A Little Bit More
Often called “water fleas”, daphnia are not insects at all, but crustaceans, related to lobster, crayfish, and shrimp. As we think of the importance of different animals in our environment, the larger, more obvious members tend to be the ones that come to mind. Yet it’s often the smaller, less conspicuous animals that exert the greatest effect on a living community; daphnia are one such important animal. Daphnia provide a critical link in the aquatic food chain by converting large numbers of minute organisms (plants and animals) into a readily available food source for animals larger than themselves. If daphnia are removed from the food chain, a wide assortment of other animals are adversely affected.
In the early spring there are few daphnia present in local waters. Those individuals that can be found are females that either over-wintered as adults or are recently hatched from “resting” eggs laid the previous fall. As temperatures warm, reproduction begins and females, there are no males at this time, begin to lay eggs that develop without fertilization. The eggs are kept in a brood pouch within the female and the embryos are nourished both by the egg yolk and by secretions from the mother’s body. As the season progresses, the daphnia population can increase dramatically; an individual may produce young as often as every two days! Often the population drops in the summer but reaches a second peak in the fall. Since daphnia populations go through cycles, it’s difficult or impossible to predict when and where they will be found. At times, however, their numbers are so great that they actually color the water.
Before any males are produced, unsuitable environmental conditions must occur. Production of male eggs seems to be induced by crowding, a decrease in available food, and/or a warming of the waters. These conditions, and probably others, cause females to produce eggs that become males. If optimum conditions remain for an extended period, however, hundreds of generations of females may be produced before any males are hatched at all.
Activities
Daphnia Galore!
Objectives: See daphnia in action
Materials: Aquarium or appropriate container and daphnia Where can they be found or obtained?
Daphnia are abundant and more common than you may think. No matter where you are in North America (actually in much of the world), you should be able to obtain Daphnia from a “local” source. There are over 150 species in North America alone and although their habitats do vary, lakes and ponds with a limestone base tend to be an especially good source for these animals. Keep in mind that their numbers fluctuate a great deal over the warmer months so although you may not be able to find any today, next week they may be especially numerous.
If you want to collect your own daphnia, you will find that a fine-meshed net works well. You can even make a homemade one from muslin curtain material and a bent coat-hanger. Sweep your net through the water in a gentle figure eight pattern and then invert the net and its contents into a bucket containing water from the collection site. The best time for collection seems to be in the evening before dusk in shallow water although you may find large numbers of daphnia at any time, it just all depends on conditions.
Your collected daphnia can be placed in a variety of containers but an aquarium probably works best. Plenty of oxygen is essential for these animals so you either need to have lots of water in relation to the number of daphnia, or use an aerator to provide adequate oxygenation. If you add pond water, especially green pond water, there should be plenty of food for the daphnia to feed on. Depending on the number of daphnia present, they may clear up the green water rather quickly and you’ll need to add more. If you do run out of enough green water, a yeast culture can be used as a substitute until more appropriate water is found. (place a bit of yeast, a little bit of sugar and warm water together for 10 minutes or so, then add this mixture to your tank)
Daphnia make excellent subjects for study under a microscope or dissecting scope. For viewing, they can be placed on a depression slide or in a few drops of water in a bottle cap. Be sure they don’t overheat while you’re observing them. Their transparent exoskeleton allows easy viewing of their internal organs; even a tiny, beating heart can be seen.
Daphnia Life Cycle –
Most of the time the daphnia population consists entirely of females that reproduce asexually. Females may produce more than 100 eggs per brood, and have new broods every few days. A female may have as many as 25 broods in her lifetime, but the average is about 6. Female daphnia start to reproduce at about 4 days of age and have a brood size of 4 to 22 eggs. Under adverse conditions, males are produced, and sexual reproduction begins. The end product of sexual reproduction is the laying of dormant eggs, that will not hatch until there are favorable conditions such as the return to warmer temperatures and/or spring. Factors that can trigger this are a lack of food, low oxygen supply, a high population density, or low temperatures.
Here are some things to look for and learn about from your daphnia:
1. Do your animals seem to interact with each other?
2. Are your daphnia more active in the middle of the day or at dawn and dusk (low vs. high light levels)?
3. Do your daphnia seem to prefer the “darker” or “lighter” side of their aquarium (give them a choice by darkening one end)?
4. Do your daphnia ever just rest (stop moving) or are they always moving?
5. If you switch a light on in the middle of the night, are your daphnia moving around or not?
6. If you place your hand or finger in their aquarium, do the daphnia react?
7. Do daphnia sink if/when they stop moving or do they remain at the same water level?
Key Concepts
Food Webs /Food Pyramids, Identification, Predator/Prey Relationships
Questions
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Posted by Tom Miller on April 09, 2006 at 11:52 AM
Q: How early in spring after a frost would it be possible for a person to be bitten by chiggers
A: Chiggers are usually a problem in late spring and summer. Of course, "late spring" comes at different times for different areas - we just had snow recently in central NY. But from what I know about Nashville, I'd say you're still a bit early for concern about chiggers. They tend to be most of a problem once grasses and brush grow to enough height so the nymphs can crawl from the plants onto your legs. Hope this helps - chiggers are no fun!!
Posted by BOB on November 20, 2004 at 11:50 PM
Q: IN NORTHEAST OHIO, IN SOME OF OUR DEEPER RESEVIORS,WE HAVE AN OVER POPULATION OFF FRESSWATER CLAMMS. BOTH LAKES ARE LOSING THE WEED BASE AND RESOLTING IN LOW FORAGE COUNTS. WHEN FALL LET DOWN OCCURS YOU REALLY NOTTICE THEM STUCK TO EVERYTHING. EVEN SOME OF THE FISH HAVE SORES. WILL THE WEEDS COME BACK?WILL THE CLAMS DIE?ANY INFO WOULD BE HELPFUL THANK YOU! BOB
A: It sounds as though you have Zebra Mussels (these are an introduced clam) as these are definitely common in some of the Great Lakes now and other nearby bodies of water. Take a look at this unit and see if the picture of this clam is like what you're seeing. www.enaturalist.org
Zebra Mussel numbers can be enormous and they are able to make a difference in water quality by straining out microscopic plants to feed. Lakes inhabited by large numbers of these clams usually become very clear. The activity of the clams shouldn't affect the larger plants however, just the small, floating ones. There may be a couple of things going on in your lakes and the clams are only part of the problem. I'll bet the Ohio Dept of Conservation can help you understand what is going on. Thanks for your question.