This cladoceran is one of many varieties of water flea and can be found along weedy margins of ponds and ditches. It has a discrete head, with antennae, and a bivalve shell that encloses all or most of the trunk and abdomen. Macrothrix filter-feeds on microscopic particles of organic matter by creating currents of water with its thoracic limbs.
Like other cladocerans, macrothrix are an important part of the food chain, filter-feeding on microscopic particles and phytoplankton and, in turn, providing food for juvenile fish. In the Great Lakes of North America, water fleas are the basic food for nearly all commercial fish species.
Cladocerans are parthenogenetic most of the year, producing unfertilized eggs that, in turn, produce females. Towards the end of seasonal population peaks, in a mechanism not well understood, parthenogenetic females produce parthenogenetic male eggs. Sexual females are also produced parthenogenetically. They copulate with the males, producing a small number of resistant eggs. These resistant eggs can overwinter, withstand drying and freezing, etc. and will hatch when conditions permit. These eggs aid in dispersal between bodies of water and allow species to endure in ephemeral habitats.