While humongous fleas were dining on dinosaurs, wee water fleas were abundant denizens of freshwater habitats. Water fleas arose during the Permian.
Water fleas are tiny crustaceans in the order Cladocera, so-called for their bouncing locomotion. They continue today to play a key role in most terrestrial ecosystems.
Like sharks, water fleas emerged with a near-ideal set of traits, including agile adaptability to survive significant ecological shifts. Their basic biomechanics have scarcely evolved since their arrival ~260 MYA, while diversifying into over 700 species that occupy almost all freshwater environments.
When conditions are favorable, female cladocerans reproduce asexually. Conversely, when environmental conditions deteriorate and variety becomes critical, males are produced and sex becomes the norm. This conditional reproductive system is known as cyclical parthenogenesis.
Though water fleas have various defense mechanisms to escape predators, some decided on a tamer domain: in groundwater. No longer needing sight, these subterranean crustaceans economically went blind and lost their eyes.