TankBud

European bitterling

Rhodeus amarus

FreshwaterFish

European bitterling (Rhodeus amarus)
Viridiflavus · CC BY-SA 3.0

Care at a glance

Temperature
55–72°F
13–22°C
pH
6.5–8.0
Adult size
3.9 in
10 cm
Minimum tank
34 gal
130 L
Temperament
Peaceful
Social behavior
Shoaling
Swim level
Middle
Difficulty
Medium
Breeding
Egg layer
Rarity
Very rare
Family
Cyprinidae
Origin
Europe

Overview

The European bitterling is a peaceful, shoaling coldwater fish for an unheated tank. It suits a keeper with a little experience.

Did you know

The European bitterling and the freshwater mussel share a two-way partnership. The mussel shelters the bitterling's eggs, while the mussel's own larvae, called glochidia, ride on the fish's skin and gills to disperse. A courting male defends his mussel like a prized territory.

Caring for the European bitterling

Give a group of European bitterlings a tank of 34 gal (130 L) or larger. Furnish it with a soft substrate, dense planting and open swimming space. Keep the water clean and well oxygenated.

Keep them cool, at 55 to 72°F (13 to 22°C) and a neutral to slightly alkaline pH of 6.5 to 8.0. They suit an unheated tank or a cool community, and a seasonal cool period benefits them. They are undemanding feeders that take flake, small pellets, frozen foods and some vegetable matter.

Keep a group of six or more, with a mix of males and females if you hope to see natural behavior. European bitterlings are peaceful and swim in the middle water. They get along well with other temperate, gentle species.

During courtship the male turns iridescent pink and purple, and the female grows a long tube called an ovipositor. Breeding needs a living freshwater mussel, with a sandy area to suit it, so you cannot spawn bitterlings casually in a bare aquarium. The female uses her ovipositor to place eggs inside the mussel's gill chamber, where the male's milt is drawn in to fertilize them. The young develop safely inside and swim free after a few weeks.

Good to know

The European bitterling, Rhodeus amarus, is a small, deep-bodied member of the family Cyprinidae. It is best known for its extraordinary breeding strategy with freshwater mussels.

In the wild

The European bitterling is native to slow, weedy lowland waters across much of Europe. Its habitats include ponds, canals, backwaters and the quiet margins of rivers where freshwater mussels live.

Symbiotic relationships

Bitterlings and freshwater mussels

Bitterlings do not lay their eggs in the open at all. In spring the female grows a long egg tube and slips her eggs through the outflow siphon of a living freshwater mussel, into its gill chamber, while the male releases sperm over the inflow siphon so the mussel draws it in. The embryos develop inside the mussel for about a month, protected from everything, then swim out. The mussel's own larvae attach to passing fish to develop, so the two use each other. Biologists now read it as a mild parasitism running both ways rather than a partnership: the eggs cost the mussel oxygen and food, and bitterlings have evolved to shed the mussel larvae that other fish carry.

Bitterlings are cool-water fish for an unheated tank and breed only with a live mussel present. Swan or duck mussels are the usual hosts; they are filter feeders that starve in clean water and need a mature, well fed tank with fine sand to sit in, which is why breeding bitterlings is a project rather than a happy accident. Watch the female's egg tube lengthen and the male stake out the mussel, and you have seen the whole ritual.

Every symbiotic relationship in the catalog

Weekly picks

Tank mates

The European bitterling is a peaceful, shoaling species that keeps to the middle. These picks share a community profile with it, are safe together in both directions, and overlap on water type, temperature and pH.

Check the full mix in the stocking calculator

Common questions

What size tank does a European bitterling need?

A European bitterling needs at least 34 gal (130 L). It spends most of its time in the middle.

What temperature and pH does a European bitterling need?

A European bitterling is comfortable at 55–72°F (13–22°C) with a pH of 6.5–8.0.

What does a European bitterling eat?

A European bitterling is an undemanding feeder. It takes flake, small pellets, frozen foods and some vegetable matter.

How big does a European bitterling get?

An adult European bitterling reaches about 3.9 in (10 cm).

Does a European bitterling need a group?

Yes. A European bitterling is a shoaling species and should be kept in a group, not singly.

How can you tell a male European bitterling from a female?

Males turn iridescent pink and purple during courtship. Females grow a long tube called an ovipositor.

How does a European bitterling breed?

A European bitterling needs a living freshwater mussel to breed. The female places her eggs inside the mussel's gill chamber with her ovipositor, and the young swim free after a few weeks.

Related species