Daphnia belongs to the phylum Arthropoda, class Crustacea, subclass Gillopoda. Commonly known as fish worms. The body is small, about 2 millimeters long, light flesh red, living in fresh water. The body is divided into the head and trunk, with dorsal armor, trunk limbs (thoracic limbs) 5 pairs, for the movement and respiratory organs.

In spring and summer, only females can be seen, unisexual reproduction, and the eggs laid are called "summer eggs", which are small, with thin shells and little yolk, and do not need to be fertilized, and can be developed directly into adult worms. These adult insects are mostly female, and then carry out solitary reproduction. Therefore, they are able to reproduce in large numbers in a short period of time and appear as a reddish color, hence the name redworm. In the fall, some small males hatch out from the summer eggs and start hermaphroditic reproduction, and the eggs laid are called "winter eggs", which are larger than the summer eggs, with thicker shells and more yolk. Fertilized winter eggs, also known as "dormant eggs", through the cold or dry environment, in the following spring when the temperature is higher for the development of new females. In addition to a few living in seawater, mostly a variety of freshwater waters in the most common zooplankton, is an excellent bait for fish.