Many small animals (invertebrates) live in and on the river bottom of the Rhine, such as bryozoa, insects, molluscs, crustaceans, hirudinea, turbellaria and freshwater sponges. They can hardly be made out with the eye and are summarized as macrozoobenthos. Since 1990, the ICPR regularly draws up an inventory of the invertebrates between Lake Constance and the North Sea.
The construction of wastewater treatment plants reduced water pollution and thus led to increased oxygen content of the Rhine, which distinctly increased the number of small animals.
In 2018, more than 500 macrozoobenthos species were detected in the Rhine between the Alps and the North Sea.
After the rapid increase in the species diversity of the macrozoobenthos following the improvement of the Rhine water quality in the 1980s and the 1990s a reverse trend is being observed since about the year 2006. In particular, the fauna of water insects was much more diverse between 1995 and 2000 than it is today. This trend is explained by the immigration of invasive alien species such as mussels, snails and small crustaceans from the Black Sea area, North America and Asia via canals and by ships. At present it is difficult to predict how stable this trend is. Since 2012, however, there has been a slight increase in mean species numbers, which is also associated with the recovery of some Rhine-typical species such as the caddisfly species Hydropsyche sp. and Psychomyia pusilla.