Your lake has “fleas” – and that’s a good thing

What might be the most important living thing in your lake is a tiny organism you probably have never seen, at least up close.

Daphnia, often called water fleas, are an essential food source for fish fry as well as for water insects and the immature forms of frogs, toads and salamanders. Creatures that eat the Daphnia become food for larger and larger fish – ultimately the perch, walleye, bluegills and others that grace your dinner table. So a healthy Daphnia population is critical to your lake’s ecosystem.

In reality, Daphnia are not fleas at all. They’re called fleas because their herky-jerky swimming patterns remind observers of the jumping of fleas. Daphnia are actually crustaceans, related to crayfish and shrimp. You don’t need a microscope to see them – the ones in your lake are probably about a millimeter in size or somewhat bigger. So if you scooped up lake water in a fruit jar and looked through it, you’d probably see a Daphnia or two kicking about.

Of course, they look much more interesting under magnification, with their translucent (actually almost transparent) shell, called a carapace. Through this you can see the innards – include a green gullet if the specimen you’re observing has just eaten its fill of algae. The heart lies just behind the head and beats roughly 180 times a minute – about three times as fast as the heart of a healthy human at rest. Under a microscope, you can watch the heart beat, watch blood corpuscles pass through the circulatory system, and even see unborn Daphnia moving in the brood pouch.

Daphnia have helmet-shaped heads that sprout long antennae, which they use (believe it or not) for swimming. A downward thrust of the antennae propels the creature upward; it then floats back down, on the way breathing and collecting food. Steady movement of the 10 legs creates a current that moves food into the digestive tract. Besides algae, Daphnia eat bacteria and protozoans (one-celled animals).

Daphnia generally undergo parthenogenetic reproduction – offspring develop from unfertilized eggs. Once hatched, the young molt (shed their shells) several times before becoming adults.

Now that you know a little about water fleas, be aware of them when you swim and fish in your lake. And don’t worry that they will infest your dog (because, of course, they are not fleas). You can see lots of cool pictures of a Daphnia at http://images.search.yahoo.com/search/images?_adv_prop=image&fr=altavista&sz=all&va=daphnia.