Crustaceans & Molluscs


Crustaceans are a group of animals which have a hard exoskeleton shell – they include things like crabs, shrimp, and lobsters. Most of them live in aquatic habitats. You’re unlikely to see crustaceans on a walk along the Thame, as they usually inhabit the riverbed. Freshwater shrimp are common in the catchment, although they prefer unpolluted water. They are thought to be beneficial to ecosystems because they eat dead matter from the riverbed. Signal crayfish are another species of crustaceans found in the Thame, which unfortunately are invasive and have outcompeted the native white-clawed crayfish.  They eat everything so are very destructive to the river habitat.


Molluscs are an extremely diverse group of animals. Slugs and snails are commonly known molluscs but things like mussels and squid are also a part of the group. The most common form of freshwater molluscs are snails. There are a range of molluscs found in the Thame including duck mussels, orb mussels, freshwater limpets, and mud snails.

Zooplankton in Rivers

Zooplankton are tiny, usually microscopic, animals which live in water. In rivers, they drift with the water column because they are not strong enough swimmers to move against the current. Zooplankton are an important link in riverine food webs because they are prey to species at higher trophic levels, such as fish and aquatic insects. Their abundance and species composition can be threatened by changes in water quality including variations in nutrient levels, temperature, and pH. Native zooplankton are also at risk from invasive species. Zooplankton are generally found close to the water surface because they eat phytoplankton which need light for photosynthesis.