of aquatic insect larvae suggests that they may serve as intermediate hosts for C. rudolphii and constitute a major reservoir of C. rudolphii larvae in aquatic habitats.

In the present study 5 grey seals (Halichoerus grypus), 3 common seals (Phoca vitulina) and 1 ringed seal (Pusa hispida) bycaught or stranded on the Polish Baltic Sea coast in years 2000-2006 were investigated for the infestation of parasitic anisakid nematodes. 749 of anisakids were found. The most common were: Contracaecum osculatum (59.3%) and Pseudoterranova decipiens (31.0%). There were also small numbers of Anisakis simplex (0.8%). After performing RFLP three sibling species were found. C. osculatum was identified as C. osculatum C, P decipiens was identified as P. decipiens sensu stricto and A. simplex — A. simplex sensu stricto. Nematodes found in seals were mostly in L4 and adult life stage — both of them were equal with some minor variations among the specimens. Sex ratio was also equal, but there was slight excess of males in some cases. There was a minority of L3 larvae belonging to A. simplex species (0.8%).

As many Arctic fish species are intermediate hosts of anisakids, they are present in the diet of the ringed seal, Pusa hispida, and the bearded seal, Erignathus barbatus. Parasitic nematodes from the stomachs of 66 seals caught in the Nunavut region (Canada) from two communities (Arviat and Sanikiluaq) from October 2007 to January 2008 have been examined in order to identify the epidemiological risk for Inuit communities who consume traditional food. In Arviat 2428 anisakids were observed in 37 seals, while in Sanikiluaq 316 Anisakidae were isolated from 29 seals. The worms were treated with a host tissue, washed in deionized water and stored until analysis in 70% ethanol. The parasites were divided into three parts. The anterior and posterior parts were stored in 70% ethanol containing 5% glycerol and were examined using a light microscope by evaporation of the ethanol/glycerin mixture. The central parts were prepared for molecular identification by fixing in 70% ethanol. Using the polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) method, the following members of the family Anisakidae were identified: Contracaecum osculatum A and C and Pseudoterranova bulbosa. In the studied material, more adult worms were noted than larval stages. The most numerous nematodes were P. bulbosa, and mixed infection was observed. The mean prevalence of anisakids infection was 43.2% in the Arviat and 37.9% in the Sanikiluaq communities.

Parasitic nematodes from the body cavity of 94 Greenland halibut (Reinhardtius hippoglossoides) caught in the Barents Sea were examined. Nematode species were identified using both morphological and molecular methods. Four morphospecies were found: Anisakis simplex s.l., Contracaecum osculatum s.l., Pseudoterranova decipiens s.l., and Hysterothylacium aduncum. The most prevalent were C. osculatum s.l. (89.4%) and A. simplex s.l. (86.2%). Mean intensities for these 2 species were 7.8 and 45.2, respectively. The number of parasites was directly proportional to the fish length. The following 6 parasite species, including 3 cryptic species of Contracaecum sp., were identified using molecular methods: A. simplex sensu stricto, C. osculatum A, C. osculatum B and C (the last 2 as host records for halibut), Pseudoterranova bulbosa, and H. aduncum. Mixed infections of C. osculatum A, B, and C were observed.

Laboratory-reproduced and bred asps were experimentally infected with Contracaecum rudolphii larvae, either directly or with previously infected copepods. In the fish exposed to larval infection, the intensity and prevalence of infection were noticeably higher than in the group exposed to copepods. The course of larvae development was similar in both groups. In the larvae measuring ca. 1000 μm in length, the gastrointestinal tract with a developed ventriculus, ventricular appendix and intestinal caecum was clearly visible. The mouth was surrounded by three lips. Over the 10-week experimental period, slightly-coiled larvae surrounded with a thin theca but no encysted larvae were found in the fish exposed to larvae. On the other hand, spirally-stranded and encysted larvae were observed after the 7th week in the fish exposed to infected copepods. The results demonstrated that in the experimentally infected asps, the intensity and prevalence of infection as well as the location of the larvae in a fish depended on the type of invasive material applied.