Could Fish Feeding Behaviour and Size Explain Prevalence Differences of the Nematode Eustrongylides excisus among Species? The Case Study of Lake Garda

The nematode Eustrongylides excisus is a parasite of freshwater fish- and fish-eating birds, with known differences on prevalence values among fish species. Thus, the present study aims to explore the hypothesis that the feeding behavior and the size of fish belonging to different trophic levels could explain such differences. For that, 14 sampling sites were selected to perform a fish parasitological survey on Lake Garda (Italy) during spring-summer 2020. Amplification of nuclear ribosomal internal transcribed spacer (ITS) rDNA sequences of nematodes morphologically ascribable to the genus Eustrongylides allowed to identify them as E. excisus. From the five studied fish species (Perca fluviatilis, Lepomis gibbous, Coregonus lavaretus, Alosa fallax lacustris and Micropterus salmoides), only three presented the parasite E. excisus: P. fluviatilis, L. gibbous and M. salmoides, with significant differences in prevalence values among species (p = 0.002). Additionally, there were differences in prevalence values within the same fish species captured from different sampling sites. Findings showed that mainly piscivorous fish were positive for E. excisus and how the prevalence was highest in M. salmoides. As regard the fish size, a negative correlation between body size and E. excisus was found in P. fluviatilis due to the feeding habit of juvenile perch which feed mainly zooplankton and benthic invertebrates (i.e., oligochaetes, which are the first intermediate hosts of E. excisus). The study findings advance novel knowledge in the field of pathogens of zoonotic importance in the aquatic environment.

Intraspecific structure of the Coregonus lavaretus complex in water bodies of Siberia: a case of postglacial allopatric origin of Yukagirian whitefish

The results of morphological and genetic analyses of forms/species of the Coregonus lavaretus pidschian (Gmelin, 1789) complex from the Indigirka and Kolyma river basins are presented in the context of there being recent postglacial speciation events. It has been found that the studied whitefishes belong to the sparsely rakered and low lateral-line forms and have previously been described as Coregonus lavaretus pidschian n. jucagiricus Drjagin (Berg), 1932. Based on these characters, this whitefish does not differ from most Arctic whitefish populations (in particular from Coregonus lavaretus glacialis Kirillov, 1972). Analysis of variability of the ND1 gene (mtDNA) showed that whitefishes from the Indigirka and Kolyma basins belong to a distant phylogenetic lineage, which is significantly different from all previously studied whitefish lineages from the Ob, Yenisei, Lena, Anadyr, and Amur river basins. Analysis of variability of the ITS1 fragment (nDNA) showed that all studied forms/species (from Ob River to Amur River basins), including C. l. pidschian n. jucagiricus, have a tandem arrangement of two identical nucleotide fragments and very similar nucleotide composition of the ITS1 region. Based on contemporary data, this phylogenetic lineage of the C. pidschian complex could be considered a young postglacial allopatric species.

Evidence of dispersal between the Yenisei and the Lena river basins during the late Pleistocene within the whitefish (Coregonus lavaretus pidschian) complex

The Coregonus lavaretus (Linnaeus, 1758) complex is a morphologically and genetically diverse group of whitefish. Its taxonomic structure has been controversial for almost a century. At least 25 forms of C. lavaretus have been described in Siberia, but there is still no consensus on their intraspecific structure and taxonomy. Coregonus lavaretus pidschian (Gmelin, 1789) was described as a subspecies of C. lavaretus. Recently, it was assumed that this subspecies is also a complex. The purpose of this study was to compare the distributions of pidschian-like whitefish haplotypes in two basins of large Siberian rivers, Yenisei and Lena, and to assess the gene flow between basins of these rivers, which were connected after the last glaciation. The sequence of the following mitochondrial DNA genes, 16S rRNA (partial), tRNA-Leu (full), NADH dehydrogenase subunit 1 (full), tRNA-Ile (full), and tRNA-Gln (partial), were used for the inference of intraspecific genetic structure of C. l. pidschian. Whitefish haplotypes were clustered into two groups according to their distribution between two large Siberian river basins; however, there were shared haplotypes indicating events of migration and hybridization, which could occur when Bolshoi Yenisei and Lena river systems were connected after the last glaciation (the Late Pleistocene).

Population niche breadth and individual trophic specialisation of fish along a climate-productivity gradient

A mechanistic understanding of how environmental change affects trophic ecology of fish at the individual and population level remains elusive. To address this, we conducted a space-for-time approach incorporating environmental gradients (temperature, precipitation and nutrients), lake morphometry (visibility, depth and area), fish communities (richness, competition and predation), prey availability (richness and density) and feeding (population niche breadth and individual trophic specialisation) for 15 native fish taxa belonging to different thermal guilds from 35 subarctic lakes along a marked climate-productivity gradient corresponding to future climate change predictions. We revealed significant and contrasting responses from two generalist species that are abundant and widely distributed in the region. The cold-water adapted European whitefish (Coregonus lavaretus) reduced individual specialisation in warmer and more productive lakes. Conversely, the cool-water adapted Eurasian perch (Perca fluviatilis) showed increased levels of individual specialism along climate-productivity gradient. Although whitefish and perch differed in the way they consumed prey along the climate-productivity gradient, they both switched from consumption of zooplankton in cooler, less productive lakes, to macrozoobenthos in warmer, more productive lakes. Species with specialist benthic or pelagic feeding did not show significant changes in trophic ecology along the gradient. We conclude that generalist consumers, such as warmer adapted perch, have clear advantages over colder and clear-water specialised species or morphs through their capacity to undergo reciprocal benthic–pelagic switches in feeding associated with environmental change. The capacity to show trophic flexibility in warmer and more productive lakes is likely a key trait for species dominance in future communities of high latitudes under climate change.

Allopatric origin of sympatric whitefish morphs with insights on the genetic basis of their reproductive isolation

The European whitefish (Coregonus lavaretus) species complex is a classic example of recent adaptive radiation. Here we examine a whitefish population introduced to northern Finnish Lake Tsahkal in late 1960s, where three divergent morphs (viz. littoral, pelagic and profundal feeders) were found ten generations after. Using demographic modelling based on genomic data we show that whitefish morphs evolved during a phase of strict isolation, refuting a rapid symmetric speciation scenario. The lake is now an artificial hybrid zone between morphs originated in allopatry. Despite their current syntopy, clear genetic differentiation remains between two of the three morphs. Using admixture mapping three quantitative trait loci associated with gonad weight variation, a proxy for sexual maturity and spawning time, were identified. We suggest that ecological adaptations in spawning time evolved in allopatry are currently maintaining partial reproductive isolation in the absence of other barriers to gene flow.