The water flea Daphnia pulex, less than Vs inch long, 

 swims by jerking the large, branched antennae. (Ja- 

 pan. Y. Fukuhara) 



dominal tail. The eggs drop out, sink to tiie bottom, 

 and either develop promptly into a new generation 

 of young or remain dormant until the pond dries up. 

 Then the eggs can stay for years if necessary, until 

 conditions are right again for hatching. They may 

 blow as dust and fall into other temporary pools, 

 starting other sudden swarms of fairy shrimps. 



Brine shrimps Vi of an inch long are almost as un- 

 behevable as they disport in salt-saturated bays of 

 Great Salt Lake, Utah, and other bodies of concen- 

 trated sea water. Often they swarm in astonishing 

 numbers in the artificial pans where man evaporates 

 brine to get salt. In these places Anemia finds abun- 

 dant microscopic plants of salt-tolerant types, and 



few enemies, since most fish cannot endure the salt. 

 Flamingoes may compete in filtering out the algae, 

 and take small brine shrimp as well. 



CLADOCERANS 



Both salt water and fresh have their water fleas, 

 members of order Cladocera. These creatures ex- 

 tend their antennae, legs, and abdominal tip through 

 the gap in a bivalved carapace saddhng them like the 

 blanket on a pet dog. A water flea's second pair of 

 antennae are remarkably long and bear a fringe of 

 bristles or hairs that make the organs effective as 

 swimming oars. A single compound eye occupies a 

 central position in the head, but it can be shifted by 

 muscles. 



The most familiar of the water fleas in ponds and 

 lakes is Daphnia, adults of which reach a total length 

 just over V» of an inch. In surface waters these pink- 

 ish or flesh-colored motes dance all day, while sweep- 

 ing into their mouths microscopic algae. 



Small fish eat enormous numbers of Daphnia and 

 other cladocerans. Yet the rapid reproduction of 

 these little crustaceans makes good all losses. For 

 much of the year only female Daphnia can be found, 

 and each releases another brood of fifty young ones 

 every eleven or twelve days entirely by virgin birth 

 (parthenogenesis). When living conditions deterio- 

 rate or autumn approaches, however, some of the 

 young released mature as males and fertilize a final 

 crop of eggs for the year. These "winter eggs" are re- 

 sistant to freezing and to desiccation. The parent 

 coats them in an extra shell (an ephippium) which 

 may have air cells providing buoyancy. As the pond 

 dries up. wading birds often become loaded with 

 these armored eggs in the mud on their feet, and then 

 wash off the living load in another pond where con- 

 ditions are more suitable for water fleas. 



Some of the larger cladocerans are carnivorous. 

 Leptodora is an especially fierce one. sometimes 

 reaching a length of Vi of an inch. It rows rapidly 

 with winglike antennae in pursuit of smaller water 

 fleas, insect young, and water mites. Like so many 

 of its crustacean victims, Leptodora tends to swim at 

 somewhat greater depths at midday, but comes to the 

 surface in the late afternoon and feeds there until the 

 sun brightens the sky in the morning. 



COPEPODS 



Members of another order of small crustaceans, 

 the order Copepoda, swim by jerky, oarlike move- 

 ments of the antennae. The name "copepod" comes, 

 in fact, from the Greek kope. an oar. But the swim- 

 ming antennae of a copepod are its second pair, not 

 the first. And the body is usually a streamlined pear 

 shape, with segmentation clearly visible through a 

 microscope. 



The largest of the free-swimming copepods is less 



230] 



