Monday, July 11, 2011


Hydra are predatory animals that belong to the phylum Cnidaria (class: Hydrozoa). They can be found in most unpolluted fresh-water ponds, lakes and streams. The easiest way to collect hydra is by using a net to gather a water sample near weeds growing on the edge of the pond.

Typically a few millimeters in length, hydra are often studied extensively by biologists because of their ability to regenerate. Hydras have a tubular body, as shown above.

Hydra tentacles (also called cnidae) are covered in stinging cells called cnidocytes. These are used when contacting prey to discharge a dart-like thread containing neurotoxins into the prey. (They are too small to injure humans). Hydra do not have a brain or true muscles, and they mainly feed on small aquatic invertebrates such as Daphnia and Cyclops.

If you have access to a microscope this summer, here is a fun project.
  1. Gather some fresh pond water (if you have access to salt water, gather some of that as well!)
  2. Make sure you label your jars of water if you collect pond water from multiple locations.
  3. Place a small drop of pond water on a depression slide and place a cover slip on top.
  4. Using a student microscope, first focus at 40x, then move up to 100x and finally 400x. You will be able to view most hydra at lower magnifications. At the higher magnifications, you may see some bacteria. 
  5. If you are unable to view any samples with your first slide, simply clean the slide and place a new drop of water on the slide and try again. Sometimes it takes a few tries before you get a good sample.
  6. Take note of the images you see. Can you identify the organisms? If you are unsure of what you are viewing draw a picture of the sample and research it, or ask your science teacher to help you identify it.