The Bitterling or (Rhodeus amarus), is a member of the Carp Family. The female lays eggs inside freshwater mussels via a long tube (ovipositor) which it develops in the breeding season. Bitterling grow to a maximum length of 3 inches. Their most striking feature is an electric blue line running down the tail root. In the breeding season the lower fins become orange and the dorsal fin of the male becomes reddish
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It is a temperate freshwater fish belonging to the Acheilognathinae sub-family of the Cyprinidae family. It originates in Europe, ranging from the Rhone River basin in France to the Neva River in Russia.

It is known simply as "the bitterling" in its native range, where it is the only species of its genus Rhodeus, and sometimes in the scientific literature also.

The fish reaches a size of up to 10 centimetres (4 in). It is found among plants over sandy and muddy bottoms in shallow waters. It feeds mainly on plants, and to a lesser extent upon worms, crustaceans, and insect larvae.

This species of fish was once used for human pregnancy tests. Female specimens were injected with the urine of the woman to be tested. If the woman was pregnant, the hormones in the urine would cause the fish's ovipositors to protrude.

The fish spawns in clear, slow-running or still water, often with a muddy bottom. The female deposits her eggs inside freshwater mussels. The male sheds his sperm into the inhalent current of the respiring mussel and thereby fertilizes the eggs. The young hatch and remain protected within the mussel for approximately one month, eventually leaving the mussel as actively swimming larvae. Both sexes reach sexual maturity in 1 year, at a length of 30 to 35 millimetres (1.1 to 1.4 in).

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