Waterflea at 10X
The waterflea is probably one of the most interesting animals to be seen in freshwater. They are about the size of a pinhead, so they can be seen swimming in a jar of water. Waterfleas are used to test water quality in sewerage treatment plants because they are highly intolerant to polluted water.
I always get a kick out of telling the boys in my microscope classes that these animals don't have to have a male to reproduce, they clone themselves. I ask the girls if they could imagine having 4 to 10 babies every week for up to 2 months and the babies would be exactly like them! Three babies can easily be seen in this waterflea's broodpouch; the small black spots on the right side are their eyes. Males can be produced when needed, such as when the water supply is dwindling or in the winter before the water freezes. When a male and female mate, a single baby is produced in a large (relatively speaking!) cyst or resting egg. The cyst settles to the bottom and stays dormant until the conditions improve enough for the baby to hatch out. The baby will be a female and she will begin to clone herself within days of hatching. Soon the pond will be full of waterfleas again!
***Update*** Thanks to Cathy, a research assistant at the Limnology Lab at Ohio State University, I now understand how the female cladocera are able to produce males:
"As for how males cladocerans (Daphnia and others) are produced, when environmental conditions change, becoming less favorable due to overcrowding, accumulation of metabolic wastes, decreasing food availability, or changes in light or temperature, some female produce parthenogenic male eggs (2n) which become males (2n) that produce sperm (1n). Some females will produce sexual eggs (1n) at this time as well, so males and females can mate to produce a fertilized egg (2n) that is released into a specialized resting case (called an Ephippium) which is resistant to freezing and drying. Ephippium look like a saddle. The ephippium may sink to the bottom of the pond or lake, and can stay in the resting state until favorable conditions return. I have read that Ephippium have been found in sediments up to 300 years old and hatched in laboratory conditions producing parthenogenetic females which will live normally."