During the 20^th century, this species was considered to be synonymous with the Siberian congener R. sericeus, and the taxonomy of Western Palearctic Rhodeus remains partially unresolved. Genetic analyses have revealed the presence of six well-supported lineages. Two of these occur throughout central and Western Europe, two are restricted to the Ponto-Aegean region of Greece, and two occur south of the Caucasus mountains in rivers draining to the eastern Black Sea and the southern Caspian Sea, respectively (Bohlen et al. 2006, Bryja et al. 2010).

The Ponto-Aegean lineage inhabiting the Vardar River system and adjacent watersheds has been described as R. meridionalis Karaman, 1924, the eastern Black Sea lineage as R. colchicus Bogutskaya and Komlev, 2001, and the southern Caspian Sea lineage as R. caspius Esmaeili, Sayyadzadeh, Japoshvili, Eagderi, Abbasi & Mousavi-Sabet, 2020. Among these, only R. colchicus has been demonstrated to exhibit obvious morphological differences to R. amarus.

There is also evidence of historical genetic admixture between several lineages (see 'Distribution'), which has further complicated their identification.

The European Bitterling is widespread and the global population is in the midst of an ongoing expansion which includes areas outside its original range.

Although some subpopulations may be threatened at the local scale, this species does not currently approach any threshold for placement in a Threatened Category. It is therefore assessed as Least Concern.

This species is traditionally considered native to much of western, central and eastern Europe, plus the Black and southern Caspian sea basins. It is naturally absent from the United Kingdom plus the Iberian and Apennine peninsulas.

Beyond the borders of this putative natural range, it has been introduced to Denmark, the Great Ouse River and a series of man-made canals in England, all major rivers in France, rivers draining to the Adriatic Sea in northern Italy, Slovenia, Croatia, Bosnia and Herzegovina and Montenegro, the Kuban, Don, Volga, Maly Uzen and Ural rivers in Russia and Kazakhstan, and the Lake Urmia and upper Tigris River basins in Iran.

However, while there is little doubt that subpopulations in western and central Europe are relatively recent colonisers, and that this was partially a natural process, the European Bitterling is in all likelihood not native to rivers draining to the Baltic Sea and Atlantic Ocean. There is strong evidence to suggest that prior to c. 1100 AD this species was restricted to the Aegean and Ponto-Caspian regions, including the lower reaches of the Danube, Dniester, Southern Bug, and Dnieper river systems. Until this time its range was limited by temperature (see 'Habitats and Ecology'), and its subsequent expansion has been aided by a warming climate plus a series of anthropogenic factors.

Research indicates that an initial period of dispersal took place between approximately 1150 and 1560, coinciding with the onset of Common Carp (Cyprinus spp.) aquaculture, and during which period the bitterling was unintentionally introduced to new locations after being transported as bycatch. The species then virtually disappeared from western and central Europe during the coldest period of the Little Ice Age, before reports again became commonplace from the end of the 18th century onwards. It initially reappeared in areas where Common Carp was being cultivated, but was widespread by c. 1850.

This latter expansion was probably driven by human translocation but also warming temperatures which allowed the species to penetrate previously inaccessible areas within major river systems. The construction of man-made canals linking many larger rivers, e.g., the Dnieper–Bug Canal connecting the Dnieper and Vistula systems was completed in 1784, is also likely to have facilitated its spread.

During the mid-20th century its range again appears to have contracted due to habitat degradation and low spring temperatures, but it has increased significantly since the 1980s. Modern transport vectors include the aquarium trade and its widespread use as a bait fish in recreational angling, plus the continued transportation of live Common Carp and other species for aquaculture.

The scenario described above is supported by genetic evidence, with the pattern of haplotypes recorded in western European subpopulations indicative of both natural secondary contact of the two lineages following colonisation of Europe and multiple human-assisted introductions.

The European Bitterling's native distribution thus comprises the Danube, Dniester, Southern Bug, and Dnieper rivers, plus a series of smaller systems draining to the Aegean, Black and Caspian seas in Greece, Türkiye, Armenia, Azerbaijan, and Iran.

Rivers draining to the northwestern Aegean Sea basin are inhabited by the congeneric Vardar Bitterling (Rhodeus meridionalis), and those to the eastern Black Sea by the Georgian Bitterling (R. colchicus).

This species apparently suffered notable declines in central and western Europe during the 20^th century, leading it to be declared as threatened in a number of countries, and protected at the regional scale (see 'Conservation'). However, it appears to have undergone a dramatic recovery since, and is currently expanding both within and outside its putative natural range. Today, the bitterling is one of the most invasive fish species in Europe.

This species is thermophilous and requires warmer conditions than the majority of native European freshwater fishes. It sometimes occurs in fast-flowing water, but mostly inhabits lowland lotic or lentic habitats including slow-moving rivers, backwaters, oxbows, lakes, ponds, and man-made canals. These requirements suggest it is likely to be benefiting from a warming climate plus the tendency towards reduced river discharge due to anthropogenic regulation and other forms of habitat modification.

Bitterlings exhibit a unique reproductive strategy whereby their eggs are deposited into the gill cavity of unionid river mussels, which act as hosts. During the reproductive period, which extends from April to August in western Europe, males are territorial and defend groups or individuals of unionid mussels. Females temporarily develop a long, tube-like structure known as an ovipositor, while mature males become brightly-coloured. They display to females with extended ovipositors in the vicinity of their territories, before leading them to a chosen mussel. The female inspects the mussel before inserting her ovipositor into the mussel's exhalant siphon and depositing a small batch of usually 1-6 eggs. The male then releases his sperm into the inhalant siphon of the mussel host, and the eggs are fertilised within its gills. Other males may also choose this moment to sneak into the territory and release their own sperm into the mussel.

A single female can utilise several mussels on a daily basis over the course of the reproductive period, and a single mussel may contain more than 100 bitterling embryos at the peak of reproductive activity. The fry hatch in c. 36 hours but do not emerge from the mussel until c. 1 month, at which point they have absorbed the yolk sac and are capable of swimming freely. Sexual maturity is reached during the first year, when the fish measure c. 30-35 mm standard length.

A variety of different mussel species are exploited across the European Bitterling's range. Extensive research has suggested that host-specificity is low, i.e., it will readily utilise mussels species with which it has no evolutionary history. In addition, native mussels in areas of ancient sympatry, e.g., the Ponto-Caspian region, have evolved specialised mechanisms to defend against oviposition that are not present in the relatively naive hosts in central and western Europe. It is likely that these factors, in combination with increasingly favourable environmental conditions and anthropogenic translocations, have advanced the ongoing expansion of the European Bitterling's distribution.

This species' diet comprises small aquatic invertebrates and detritus, including plant material.

Some European Bitterling subpopulations are threatened by water pollution, the clearing of macrophytes, introduction of predatory, often non-native, fish species, and the ongoing decline of freshwater mussels throughout its range.

Research also suggests that presence of the invasive non-native Zebra Mussel (Dreissena polymorpha) limits the bitterling's reproductive potential. The Zebra Mussel colonises, or 'fouls', the exposed portion of native unionid mussel shells, and bitterling eggs tend not to reach the gills of fouled hosts.

This species is not used or traded, except in relatively small numbers within the aquarium hobby trade. It is bred in captivity for the latter purpose.

This species is included (as Rhodeus sericeus/R. sericeus amarus) in Appendix III of the Bern Convention and Annex II of the European Union Habitats Directive.

It occurs within the boundaries of numerous protected areas of varying designation and size throughout its range, and has been the beneficiary of dedicated conservation management in some cases.

Efforts to stem the decline of native freshwater mussels are likely to benefit the European Bitterling, should they prove successful.