Bitterling

A small, deep-bodied freshwater fish found in slow-flowing rivers, canals and ponds across much of Europe, notable for an unusual breeding partnership with freshwater mussels: females use a long tube to lay eggs directly inside a living mussel, where the young develop protected from predators. Breeding males develop a striking rosy-pink flank flush in spring, making this the best season to spot them in clear, weedy shallows. Look for small shoals hovering near mussel beds in clean, slow-moving water with a silty or sandy bottom.