Breeding European Bitterling

How to breed Rhodeus amarus: the female uses a long ovipositor to lay eggs inside a living freshwater mussel, the male fertilises through the mussel's current, and fry develop inside.

Overview

The European Bitterling (Rhodeus amarus) is a small native European cyprinid with one of the most specialised reproductive strategies among freshwater fish: it spawns inside living freshwater mussels. The presence of nearby freshwater mussels is of vital importance for breeding.

Sexing

In breeding condition the female develops a long egg-laying tube (ovipositor), historically so distinctive that females were once used in pregnancy tests, protruding their ovipositors after injection with urine from pregnant women. Both sexes reach sexual maturity in one year, at a length of 30 to 35 mm.

Breeding Setup

Provide clear, slow-running or still water, often with a muddy bottom, and house live Unio mussels as the spawning substrate. A cool, unheated tank suits this temperate species. Lifespan is exceptionally up to five years, but most individuals do not survive the year of their first reproduction.

Spawning Behavior & Trigger

The female deposits her eggs inside the valves of a living mussel using her ovipositor. The male sheds his sperm into the inhalant current of the respiring mussel, and the eggs are thereby fertilised within the mussel's gill chamber.

Egg & Fry Care

The young hatch and remain protected within the mussel for about a month, eventually leaving it as actively swimming larvae. The fry remain in the mussel until protection is no longer required, so no separate fry tank is needed during this in-mussel stage.