Daphnia Pulex

Daphnia pulex, the most common species of water flea, is a small freshwater crustacean in the family Daphniidae, formally described by Leydig in 1860. Found worldwide across the Americas, Europe, and Australia, it favors small, shaded pools, and has a head with six fused segments and two pairs of antennae, the second pair enlarged for swimming, with a shell extending backward into a spine. It is a model species and was the first crustacean to have its genome sequenced, a genome of about 200 megabase pairs across 12 chromosomes carrying some 31,000 genes, and it reproduces by cyclical parthenogenesis, alternating between asexual and sexual generations.