DAPHNIA: THE WATER FLEA
Daphnia, commonly known as water fleas, are a fantastic organism with which to explore many aspects of biology. We can:
Dr Jenny Koenig is a Science and Engineering Ambassador.
- look at their body shape and organisation - we can see the gut, eggs, heart etc through the microscope
- compare their body shape and organisation with ours
- understand how they are adapted for their environment, investigate the effects of changing their environment
- see, through the microscope, how their heart beat changes after addition of drugs or changes in temperature.
In 2008 we ran a series of "Water flea Workshops" for primary schools and a practical display for the Cambridge Science Festival.
Here you can find out
- how to grow them,
- what they look like,
- the experiment ...
- the experimental results
We are grateful for funding from the Wellcome Trust, the University of Cambridge, Department of Pharmacology and with help and advice from the STEMTEAM Cambridgeshire and the University of Cambridge Science Festival Office.
Image credit: Functional Genomics Thickens the Biological Plot. Gewin V, PLoS Biology Vol. 3/6/2005, e219.
How to look after Daphnia
Any clear plastic or glass container is fine. A large food storer (say 2 - 3L) is ok or old fish tank. The container needs to be loosely covered so that dust and junk doesn't get into it but fresh air (containing oxygen!) can get in. If you're keeping them outside this is very important otherwise you might get unwelcome guests.
Where to put them.
They need light but not too much direct sunlight.
Anything between 20-25 C to be comfortable and to reproduce.
This is important. Daphnia are freshwater creatures. Do not use tap water as it has chlorine in it. However you can leave tap water until the chlorine has evaporated or you can buy dechlorinating kits from aquarium supplies shops. Or you could boil it and then allow it to cool. Rainwater is good if collected from a source that doesn't have too much air polution. Filtered lake or stream water is good.
A good aquarium supplies shop can supply a powdered salt mixture that has the same chemical components as sea water. You can use this at a much more dilute concentration than you would for sea water - usually about 10 g in one litre.
Distilled water will the following salt has also been recommended (Cornell University - see link below). NaHCO 3 0.192 g/L; CaSO 4•2H 2O 0.120 g/L; MgSO 4 0.120 g/L; KCl 0.008 g/L.
The pH should be between 7 - 8.6.
The water will last 2 - 3 months with the Daphnia growing in it. After that there may be build up of waste products and it can help to make a fresh culture.
They don't like air bubbles (the air gets caught under their carapace and they float to the surface where they die).
Daphnia eat very very small organisms such as bacteria, yeast and algae. They don't have teeth so can't chew up anything big. We have used yeast (the stuff used to make bread!) and algae. If you place a bottle of clean water with a few drops of liquid fertiliser (the stuff you feed tomato plants or house plants with is fine) on a sunny windowsill it will start to go green within a few days. The green colour is due to the growth of algae which are so small you can't see individual organisms. Some people put a little bit of sterilised manure or compost. (The compost you buy in bags at the garden centre is usually sterilised).
It is a good idea to keep a bottle of water growing algae alongside the Daphnia and occasionally add a little of the algae water to the Daphnia water. The Daphnia water should be just a little bit green, showing that there's still plenty of food for them. If you have a lot of Daphnia you may need to add algae often. Don't add so much algae that the water goes all murky and dark, the Daphnia don't like lots of gungy stuff in the water.
There is lots of information available on the web, for example:
What do Daphnia look like?
The body is transparent which means the shell (properly known as the carapace) is see-through and not coloured at all. This means you can see all the internal organs such as the heart and the gut. You can also see eggs.
Why do you think the gut is green? What might Daphnia eat? Find the answer in How To Grow Them
Here is a little (2 min) video clip showing the parts of the body. This may take a while to download if you're on a slow connection. It requires the Flash plug-in.
Our Experiment... The details
This is a stereomicroscope used for taking the video clips.
The microscope is connected via a camera to a little box which converts the signal to something that can be viewed either on a TV screen or a computer. The device we used was called WinTVUSB2.0 available from Hauppage http://www.hauppauge.co.uk/ and many electronics shops. This connects via a USB port and cost roughly £50. Other similar products are available and this technology is advancing very rapidly.
When connected to the laptop we could record still images and videos onto the laptop hard disk. It is also possible to display onto the TV screen and record the videos onto a DVD Recorder with a hard disk.
In the schools visits the laptop was connected to the interactive whiteboard to give a beautifully-magnified moving image!
Daphnia move about very quickly so we needed to have some way of keeping in the field of view of the microscope. One way of doing this was to make a very small chamber to keep the Daphnia in place. This was made out of two microscope slides as in the sketch. One microscope slide was cut in half with a glass cutter then glued so as to leave a channel about 2mm wide.
An alternative way of keeping the Daphnia from moving about too much is to use a tiny spot of petroleum jelly applied using a syringe or even just a dab from a cocktail stick.
Moving the Daphnia
We used a plastic pasteur pipette with the end of the tip cut off to suck up the Daphnia and transfer it to a small dish to put under the microscope. You could also use a turkey baster (available from kitchenware shops - it's a small glass tube with a rubber bulb at one end).
To see the heart beat and measure heart rate accurately it is really necessary to cool the Daphnia to between 10 - 15 C. To measure the heart rate we counted the number of beats in a 20 second video clip.
The Experimental Results
We added drugs to the water surrounding the Daphnia and measured the heart rate before and afterwards. We made two or three measurements with the same water flea and same drug and then calculated the average.
Here is a 20 second movie clip of Daphnia with just water. There are two different file formats one of which should work on your computer!
just water as a Flash movie (should run on any browser with the Flash plug-in downloaded)
just water as a mpg movie (3.2 MB mpg file)
Here is the same Daphnia with diet cola added to the surrounding water. We waited 5 min from adding the cola to taking the measurement.
with cola as a Flash movie (should run on any browser with the Flash plug-in downloaded)
with cola as a mpg movie (3.2 MB mpg file)
Here are some of the results.
- Ma Huang is a chinese herbal medicine used for treating colds and asthma. We obtained the herb from Dr Tai-Ping Fan of the Department of Pharmacology and made a solution at a concentration of 1 mg/ml. The treatments were paired so that the same Daphnia was used for just water and then the drug.
- Coca-Cola was diluted one part water to one part coca-cola.
- Red Bull was diluted one part Red Bull plus 2 parts water.
treatment number drug number of beats in 20 seconds average heart rate
1 just water 58 61 61 60
2 Ma Huang 74 71 68 71
3 just water 46 52 49 49
4 Ma Huang 59 59 52 57
5 just water 54 52 . 53
6 Ma Huang 67 66 . 67
7 just water 58 60 61 60
8 Coca-Cola (diet) 63 70 70 67
9 Red Bull 69 66 . 68