The microcrustacean arthropod Daphnia pulex, commonly called the water flea, has been subject to intense biological investigations for over a century as a keystone species of freshwater ecosystems. Their populations are sensitive to toxicants in the environment, and are used to assess the ecological impact of environmental change.

Daphnia pulex was the first published crustacean genome, and was sequenced and assembled by the Joint Genome Institute. The assembly consists of about 197 Mb in 5,191 scaffolds, is high-quality, and covers approximately 80% of the Daphnia nuclear genome.