European Bitterling

About

First, the name, because it is genuinely confusing and Lithuanian shops sell the fish under the wrong one. The European bitterling is Rhodeus amarus (Bloch, 1782), a small, deep-bodied, silver fish of still, weedy lowland water that Lithuanian Wikipedia calls common in this country's lakes and rivers. Rhodeus sericeus (Pallas, 1776) is a different species: FishBase places it in the Amur basin and Sakhalin Island in Russia. The two were treated for a long time as one species with the European form as the subspecies Rhodeus sericeus amarus, and English Wikipedia is blunt that calling the European fish 'R. sericeus' today is technically wrong — a leftover from that arrangement. The leftover is everywhere: Lithuanian Wikipedia's article, Lithuanian shop listings, and even Annex II of the EU Habitats Directive, which still lists 'Rhodeus sericeus amarus'. When a Lithuanian shop offers 'Rhodeus sericeus, kartuolė', the animal in the tank is R. amarus. Second, the thing this fish is famous for. The bitterling cannot spawn without a living freshwater mussel. The female grows a long tube — an ovipositor — and places her eggs inside the gill cavity of a live Unio or Anodonta; the male releases sperm into the mussel's inhalant current, fertilisation happens inside the mussel, and the young stay in its gills for three to five weeks before swimming out at about 10 mm. Kujawa and Piech put it flatly: bivalves are indispensable for bitterling reproduction, because the eggs can only be incubated in the gill cavities of mussels. No mussel, no spawning — not fewer fry, none. Older accounts, FishBase's page on R. sericeus among them, called this a fair trade in which the fish cleans parasites off the bivalve. The modern work does not: Halabowski, Reichard and colleagues describe it as parasitism, with the embryos competing for resources with the mussel, damaging its gills and reducing its fecundity. One more twist completes the picture — the mussels return the favour, since unionid larvae (glochidia) attach to the gills of fish for weeks before dropping off as juvenile mussels. FishBase records a maximum of 11.2 cm, a common length of 5 cm and a maximum age of five years, most fish not surviving the year of their first spawning; and, historically, that females injected with a pregnant woman's urine extend their ovipositors, which is why this fish was once a pregnancy test.

Care

As a fish it is easy: eight or more in 120 litres, 80 cm of length, a sandy or silty bottom, dense planting, gentle flow and no heater needed in an ordinary room. It is the one fish in this group that is not strictly coldwater — Kujawa and Piech record wild spawning at 17–26 °C and reared it successfully at both 20 and 26 °C — so a temperate or warm unheated tank suits it, and 17–23 °C is the sensible band. It is peaceful, it does not damage plants, and it needs no special food. Keep a lid or cover glass on it: no source calls the bitterling a jumper, but nothing says it is not one either, and every other small shoaler in this group is covered. The hard part is breeding, and it is not optional if you want the fish to be what it is. That means a living Unio or Anodonta, and the mussel is a harder animal to keep than the fish. Maidenhead Aquatics is clear about the swan mussel: it belongs in a mature, unfiltered wildlife pond with soft substrate and a continuous supply of suspended infusoria, is explicitly not recommended for clear, heavily filtered ponds because they will not feed it, and is not always advisable alongside fish, because its own parasitic larvae can damage their skin. A swan mussel starving to death unseen in a bare tank is the usual outcome of a good intention. There is also a legal edge. In Lithuania the bitterling itself is not on the protected species list, but the thick-shelled river mussel Unio crassus — one of its host species — is, and the same mussel appears in Annexes II and IV of the EU Habitats Directive, Annex IV being the strict-protection list. So collecting a host mussel from a Lithuanian river is not a casual matter; buy the mussel, do not dig it up. The bitterling itself is listed EU-wide in Annex II of the Habitats Directive under its old name, which obliges member states to designate protected sites for it rather than to protect individual fish (Annex II entries marked '(o)' appear in neither Annex IV nor V). Poland goes further: Kujawa and Piech record that the bitterling there is covered by strict all-year protection even though its Polish populations are not currently endangered. Check the rules where you live before taking any bitterling, or any mussel, out of the wild.

Diet

Undemanding omnivore. FishBase says it feeds mainly on plants with worms, crustaceans and insect larvae taken as well; the rearing literature describes juveniles on mixed plant and animal food and larger fish on detritus and plant residue. Small frozen foods, sinking granules and a vegetable flake cover it.

Telling males from females

What works for this species, most reliable first.

• Belly depthA hint only · only on well-fed adults, and only as a comparison

  For most tetras, barbs and rasboras this is all there is: a conditioned female is deeper through the belly than a male of the same length.

Also in these lists

Similar species

Same category and the closest water parameters — if this species isn't quite it, start here.

Sold in Lithuania

These Lithuanian shops listed this species in their public catalogue (checked August 8, 2026). Stock moves — ask before you travel.

These links are unpaid: we get nothing for them and ask for nothing.

Frequently asked questions

What tank size does a European Bitterling need?

A European Bitterling needs an aquarium of at least 120 litres (31.7 US gallons) with a footprint at least 80 cm (31.5 in) long — for this species, swimming length matters as much as raw volume.

What water parameters does a European Bitterling need?

European Bitterling does best at 4–26 °C (39.2–78.8 °F), pH 6.5–8.5 and a general hardness of 8–25 °dGH.

What group size does a European Bitterling need?

European Bitterling is a social species — keep a group of at least 8 so natural behaviour emerges and stress stays low.

Is the European Bitterling suitable for beginners?

European Bitterling sits in the middle of the difficulty scale — manageable for a careful beginner who has done the reading, but it has specific needs worth researching first.

Sources

• FishBase — Rhodeus amarus (European bitterling)
• FishBase — Rhodeus sericeus (Bitterling)
• Wikipedia — European bitterling
• Kujawa & Piech 2021 — Rearing of Bitterling (Rhodeus amarus) Larvae and Fry under Controlled Conditions, Animals 11(12):3534
• Halabowski et al. 2025 — The impact of invasive Sinanodonta woodiana (Bivalvia, Unionidae) and mussel macroparasites on the egg distribution of parasitic bitterling fish in host mussels, Scientific Reports 15:9417 (Reichard is the last author)
• Maidenhead Aquatics (Fishkeeper) — Swan Mussel, Anodonta cygnea
• Council Directive 92/43/EEC (Habitats Directive), Annexes II and IV — as published by legislation.gov.uk
• Lietuvos raudonoji knyga — Lietuvos Respublikos saugomų gyvūnų, augalų ir grybų rūšių sąrašas 2020 m.
• Wikipedia — Unio crassus
• Vikipedija — Paprastoji kartuolė