HydraVs Daphnia


Christine Quach and Nic Miles


CU Boulder, Fall 2006


Hydra eating behaviors may change in different temperatures. Hydra is a 4-30 mm long multicellular organism, belonging to the Hydridae family.  Hydra paralyze their food with nematocysts that extend from their mouth. Hydra are hermaphroditic and have the ability to reproduce both asexually and sexually. They are filter feeders that prey on small arthropods called daphnia, also known as water fleas. Daphnia range between 0.2-5 mm in size and belong to the family Daphniiade. Daphnia reproduce parthenogenetically,  resulting in hatchlings that will moult several times before considered adults. Given favorable environmental conditions, females may do this every ten days. Daphnia are a food source for not only hydra but for a variety of  various fish and dragon flies. It is well known that metabolism rates are correlated with temperature.  Since hydra has been observed to consume daphnia and metabolism is temperature-dependent, I predict that hydra will consume daphnia faster in warm water than in cold water.

         Several hydras were placed in a small watch glass at room temperature (23 degrees Celsius) with daphnia, and timed how long it took the hydra to capture and subdue the daphnia. This procedure was repeated five times and once for our experimental control. The hydra was then placed in a watch glass within a petri dish of warm water set at 43 degrees Celsius. Once the water reached 43 degrees Celsius, the daphnia and hydra were introduced. The time it took for hydra to capture and subdue the daphnia was recorded. This protocol was repeated in cold water set at 4 degrees Celsius.

         The results that we obtained from five trials at room temperature (23íC) were: 3:48.63 minutes 3:57.83 minutes, 4:10.46 minutes, 3:53.74 minutes, and 3:32.38 minutes. At 43íC we obtained: 1:18.44 minutes, 1:27.68 minutes, 1:06.21 minutes, 1:36.02 minutes, and 1:11.39 minutes. At 4íC we obtained: 14:02.57 minutes, 13:48.09 minutes, 13:53.46 minutes, 13:39.04, and 13:43.93 minutes. The mean time to capture and subdue at room temperature was 3.61 minutes, for warm temperature, 1.20 minutes, and for cold temperature it was 13.57 minutes.  We ran a statistical linear regression. The line of best fit was y = -0.3186x + 13.77 with an R^2 value of 0.8863.  This statistical test is the most accurate for our experiment.

         As predicted, the Hydra captured and subdued the daphnia faster in warm water than in cold water. In warm water the hydra captured and subdued the daphnia at an average time of 1.20 minutes and in cold temperature it was 13.57 minutes. The difference between the times was 12.37 minutes.  Potential problems that were accounted for in this experiment were the hydra were small in size compared to the daphnia. This made it difficult for the hydra to eat the daphnia. To account for this, the time it took for hydra to capture and subdue daphnia was recorded as an indicator of intended consumption. Also, there were problems with maintaining temperatures throughout the experiment. During the time it took to capture and subdue in cold water, the ice melted, causing the temperature to rise and the hydra bodies to warm. Also, during the warm water treatments, the warm water was surrounded by room temperature, causing the hydra to cool down.

         A modified hypothesis might be to place the hydra in a different pH level to see how acidity or alkalinity may change their eating habit and what pH level they optimally feed  in. The new idea is opposite with the results of this experiment in that when the pH level is lower at 6, the hydra react faster than when the pH is higher at 7 or 8. In this experiment the higher the temperature, the faster the hydra react and the lower the temperature the slower the hydra react.