Re: Why and how does sugar increase a daphnia's heart rate?

Dear Georgia and Theresa, 

The sugar that you gave your Daphnia is a simple carbohydrate. Just like 
humans Daphnia use carbohydrates for energy. The sugar acted as a 
stimulant, it is like you eating a chocolate bar as a 'pick me up'.

 As a whole sugar cube is a huge amount of energy for the daphnia to take 
at one time, it could be that the heart has to work hard to keep up with 
oxygen supply needed for the activity caused by the energy. 

It could be however that the sugar causes an increase in heart rate because 
 the process of digesting all the sugar reqires increased work by the 
heart.

Hope some of this is useful to you