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Rhodeus sericeus


This minnow-like fish is about 3 inches long. The normal coloring is gray-green on the back with silver flanks and belly. The dorsal fin is blackish, the other fins are reddish or yellowish. Both sexes are normally colored similarly, but during the breeding season the male becomes brilliantly colored. He turns olive-green on the back, his flanks becoming iridescent with all the colors of the rainbow but with violet and blue predominant. His throat and belly turn red or orange, his paired fins yellow, tail fin green and yellow, and unpaired fins red with black edges.

Distribution and Habitat

Bitterlings live in ponds, lakes, marshes, muddy and sandy pools, and backwaters of rivers. They range from Central and Eastern Europe through Asia Minor.


The bitterling's breeding behavior is by far its most distinguishing feature. Bitterlings spawn in April, at which time the male takes on his brilliant colors, and the female develops an ovipositor that is about two inches long.

The male keeps close beside the female, and the pair swims around until they find a freshwater mussel. The pair approach the mussel and the female positions herself head downwards over its siphon. The female lays her eggs in the siphon. The male then discharges his milt over the mouth of the siphon and it is taken in by the ingoing current to fertilize the eggs. The young bitterlings remain inside the mussel for a month after hatching, benefiting from shelter provided by the mussel's shell without encumbering the mussel to any degree.

the female bitterling is on the left, with her long trailing ovipositor ready for deposition of eggs
the female bitterling is on the left, with her long trailing ovipositor ready for deposition of eggs

What makes this unusual breeding behavior even more remarkable is the fact that the presence of an excited male is not enough to get the female to lay her eggs. The female needs the added stimuli of the sight of a mussel's shell and the feel against her body of the current entering the siphon or she will not lay. Equally, if there is a mussel but no male near she will lay infertile eggs.

The above arrangement is also beneficial to the mussel. As the bitterling is laying her eggs the mussel is discharging its larvae. These, known as glochidia, attach themselves to the fins or tail of the bitterling. Once they have taken hold the bitterling's skin grows over and encases each larva, which is carried about by the fish for about three months before finally dropping from its host as a fully formed mussel. The mussel is not, however, nearly as dependent on the bitterling as the bitterling is on the mussel. The glochidia will attach themselves to any floating debris present, but they do stand a better chance of survival if there is some species of fish present to which they can attach themselves.


The main food of the bitterling consists of small worms and midge larvae, but a variety of animal and plant food is taken.

Scientific Classification

phylum Chordata
subphylum Vertebrata
class Actinopterygii
order Cypriniformes
family Cyprinidae
genus & species Rhodeus sericeus

Maurice Burton and Robert Burton Funk & Wagnalls Wildlife Encyclopedia New York: Funk & Wagnalls, Inc., 1974

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The Robinson Library >> Science >> Zoology >> Fishes >> Class Actinopterygii

This page was last updated on October 30, 2017.