The Bitterling is a small fish with a high-backed body. On its sides it has a blue-green stripe, which widens out towards the tail. It is quite abundant in the stagnant waters of the lower reaches of rivers, in shallow creeks, in old river backwaters and pools. However, it lives only in waters inhabited by the freshwater mussel, into which the bitterling lays its eggs between April and June. During this period the male becomes reddish-purple at the sides and the female develops a pink ovipositor by means of which she places the eggs within the mantle cavity of the mussel.
It is found all over Europe from north-eastern France as far as the Caspian Sea, but is not present in Denmark, Scandinavia, or the Mediterranean countries. It has been introduced into a number of waters in north-west England. It lives on planktonic crustaceans, insect larvae and worms.
Maximum size and weight: 8 cm, 15 g.
Identifying characteristics: A blue-green stripe on the sides, widening towards the back. During mating female with long ovipositor, male reddish-purple on sides. Mouth in semi-inferior position; lateral line extends to only 5-6 scales.