Rhodeus amarus

Common name. Bitterling.

Diagnosis. Distinguished from R. colchicus by: ● second infraorbital bone narrow ● 35−38, usually 36−37, total vertebrae / ● 34−41, usually 36−40, scales in lateral series. No morphological character known to distinguish from R. caspius. Size up to 95 mm SL.

Distribution. Marmara and Black Sea basins (except easternmost) and all rivers draining into Aegean south to Büyük Menderes. Very widespread and invasive in Europe, where it is native in northern Aegean basin west of Nestos. Widespread from France to Volga, in southern Russia in Don and Kuban drainages, Crimea, Great Britain, north of Italy, and possibly elsewhere. Suspected to be native only to middle and lower reaches of Black Sea tributaries and Aegean basin.

Habitat. Still or slow-flowing waters with dense aquatic vegetation and sandy-silt bottoms, such as lowland ponds, canals, streams, rivers, backwaters, and oxbows where mussels are present. Often in artificial waterbodies, rare in fast-flowing habitats.

Biology. Spawns first time at 1 year and about 30–35 mm SL. Exceptionally lives up to 5 years, but most individuals do not survive year of first reproduction, and population sizes might fluctuate greatly over years. Spawns April–August at water temperatures above 15°C. Males defend small territories around one or a few bivalves (mostly Unio spp.), moving with bivalves. Females select bivalves with high oxygen concentrations in exhalation siphon. Avoid bivalves that already contain high numbers of R. amarus larvae or are gravid with glochidia (bivalves’ own larvae). Also, avoid Anodonta cygnea, which can expel most eggs and larvae and usually has low oxygen concentrations in its exhalation siphon. Males display pre-oviposition ejaculation when females inspect or approach mussel.When a female is ready to spawn,she extends ovipositor and muscular conical structure at its base.A batch of eggs is positioned at basal opening of ovipositor, behind which urine collects. She then pushes cone into mussel’s siphon. The cone contracts, and urine, under pressure, forces eggs through ovipositor. Oviposition is a rapid process. At same time, male releases sperm into inhalant siphon. After spawning, sneaking males attempt to fertilise eggs. Females typically lay 80– 250 eggs yearly, spawning up to five times per season. Eggs are oval, about 2.5–3.0 mm long, and are incubated within gill chamber of mussel. Yolk sac has lateral outgrowths, which allow larvae to attach to mussel’s gills. After 20–30 days, feeding larvae leave mussel. Larvae show adaptations to very low oxygen concentrations in mussels, such as very rapid hatching (36 h) and enlarged skin respiratory system. Wing-like yolk projections hold larvae in mussel’s gills. Bitterlings and mussels have a parasitic relationship. Mussels suffer a reduction in fitness when carrying R. amarus larvae. Larvae and mussels compete for oxygen, and larvae appear to inhibit free water circulation through mussel gills, which may damage them. Bitterlings are rarely infested by glochidia (ectoparasitic mussel larvae). Feeds mainly on algae, diatoms, and other plant material. Frequently translocated, bred, and used for human pregnancy testing before development of modern techniques, resulting in numerous introductions. Females injected with a pregnant woman’s urine protrude their ovipositor. Has spread since Middle Ages and is widely introduced with carp stocked from fish farms inhabited by bitterlings. Warm-water species expanding its range due to climate change.

Conservation status. LC; abundant and expanding throughout most of its range. A major invasive species in Europe.

Further reading. Abdurakhmanov 1962 (morphology, biology); Holčík & Jedlička 1994 (morphology); Kottelat 1997 (systematics); Holčík 1999 (biology); Bogutskaya & Komlev 2001 (morphology); Smith et al. 2004 (reproduction); Bohlen et al. 2006 (genetics); Van Damme et al. 2007 (invasion); Bartáková et al. 2019 (phylogeny).