Volodimir Sarabeev

Biological invasions are among the most disruptive forces affecting global biodiversity. The “killer shrimp” Dikerogammarus villosus , a pervasive Ponto‐Caspian invader across Europe, serves as a key model for understanding the ecological consequences of such expansions. However, despite its extensive spread, the helminth fauna associated with this invader remains poorly characterized, obscuring potential spillover effects and the evolutionary history of its symbionts. To address this gap, we surveyed 54 European freshwater localities (2020–2025) and screened 1271 D. villosus specimens from 10 positive stations. These sites span the Danube (Slovakia) and Baltic (Poland) drainage basins, representing the southern and northern invasion corridors. Using an integrative taxonomic approach combining detailed morphology (12 fixed adults and 4 live metacercariae) and multi‐locus phylogenetics (18S, 28S, ITS, and COI), we formally describe Maritrema dikerogammari sp. nov. infecting D. villosus . Our analyses place the new species within a distinct freshwater clade as the sister taxon to the Carpathian endemic Maritrema neomi . Historical records of Maritrema inusitatum from the Dnieper River were misidentifications, and we reassign them to the new species , confirming the parasite's longstanding presence in both the Danube and Dnieper basins. This study documents a clear case of “hidden co‐invasion,” where a cryptic parasite established itself in the invasion corridor undetected. Phylogenetic reconstruction reveals that the evolution of this group is driven by a fundamental marine‐to‐freshwater transition, shaped by ancient vicariance and recent host‐facilitated dispersal. We refine the taxonomy of the genus Maritrema and underscore the critical role of invasion routes in the spread of associated fauna.

Accurate testing and interpretation of interaction effects are essential for a robust understanding of the dynamics of biological systems. However, exclusive reliance on the statistical significance of the interaction term in regression models can lead to either an underestimation or an overestimation of interaction effects. Using empirical data of monogenean ectoparasite abundance in the Pacific so-iuy mullet host, Planiliza haematocheila (Temminck & Schlegel, 1845), we detected context-dependent patterns that standard coefficients failed to capture fully. Specifically, our findings suggest that the temperature–abundance relationship is not uniform across all body length–age classes but is modulated by host ontogeny/size, a pattern only resolved by explicitly evaluating conditional effects.

The host diversity, complex life cycle and distribution of digeneans underline their value as bioindicators of ecosystem health. We examined the intestinal digenean trematode Cephalotrema elasticum of insectivorous mammals, sampled from Gammarus balcanicus (Crustacea: Amphipoda) as second intermediate hosts. The species is particularly relevant as part of the helminth species complex associated with protected mammals. Our study is based on an intensive survey of helminths in 8920 gammarids collected from 38 localities in Slovakia, Poland, and France. DNA sequences for three markers (18S rDNA, 28S rDNA, and CO1 mtDNA) were analysed using Bayesian inference. Molecular data suggest that C. elasticum represents a distinct genetic lineage closely related to Cryptotropidae. Rare anatomical traits, such as the marginal or submarginal genital pore arrangement, may influence mating success and reproductive isolation within Microphalloidea. Genetic and morphological evidence supports establishing a new family, Cephalotrematidae, within Microphalloidea, to include Cephalotrema, Pseudocephalotrema, and Opisthioparorchis. A family diagnosis and a key to the genera are provided to facilitate further taxonomic studies. The geographic distribution of C. elasticum highlights its utility as a biological indicator, reflecting the decline of water shrew populations due to climate-induced ecological changes, particularly in the French Pyrenees.

A major current challenge related to invasions is the absence of a comprehensive theoretical basis for preventing the introduction of alien species and controlling their populations. By using a macroecological framework, we aim to examine patterns of symbiotic species diversity across native and invasive hosts to understand the mechanisms underlying the host–parasite system stability. The symbiotic communities of seven gammarid species, two native and five invasive, were analyzed at 16 fresh and brackish water sites along the Baltic coast of Poland. We investigated the influence of four factors—locality, habitat, host species, and host origin—on diversity patterns and assessed their relative contribution to diversity variation. Our results indicate that all factors are crucial in determining the composition and abundance of symbiotic communities in gammarids. Among these factors, locality and host species apparently have a greater influence on symbiotic communities than habitat conditions and host origin. Comparative analysis of diversity indexes of symbiotic organisms from native and invasive hosts showed that the richer communities of native gammarids were paradoxically less diverse. We assume that these rich and uneven symbiotic communities keep the host–parasite system of native gammarids in equilibrium. Highly dominant symbiotic species with a high load per host individual may stabilize the growth of the host population. Symbiotic communities of the invasive host exhibited lower species richness but displayed a relatively even distribution of species with moderate loads per individual, resulting in a scarcity of heavily infected gammarids within the population. Thus, the survival rate of invasive gammarids is expected to be relatively high, facilitating their population growth and further spread. We conclude that complex communities of native gammarids constructed from many symbiotic species may prevent host populations from undergoing explosive growth, while such mechanisms may be completely or partially reduced in invasive hosts.

A checklist of 113 monogenean species parasitizing marine fishes (60 species) from different localities in Egypt is provided. The list is supplemented by eight newly collected monogenean species from Red Sea fishes, off Safaga and El-Quseir. Five of these species are new Egyptian records: Calydiscoides euzeti Justine, 2007, Calydiscoides rohdei Oliver, 1984, Lethrinitrema austrosinense (Li & Chen, 2005) Sun, Li & Yang, 2014, Pseudohaliotrema sphincteroporus Yamaguti, 1953, and Pentatres sphyraenae Euzet & Razarihelisoa, 1959. Furthermore, Lutjanus ehrenbergii (Peters), Lethrinus nebulosus (Forsskål), Lethrinus mahsena (Forsskål), Siganus stellatus (Forsskål), and Sphyraena flavicauda Rüppell represent new host records. The current study also lists nine monogenean species from the Gulf of Aqaba for which the coordinates of the sampling localities were not clearly defined, but which could also belong to the Egyptian fauna as the gulf is part of the Red Sea basin. Dactylogyrus aegyptiacus Ramadan, 1983 is transferred to Ecnomotrema Kritsky, 2023 as E. aegyptiacum (Ramadan, 1983) n. comb. Entobdella aegyptiacus Amer, 1990, Polylabroides aegyptiacus Mahmoud & Shaheed 1998, Gotocotyla sigani Abdel Aal, Ghattas & Badawy, 2001, Neohexostoma epinepheli Abdel Aal, Ghattas & Badawy, 2001, Neothoracocotyle commersoni Abdel Aal, Ghattas & Badawy, 2001, Acleotrema maculatum Morsy, El Fayoumi & Fahmy, 2014, Diplectanum harid Morsy, El Fayoumi, Al Shahawy & Fahmy, 2014, and Pseudorhabdosynochus chlorostigma Morsy, El Fayoumi, Al Shahawy & Fahmy, 2014, are considered species inquirendae. Paranaella diplodae Bayoumy, Abd El-Hady & Hassanain, 2007 is considered incertae sedis. Allencotyla lutini El-Dien, 1995 and Lamellodiscus diplodicus Bayoumy, 2003 are regarded as nomina nuda.

The major current challenge related to alien species is the absence of a comprehensive theoretical basis to explain the resistance from bioinvasions. One of the factors conferring resistance to invasion is the native species diversity of a recipient ecosystem. We aim to examine patterns of species diversity to understand the mechanisms underlying host-parasite relationships in alien species. The dataset represents 20 host-locality samples, including over 15,067,600 symbiotic organisms counted from 612 host individuals belonging to 7 gammarid species, two native and five invasive, sampled at 16 localities in the Baltic region of Poland. We asked whether diversity patterns depended on locality, habitat and host species, and which factors are deterministic in the variation of diversity. Our results suggest that both locality and host phylogeny are critical factors in determining the composition and abundance of symbiotic communities in gammarids. Out of these two factors, locality appears to be more determinant of the symbiotic community of gammarids than host species, whereas habitat condition was a stronger determinant of the symbiotic communities than geographic distance. The rich and uneven symbiotic communities of native gammarids keep the host-parasite system in equilibrium. Sharply dominated symbiotic species with a high load per host individual stabilize the host population. Simplified symbiotic communities of the invasive hosts showed a relatively even distribution of species with moderate loads per individual, with heavily infected gammarids being rare in the population. The survival rate of invasive gammarids is expected to be relatively high, facilitating their population growth and further spread. Complex communities of the native gammarids constructed from many symbiotic species may prevent host populations from undergoing explosive growth, while such mechanisms may be completely or partially reduced in invasive hosts.

Bipartite network analysis is a powerful tool to study the processes structuring interactions in ecological communities. In applying the method, it is assumed that the sampled interactions provide an accurate representation of the actual community. However, acquiring a representative sample may be difficult as not all species are equally abundant or easily identifiable. Two potential sampling issues can compromise the conclusions of bipartite network analyses: failure to capture the full range of interactions (sampling completeness) and use of a taxonomic level higher than species to evaluate the network (taxonomic resolution). We asked how commonly used descriptors of bipartite antagonistic communities (modularity, nestedness, connectance, and specialization [H₂′]) are affected by reduced host sampling completeness, parasite taxonomic resolution, and their crossed effect, as they are likely to co‐occur. We used a quantitative niche model to generate weighted bipartite networks that resembled natural host–parasite communities. The descriptors were more sensitive to uncertainty in parasite taxonomic resolution than to host sampling completeness. When only 10% of parasite taxonomic resolution was retained, modularity and specialization decreased by ~76% and ~12%, respectively, and nestedness and connectance increased by ~114% and ~345% respectively. The loss of taxonomic resolution led to a wide range of possible communities, which made it difficult to predict its effects on a given network. With regards to host sampling completeness, standardized nestedness, connectance, and specialization were robust, whereas modularity was sensitive (~30% decrease). The combination of both sampling issues had an additive effect on modularity. In communities with low effort for both sampling issues (50%–10% of sampling completeness and taxonomic resolution), estimators of modularity, and nestedness could not be distinguished from those of random assemblages. Thus, the categorical description of communities with low sampling effort (e.g., if a community is modular or not) should be done with caution. We recommend evaluating both sampling completeness and taxonomic certainty when conducting bipartite network analyses. Care should also be exercised when using nonrobust descriptors (the four descriptors for parasite taxonomic resolution; modularity for host sampling completeness) when sampling issues are likely to affect a dataset.