### Re: How do I calculate the average body fluid of a daphnia?

Date: Sun Sep 18 03:29:53 2005
Posted By: Rob Campbell, Postdoctoral researcher, Biological Oceanography
Area of science: Zoology
ID: 1127034612.Zo
Message:

Hi Laura:

The way to tell how much water is in an organism is to weigh it while it is "wet", then take all the water out (by putting in in an oven for a while and allowing it to dry out), and then weighing it "dry". The difference between the "wet" and "dry" weights equals the amount of water that was lost.

I had a look to see if anyone has measured these things in Daphnia, and I found this paper (warning: it's a pdf and may take a while to download). It has some data for Daphnia magna, one of the larger and more common Daphnia species. They measured wet and dry weights for a number of treatments, and I just took the wet and dry weights from the "control" group in figure 4. Here is the data:

 Time Wet Weight Dry Weight Water Weight Percent Water 5 days 1.36 mg 0.09 mg 1.27 mg 93.60% 8 days 1.82 mg 0.10 mg 1.72 mg 94.55% 11 days 2.52 mg 0.11 mg 2.41 mg 95.51% 15 days 3.40 mg 0.18 mg 3.23 mg 94.83%

The wet and dry weights I took directly from the figure (they are the triangles in the bottom two panels). The water weight is the weight of the water that was removed by drying (wet weight minus dry weight), and I have also calculated that as a percentage of wet weight that was water (i.e. water weight divided by wet weight, times one hundred). You can see that Daphnia magna are mostly water.

Obviously this technique does not work with humans (most people are not going to be willing to be dried out so a scientist can measure their water content!). In humans a more indirect approach is used, estimating the water content of the different parts of the body (and it usually comes out to something like 60% to 70% water). You can read more about that here.