Order:	Cypriniformes
Family:	Cyprinidae
Genus and species:	Rhodeus sericeus amarus

Alwyne Wheeler (1969) in "The Fishes of the British Isles and North West Europe":

Bitterling

Rhodeus sericeus amarus (Bloch, 1782)

NAMES Fr. Bouvière; G. Bitterling; Du. Bittervoorn.

IDENTIFICATION A small, deep-bodied, laterally compressed fish with a short lateral line, confined to the first five or six scales. The mouth is small and oblique, inferior or just terminal, but the lower jaw is never longer than the upper. The dorsal fin is long, commencing just behind the pelvic origin, and ending over the fourth or fifth anal ray. It grows to 3¾ in (9.5 cm), but usually to 2½-2¾ in (6-7 cm).
A brightly coloured fish, its back is grey-green, there is a metallic streak on the hind end of the sides, and the belly and lower sides are silvery white. The fins are pale orange with grey shading. At spawning the coloration of the males is brilliant; the sides are strikingly iridescent, and the dorsal and anal fins bright red. A triangular area each side of the snout bears dense white tubercles.
D. III/9-10; A. III/8-9; scales on side 34-8; transverse scales 10-12; pharyngeal teeth five each side.

BIOLOGY The bitterling is found in small lakes, marshes and slowly flowing back-waters, mostly where the bottom is sandy or muddy. It breeds amongst bulrushes and marginal weeds from April to June, when the males acquire their breeding colour and the females develop a long fleshy tube from the genital opening. The eggs pass down this ovipositor and are laid two at a time in the gill chamber of mussels belonging to the genera Anodonta (the swan mussel), Pseudanodonta, or Unio (the pearl mussel). To fertilise the eggs the male ejects sperm into the inhalent current as the mussel respires. The process is repeated until all the eggs have been deposited in the mussel's gill chamber. The bitterling larvae stay inside the mussel for three or four weeks until their yolk sacs are absorbed at a length of about ¾ in (2 cm). Bitterling frequently act as the host to the parasitic early stage of these mussels.
Bitterling feed on phytoplankton, small animals, crustaceans, insect larvae, tubificid worms and, it is said, vegetable detritus.
The bitterling has value neither as a food fish nor as a sport fish, and indeed its flesh is said to be bitter and unpalatable. It has some use as bait, but its chief appeal is as an aquarium fish, both for its colouring, lively behaviour and interesting spawning habits (although it is difficult to breed in captivity). Its local establishment in British waters may have resulted from the liberation of pet fish. It was first noticed in the 1920s and, probably mostly by artificial introduction, became well established in south Lancashire and Cheshire. Its present status is obscure, and it may have declined.

DISTRIBUTION

Introduced to Britain. A subspecies R. s. sericeus (Pallas, 1776) in central and north-east Asia.