During the pilot project of Summer 2001, students attempted to look at the effects of changes in salinity (the concentration of salts in the water)
and temperature on the heart rate of the striped shore crab, Pachygrapsus. Students began by measuring the resting heart rate of crabs in plastic containers of natural seawater. Then they
began to change either the salinity or the temperature. To change the salinity, students mixed seawater with more salt to increase the salinity, or mixed seawater with freshwater to decrease salinity. The crabs were then transferred to different containers with the new salinity mixture at regular intervals. To change the temperature, students put the plastic container of seawater with the crabs into a larger container of water that was either heated with a heating plate or cooled on a bed of ice. This method of using a larger contained with water to heat or cool the original container is called a "water bath" and is a good way to change the temperate gradually and safely.
The students recorded the heart rate of both their "experimental" group of crabs - those that experienced a change in either salinity or temperature, and their
"control" group of crabs - those that remained in the original natural seawater at a stable temperature for the entire experiment. The reason we use both
"experimental" and "control" groups during the experiments is so that we can compare the reaction of the experimental crabs with the control crabs to see if the changes that we made (salinity or temperature) had any affect on the heart rate. It is also important to change only one parameter during an experiment, either salinity or temperature, but not both. If you were to change both the salinity and temperature at the same time, you would not know which one had an affect on the heart rate.
Students preparing the reports
Read the students' reports from their experiments