The evolution of groundwater Cladocera

Cladocera occur in various types of groundwater, but are most common in the underflow of rivers. Numerous surface water species occasionally venture into groundwater; some chydorids are more common in groundwaters than in surface waters; at least three groups within Alona, finally, have evolved exclusive groundwater species. The latter show few obvious adaptations to the subterranean habitat, except loss of an eye or total blindness. Some, however, have conserved an array of primitive characters (e.g, on the end-claw of the postabdomen, and the setation of the valve rims) which suggest that the physical protection and relative constancy of the hyporheic has permitted the survival of some ancient taxa