Invertebrates are animals that have no internal supporting structure or skeleton.

Invertebrates do not possess well-developed senses. Their circulatory and respiratory systems are inefficient. These factors explain why invertebrates are often quite small. 

Water ecosystems are home to a large number of invertebrate species. In addition to the very small zooplankton, many larger invertebrates are also found in water ecosystems. Many water-dwelling invertebrates, such as snails, crabs, and clams have hard shells. 

Species of crustaceans, molluscs, and worms are common in water ecosystems. In addition to these, insects and spiders can also be found living in the water. The majority of insects live in the water during their larval stage, but for some species water ecosystems are an important habitat also in maturity.


The liver fluke snail is an invertebratic mollusc.

Most small invertebrates live on the surface of the water. Pond skaters and whirligig beetles are examples of such species. Other invertebrates live under the water's surface, where algae and aquatic plants give them protection from predators. Clams, molluscs, and crabs dwell on the bottom of water ecosystems. Jellyfish can be found in the Baltic Sea.