Macrothrix is very closely related to the Daphnia (both are in the Order Cladocera but in different Families). They are invertebrates with a hard outer shell. They swim freely about and are from 1/2 to 3mm long. The Macrothrix has two separated and distinct compound eyes and is somewhat larger than the Daphnia. It can be differentiated from the Daphnia by the long spines extending from the lower part of its body.

Macrothrix are food for Hydra and small fish.