The bitterling is one of the smaller members of the carp family. Widely distributed throughout Europe from the English Channel to the Caspian Sea, the bitterling was introduced to Britain as an ornamental fish. Having escaped – or been released – it is now found in Lancashire, Cheshire, East Anglia (notably on the Great Ouse system) and a number of ponds in southern England. Living in slow-flowing rivers and streams and a variety of still waters, the bitterling prefers a pollution-free enviroment.
Unlike many of the smaller carp-like fishes the bitterling is fairly easy to recognize. A deep-bodied little fish, with a small head and mouth, it is quite similar to a small crucian carp, but has silver scales and fewer (8-10 versus 14-21) rays in the dorsal fin. The body scales are rather large with between 32 and 40 from the back of the head to the tail fin.
It is unique among the carp family in that its lateral line has pores on only the first five or six scales from the head. Both sexes have a bluish streak near the tail. However, during the mating season, this becomes iridescent and the male develops a pink belly and bright red fins.
The bitterling is unlike any other European fish in that it relies on the freshwater mussel for its reproduction. In the spring the female develops a long tube from her vent, while the male sets up a territory on the lake bed very close to a mussel. After a showy display by the colourful male, the female nudges the mussel with her snout to condition it to her presence. This ensures that when she inserts her egg-laying tube into the mussel – to deposit her eggs on to its gills – it doesn’t clamp shut. The male then releases his sperm, which is sucked in when the mollusc inhales.
In this way the eggs are fertilized. They hatch after two to three weeks. Spawning is repeated until all the female’s 40-100 eggs are laid. Several mussels are used as nurseries. The young fry leave their host within a few days of hatching, once their supply of egg yolk is used up.
This fascinating use of one animal to act as a nurse to another is very rare in nature. The advantages to the bitterling are several: the eggs are safe from predators inside the shell, they are well aerated during their development, and if there is a risk of drought, the mussel moves itself and the fish’s nursery to deeper water.
The mussel too gains from this relationship: frequently their own strange parasitic larvae attach themselves to the fish, so ensuring a much wider distribution for the mollusc. This type of mutually beneficial partnership between animals of different kinds is known as symbiosis.
The bitterling feeds mainly on small insect larvae, worms and crustaceans. It also eats some plant matter, particularly algae. Such unfiissy feeding habits are an advantage when living in small ponds where potential food sources can be limited.