Water Flea is a popular name for minute aquatic Crustaceans, such as
Daphnia among Cladocera
Cypris among Ostracoda and
Cyclop among Copepoda. The common
Daphnia pulex, abundant in fresh water, is a good representative. The body is enclosed in a bivalve shell; there is a large single eye; a pair of large antennae is used an swimming organs. The daphnoids are marvelously prolific, and for prolonged periods parthenogenetic. There is an interesting difference between the winter eggs which require fertilization and the summer eggs which do not. The females have a dorsal brood chamber between the shell and the back. Of related genera
Sida,
Sloina,
Lynceus,
Polyphemus, and
Leptodora may be noted. In cypris also the shell is bivalve; there are five pair of appendages on the head and two on the body; most of these are used in swimming or creeping. Related to cypris but living in the sea are
Cythere,
Halocypris, and
Cypridina. Among cyprides parthenogenesis again occurs. and in some species males have never been observed, while parthenogenetic development has been traced for as many as 40 successive generations. The females bear large egg sacs. In cyclops the body is more distinctly segmented and the shell is not bivalve; the head bears antennae mandibles, and maxillae, and the first five segments of the thorax bear swimming appendages. Resembling cyclops and also a fresh-water form is
Canthocamptus, while
Cetochilus and
Clausocalanus represent numerous marine
Copepods. Water fleas feed on microscopic plants and animals and on organic debris, while they themselves — often occurring in countless swarms—form an important part of the food supply of certain fishes.
