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IVA 22. What Are Zooxanthellae?
Background
Corals belong to the phylum Cnidaria, an exclusively aquatic division of animals that also includes
anemones and jellies. This group is radially symmetrical (round) and hollow, with one body opening for
the mouth. Surrounding the mouth are tentacles armed with tiny stinging capsules called nematocysts. All
cnidarians are carnivores, and the tentacles and nematocysts are used to capture prey.
The body and tentacles consist of two cell layers that function as an inner and outer skin: the endoderm
forms the lining of the gut and the ectoderm forms the outer covering. Between the two lies a noncellular
substance called mesoglea, which is best developed in the jellies.
Also characterizing this group is the presence of symbiotic organisms called zooxanthellae. Many cnidarians
have zooxanthellae, but they are almost always present in anemones and corals. Zooxanthellae are tiny,
one-celled plants classified as dinoflagellates. They characteristically have two hairlike structures called
flagella. Like all plants, they convert nutrients into food using the sun’s energy. Because they need sunlight
to survive, they are found in the skin or surface areas of their hosts. Zooxanthellae live in the tissues of a
number of animals, including sea anemones, gorgonians (also known as sea fans), giant clams, and many
nudibranchs (sea slugs). But they are best known for their relationships with corals.
It is this complex relationship that has captured scientists’ attention as they try to understand the process of
bleaching. Bleaching and reef deterioration are widespread in the tropics. When reefs die, islands and their
human inhabitants are left exposed to inundation by salt water during storms. This pressing issue requires
the analysis of the bleaching process and focuses on the role that zooxanthellae play.
Focus Question
What are zooxanthellae and where do they live?
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Learning Objective
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Students will demonstrate an understanding of the shared characteristics of Cnidarians, i.e., presence of live
zooxanthellae and nematocysts.
Materials
Anemones from a biological supplier
Microscopes or video microscopy setup
Slides and cover slips
Eye dropper
Vinegar
This activity works well using the giant green anemone found on the Oregon coast or the anemone
Aiptasia sp., or green hydra, that are available through biological supply houses such as Carolina Biological
Supply. Green anemones are difficult to collect and regulations apply to their collection.
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Teaching Time
20 minutes
Procedure
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1. Clip a small bit of tissue from the tip of the sea anemone tentacle. Place on a glass slide with a
drop of water and cover with a coverslip.
2. Tap the cover slip until the tissues are crushed and release the zooxanthellae. Spread the body
fluids of the anemone over the surface of the slide. Place under a microscope and look for round
cells of the zooxanthellae.
3. Look for tiny rods in the tissues. These are unfired nematocysts (stinging cells common to
cnidarians). In the Pacific giant green anemone, Anthopleura xanthogrammica, the zooxanthellae
are tiny bright green orbs.
4. Clip another bit of tissue from the tip of the anemone tentacle and place on a glass slide and
position a cover slip over the tissue. Place under the microscope and observe the tissue.
5. Drip several drops of vinegar onto the glass slide and watch as the acid seeps under the cover
slip to the tentacle and the nematocysts are fired. Most of the tentacles will eject tiny hairs (the
nematocysts). The tip is an especially good place to look. It will look like a bad hair day on the
tentacle.
What Are Zooxanthellae?
Student Work Sheet
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Questions
1. What functions are performed by the nematocysts?
2. What animals with nematocysts are harmful to people?
3. What are zooxanthellae and why are they important to corals?
4. Why are scientists interested in investigating the relationship between corals and zooxanthellae?
What are Zooxanthellae?
Teacher Answer Key
Questions
1. What functions are performed by the nematocysts?
Protection, stunning prey for food.
2. What animals with nematocysts are harmful to people?
Box jellyfish of Australia can kill people; fire coral in the Caribbean can give skin divers a nasty rash;
Portuguese man-of-war can give painful stings
3. What are zooxanthellae and why are they important to corals?
They are small, one-celled organisms with two tails that can produce food. The excess food zooxanthellae produce
is an important food source for coral.
4. Why are scientists interested in investigating the relationship between corals and zooxanthellae?
Coral bleaching occurs when corals expel their symbiotic zooxanthellae. Many islands in tropical zones are
dependent on coral reefs for protection against storm surges. Unlocking the mechanism of bleaching may provide
clues to control it.
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References and Further Reading
Santos, S. and NOAA. Photos of zooxanthellae. http://www.coral.noaa.gov/themes/zoox_pics.html.
Accessed July 2006.