Anatomy of Coral

Part A: Coral's Cousin Hydra

Hydra viridis at 100x magnification. Image courtesy of Wikipedia.

Although it is extremely difficult to obtain and study live coral polyps in the classroom, hydras, freshwater cousins of corals, are readily available. Luckily, there are many similarities between these two animals, so studying hydras can help us understand more about corals. Hydras belong to the phylum Cnidariacnidaria: an animal phylum characterized by stinging cells called nematocysts, which also contains stony (hard) corals, anemones, sea fans, sea pens, hydras, and jellyfish.("nī-DARE-ee-ah"), which comes from the Greek word for "stinging needle". These coral relatives are found in most unpolluted freshwater ponds, lakes and streams in temperate and tropical regions. They are typically only a few millimeters long and are therefore best studied under a microscope.

  1. Using a medicine dropper, carefully place a live hydra culture and a few drops of water into the indentation of a well or depression slide.
  2. Without using a microscope, examine the hydra specimen, taking note of its color, size, shape, and any other distinguishing features.
  3. With your fingernail or the tip of a pen, gently tap the side of the slide. Does the hydra respond?
  4. Now observe the hydra under the microscope at low or medium power. Do NOT exceed 100x magnification to avoid dipping the objective lens into the water and potentially damaging the hydra.
  5. Look through the microscope while another group member gently touches the hydra's tentacles with a pin or straightened paper clip. How does the hydra react? Look closely at the tentacles to see if you can identify small round stinging cells (nematocysts).
  6. Place a daphnia or brine shrimp on the slide near the hydra's mouth and observe what happens.

    Checking In

    • How did the hydra respond to the straight pin?
    • How did the hydra respond to the daphnia or brine shrimp? Describe the feeding behavior of hydras.