The environmental DNA (eDNA) metabarcoding approach has become a useful tool for detecting the species diversity of different animal groups, including parasites. Myxozoa (Malacosporea and Myxosporea) represent a unique group of morphologically simplified endoparasites that mainly infest fish and whose diversity remains largely unexplored. Metabarcoding of DNA from the aquatic environment is a promising non‐invasive method that allows us to assess myxozoan biodiversity at a given site. This is essential not only for describing myxozoan communities, but also for the development of disease control methods. Using an alignment of 330 sequences, we employed in silico PCR to score primer pairs, designed to amplify the V4 region of the SSU rDNA of different myxosporean sublineages comprising the entire diversity of oligochaete‐infecting (freshwater) myxosporeans. We selected eight clade‐specific primer sets for metabarcoding, avoiding amplification of DNA from other organisms present in eutrophic freshwaters. The metabarcoding approach used in the analysis of eDNA sediment samples detected a high myxosporean diversity even in small sample volumes (in total 44 OTUs). Furthermore, metabarcoding analysis of myxosporeans in fish tissue samples selected for primer testing revealed 91 different myxosporean OTUs, more than double the number obtained by classical PCR screening and Sanger sequencing with general myxozoan primers and almost seven times higher detection than by microscopic examination. Our results further suggest quantitative sampling requirements for realistic future diversity estimates by comparing OTUs from fish tissue metabarcoding and eDNA samples. The use of specific primer sets enabled the detection of a high proportion of myxosporean reads (63–100%) in all datasets, even in highly eutrophic habitats. This shows our metabarcoding approach as an excellent tool for non‐invasive and sensitive detection of myxosporean biodiversity in aquatic sediments, potentially useful for monitoring myxozoan disease agents that threaten economically important fish in aquaculture.

Ligula intestinalis (Linnaeus, 1758) is a tapeworm parasite with a worldwide distribution that uses a wide variety of fish species as its second intermediate host. In the present study, we investigated the prevalence and population genetic structure of plerocercoids of L. intestinalis in five common cyprinoid species, roach Rutilus rutilus (Linnaeus), freshwater bream Abramis brama (Linnaeus), white bream Blicca bjoerkna (Linnaeus), bleak Alburnus alburnus (Linnaeus), and rudd Scardinius erythrophthalmus (Linnaeus), collected in six water bodies of the Czech Republic (Milada, Most, Medard, Jord?n, ??mov and Lipno). Of the six study sites, the highest frequency of parasitism was recorded in Lake Medard (15%). The overall prevalence rate among the species was as follows: roach > rudd ≥ freshwater bream > bleak > white bream. Two mitochondrial genes (cytb and COI) were used to compare the population genetic structure of parasite populations using selected samples from the five fish species. The results of the phylogenetic analysis indicated that all populations of L. intestinalis were placed in Clade A, previously identified as the most common in Europe. At a finer scale, haplotype network and PCoA analyses indicated the possible emergence of host specificity of several mtDNA haplotypes to the freshwater bream. Moreover, pairwise Fixation indices (F_ST) revealed a significant genetic structure between the parasite population in freshwater bream and other host species. Parasite populations in roach not only showed the highest rate of prevalence but also depicted a maximum number of shared haplotypes with populations from bleak and rudd. Our results suggest that recent ecological differentiation might have influenced tapeworm populations at a fine evolutionary scale. Thus, the differences in prevalence between fish host species in different lakes might be influenced not only by the parasite's ecology, but also by its genetic diversity.

Intra-species variability in isotopic niches, specifically isotopic total niche width (ITNW), isotopic individual niche width (IINW), and isotopic individual specialization (IIS), was studied using an innovative approach without sacrificing the vertebrates. Stable isotopes (δ¹³C, δ¹⁵N) in four body tissues differing in isotopic half-life were analyzed from four freshwater fish species representing different trophic positions. ITNW was widest for the apex predator (European catfish) and narrowest for the obligate predator (Northern pike). IINW exhibited a polynomial trend for the European catfish, Northern pike, and Eurasian perch (mesopredator), decreasing with body mass and increasing again after exceeding a certain species-dependent body mass threshold. Thus, for ectotherms, apex predator status is linked rather to its size than to the species. In herbivores (rudd), IINW increased with body mass. The IIS of predators negatively correlated with site trophic state. Therefore, eutrophication can significantly change the foraging behavior of certain species. We assume that the observed trends will occur in other species at similar trophic positions in either aquatic or terrestrial systems. For confirmation, we recommend conducting a similar study on other species in different habitats.

Year to year fluctuations in 0+ fish cohort strength are a common phenomenon. Many factors can affect cohort strength during the fish's early life period. In this study, development of a 0+ pikeperch Sander lucioperca cohort in the pelagic zone was studied by trawling for 50 days from first larvae hatching, in two consecutive years. In 2007, an abundant S. lucioperca cohort collapsed suddenly soon after hatching. After the incident, slow-growing S. lucioperca prevailed in the catch. In 2008, the catch gradually increased during the whole study period because of prolonged hatching. Environmental factors differed mainly in a slower temperature increase, higher water level and higher zooplankton abundance in 2008 compared to 2007. Our study revealed that a strong 0+ S. lucioperca cohort at the time of hatching might not result in a strong S. lucioperca cohort in general.