Look into the water beneath the bridge. With a bit of luck you may be able to see some well camouflaged mussels. What you will probably barely be able to see are the smallest European species of carp - bitterlings - which are around 6cm long. But the mussels are an essential prerequisite for their presence, because life is dangerous for the fry. This is one of the reasons why the river meadow waters are so important as a nursery for the Rhine fish. It is quiet here and there are lots of places to hide in the shallow waters. But this safety is not enough for some species of fish. During the course of evolution they developed astonishing strategies to better protect their offspring. The bitterling has found an especially spectacular/eye-catching method. At spawning time, the males seek out freshwater mussels. The female then lays around two eggs in their gill chambers - a total of 100 in different mussels. The male then releases his sperm in front of the breathing apertures and the mussel then involuntarily takes care of fertilisation. After two to four weeks, the fry, which are one centimetre long, then leave their nursery. This surrogacy weakens the mussels and the little stowaways get away without doing anything in return. However, mussel larvae can stick to the bitterling mother, which then helps to spread the otherwise leisurely mussels. A spectacular trick of evolution can quickly become a real problem when the number of mussels falls. This happens quickly if suitable bodies of water disappear or the water quality deteriorates. Mussels are very sensitive to this and, without them, it's all over for the bitterlings.