MadSci Network: Zoology
Query:

Re: Questions on testing green tea and beta-blockers in daphnia

Date: Mon Feb 12 12:19:50 2007
Posted By: Kerri Finlay, Post-doc/Fellow, biology, University of Quebec at Montreal
Area of science: Zoology
ID: 1170611121.Zo
Message:

1)	Daphnia thrive in moderately sized aquaria (10 to 40 l), so I'd suggest
growing and keeping your Daphnia cultures in a few different aquaria (in
case one crashes, you won't have to start an entirely new culture) until
you're ready to start your experiments.  Once you start your experiments,
however, I'd suggest keeping the animals separate from each other, just
try to keep them as similar as possible and make note of any differences in
container size, or distance to a heat source that may confound your results.
2)	Daphnia are very small organisms and will likely respond very quickly to
your beta blocker and green tea treatments.  I think that two weeks will be
too long for your study, and you should see an effect within a few hours of
treatment (Campbell et al 2004 Comparative Biochemistry and Physiology 139
(2): 225-234) - they noticed an effect of their treatments after 15
minutes, and continued to follow the heart rate of the Daphnia for 2
hours).  Fortunately, because Daphnia respond so quickly, you can easily do
some preliminary studies to determine the amount of time necessary to see
an effect.  I'd suggest checking the heart rate of a few individuals before
adding beta blockers or green tea, then add your treatment, and examine the
animals every 15 min for a few hours after the treatment.  Once you know
the necessary amount of time to see an effect, you can conduct all of your
experiments for this length of time.  Alternatively, you can follow each
individual every 15 minutes for a few hours and plot graphs with Daphnia
heart rate on the y-axis and time on the x-axis.

Your question of the length of time to detect significance is a question of
statistical power (Wikipedia explains this quite well if you want to
explore it in more detail http://en.wikipedia.org/wiki/Power_%28statistics%29).
 I believe you're
more interested in the number of individual Daphnia (known as your "sample
size") to include in your experiments rather than the length of time to
observe them.  You can calculate the sample size required in order to
observe a statistically significant effect, but this requires knowledge
beforehand of the size and range of the effect of the beta-blockers on
Daphnia heart rate.  Instead of getting deep into these statistics, though,
I think you will be fine if you just plan to look at about 20 individuals
that receive the beta-blocker treatment, and 20 that serve as controls. If
the effect of the beta-blockers is large, then you may need far fewer
individuals (the article I referenced above used only 3 individual Daphnia
per treatment).

3) If you keep your Daphnia together in an aquarium until you start your
experiments, you won't need to change the water.  Daphnia tend to thrive in
pond water which has lots of algae and often low oxygen concentrations and
they can live for many months in the same water in aquaria.  Just keep your
aquaria near a light source (on a window ledge, or with a grow light for
algal growth) and keep some oxygen bubbling in the tank.  Since your
experiments will only last a few hours, it won't be necessary to change the
water for this either.  




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