The Phylogenetic Imperative: A Genetics-Informed Framework Delineating Conservation Units for Carassius carassius in Czechia

The conservation of declining wetland fish species in Central Europe necessitates a paradigm shift from species-level management to the preservation of evolutionarily significant phylogenetic lineages. The crucian carp ( Carassius carassius ) is a compelling case study of the challenges of modern conservation, where a species' apparent widespread distribution belies a complex, fragmented genetic structure. Within Europe, the species is not a monolithic entity but rather comprises two distinct and highly divergent phylogenetic lineages. The first, a northern lineage, is found across a vast expanse of Northern and Eastern Europe, including the Labe (Elbe) and Odra (Oder) River Basins. The second, a Danubian lineage, is geographically restricted to the Danube River Basin. Critically, both of these natural, evolutionarily distinct lineages are native to the Czech Republic, with some regions serving as a vital contact zone where their historical ranges overlap. The explicit recommendation to classify this Central European lineage as "critically endangered" and to prioritize its protection is a powerful argument for a new framework of conservation. It moves the discussion beyond simply asking, "Is the species endangered?" to a more nuanced question: "What specific, irreplaceable genetic units are at highest risk?" This approach elevates a population's conservation priority based not only on its vulnerability but also on the irreplaceability of its unique evolutionary history. This model is directly applicable to the conservation of C. carassius and other species with similar fragmented genetic structures. It suggests that policy and funding decisions should be guided by a species' unique evolutionary trajectory and the critical need to preserve its genetic subgroups as distinct ecological and evolutionary entities.