This practical investigates how a stimulant changes heart activity in a living organism while developing skills in observation, variable control, repeat measurement, and interpretation of biological data.

Purpose of the practical

The aim is to determine whether caffeine concentration changes heart rate in Daphnia. Daphnia are tiny freshwater crustaceans with transparent bodies, so the beating heart can be seen directly under a microscope. This allows a fast, non-destructive test of a physiological response.

The prediction is usually that caffeine, a stimulant, increases heart rate, but the investigation must test this using measured data rather than assumption.

Why Daphnia are suitable

• transparent body allows direct observation of the heart

• small size makes them easy to handle in a laboratory

• rapid heartbeat produces measurable changes in a short time

Apparatus and setup

Typical apparatus includes:

• living Daphnia in culture water

• cavity slide

• pipette

• microscope

• stopwatch

• caffeine solutions of known concentration

• water for a control

• cotton wool fibers to reduce movement

Transfer one Daphnia in a small drop of culture water onto the cavity slide. Add a few cotton wool fibers so it remains in view without being damaged. Place the slide under low magnification and identify the heart, usually on the dorsal side of the body.

Labeled anatomical view of a Daphnia showing major internal and external structures, including the heart positioned dorsally. Use it as a reference map when you first scan the specimen under low power to orientate the head–thorax–abdomen and then pinpoint the heart region before counting beats. Source

A baseline heart rate should be measured in water first. Then add a known concentration of caffeine solution, wait a fixed time, and count the heart rate again. This can be repeated with several concentrations to see whether there is a pattern.

If the same Daphnia is used more than once, it should be handled consistently between trials. Repeated exposure can influence later readings, so the order of testing matters when results are evaluated.

Suggested method

• place one Daphnia onto the slide in culture water

• observe under the microscope and locate the heart

• count beats for a fixed period, such as 15 or 30 seconds

• add a measured volume of caffeine solution

• wait the same time before counting again

• repeat for a range of concentrations

• carry out repeats and calculate a mean for each concentration

Variables and fair testing

The independent variable is caffeine concentration. The dependent variable is heart rate.

When planning the investigation, other factors must be kept constant.

Important control variables include:

• temperature, because heart rate changes with temperature

• light intensity, especially if the lamp warms the slide

• volume of solution added

• time interval between adding caffeine and counting

• length of the counting period

• the size or age of the Daphnia where possible

Keeping the counting period constant is particularly important because even a small timing error can affect the calculated heart rate.

A control treatment using water with no caffeine is also needed. This shows whether handling, added liquid, or time under the microscope affects heart rate by itself.

Recording and processing data

Because the heart beats quickly, it is practical to count for a short fixed time and convert the result to beats per minute.

Repeats are important because counting can be difficult and living organisms show natural variation. Mean values are more reliable than single readings. Results can be presented as concentrations with mean heart rates, or plotted on a graph to identify a trend.

A graph may show a steady increase, little change, or a less regular pattern at higher concentrations. A valid conclusion should be based on the overall pattern across repeats, not on one unusual result.

Sources of error and improvements

Common limitations include:

• movement of the Daphnia, making the heart hard to follow

• manual counting errors, especially at high heart rates

• warming from the microscope lamp

• natural variation between different Daphnia

• uneven mixing of the caffeine solution around the organism

Possible improvements include:

• using a cool LED light source

• allowing the same acclimation time in every trial

• testing more concentrations

• using several Daphnia and calculating means

• recording the heartbeat on video for slower playback

• repeating any anomalous result

Using the same Daphnia for several concentrations can reduce variation between individuals, but earlier exposures may affect later measurements. Using a new Daphnia each time avoids this carry-over effect, but increases natural variation. This trade-off should be considered when evaluating the method.

If an anomalous result appears, the method should be checked first. Timing errors, poor visibility of the heart, or inconsistent mixing may explain an outlier better than biology alone.

Safety and organism care

Handle Daphnia gently and keep exposure to caffeine as short as possible. Do not trap or crush them under the slide setup, and return active organisms to fresh culture water after observation. Clean apparatus between trials to avoid contamination.

Caffeine solutions should be handled carefully, and spills should be wiped up promptly. Careful technique improves the quality of the data and reduces unnecessary stress to the organism.