Christine Ream


The symbiotic association between the invertebrate phylum Cnidaria (Coelenterate) and the unicellular dinoflagellate algae, called zooxanthellae, is very common. The most well-known relationship is between zooxanthellae and hermatypic, or reef-forming, corals. In this paper, a closer look at the associations between these unicellular algae and different members of the phylum Cnidaria will be presented. During the research of this subject, it was found that hard corals were not the only cnidarians discovered as having a symbiotic relationship with dinoflagellates. Sea Anemones and jellyfish have been found to have symbiotic associations with dinoflagellates. Hydra have a symbiotic association with another type of algae that will be discussed briefly. This covers all three extant classes belonging to the phylum Cnidaria: Anthozoa (corals and sea anemones), Scyphozoa (jellyfish), and Hydrozoa (hydra).


The relationship between cnidarians and dinoflagellate algae is termed as "symbiotic", because both the animal host and the algae are benefiting from the association. It is a mutualistic interaction. For some cnidarian species, it has been studied whether or not they could even survive without the dinoflagellate algae. Before discussing the different advantages between the symbiosis of cnidarians and dinoflagellate algae, a brief overview of dinoflagellate characteristics must first be presented. Dinoflagellate algae make up the division Pyrrophyta. Only about half of the species of dinoflagellates have photosynthetic pigments, the species that do not have photosynthetic pigments are heterotrophs, meaning they are able to use organic compounds in the dark as energy sources (Sze 1993). The symbiotic dinoflagellate algae known as zooxanthellae contain photosynthetic pigments and use sunlight to produce nutrients for both itself and its host. Most dinoflagellates are unicellular flagellates, but there are some colonies of flagellated cells, non-flagellated cells, palmelloid aggregations, and filaments known (123). Zooxanthellae are unicellular flagellates. The chloroplasts of dinoflagellates may be red, green, or blue-green, but are most commonly brown in color. Zooxanthellae are usually a brownish color (Smith 32). Dinoflagellates can reproduce either by division into two daughter cells, or by formation of zoospores (Sze 1993). Most dinoflagellates have two flagella, one that trails in the water; the other is wrapped around the "waist" of the cell like a belt confined to a groove in the plates. The flagella make the dinoflagellate motile, and they can move up to three meters in one day (Milne 139). The majority of dinoflagellates are marine, but there are several hundred freshwater species. Dinoflagellates have been known to cause illnesses due to toxins some species manufacture, and they also cause episodes such as red tides. Ciguatera is an illness that is widespread through the tropics this is acquired by eating raw or cooked fish. The dinoflagellate that causes this is a rare species called Gambierdiscus toxicus. Red tides are caused under some conditions that allow species to grow from one hundred or fewer organisms per milliliter to some million or more and this gives the water a red tint known as a red tide. The toxins involved in red tides, cause widespread kills of fishes and other organisms. The species known to cause red tides are Gymnodinium and Gonyaulax found in the U.S. Atlantic and Pacific, respectively (140). Zooxanthellae, which are the symbionts involved in the interaction with cnidarians (and other phyla, such as Mollusca), are important in many ways to their hosts. Each class of Cnidaria will be discussed separately, and the advantages to each unique symbiotic interaction with zooxanthellae (with the exception of Hydra) will be elaborated upon. The dinoflagellate algae that is found in cnidarians, is from the genus Symbiodinium. This is the genus that is also known as "zooxanthellae" (Day 1994).

Class Anthozoa (Sea anemones and Corals) Sea Anemones

 Both sea anemones and corals are exclusively marine. Sea anemones are known to have symbiotic algae living within their gastrodermal cells. Some sea anemones contain both zooxanthellae and zoochlorellae (single-celled green algae). An example of a sea anemone containing both species is Anthopleura xanthogrammica. The proportion of each symbiont is dependent upon the water temperature. At high temperatures (26 degrees Celsius), zooxanthellae are more abundant, and at low temperatures (12 degrees Celsius), zoochlorellae are more abundant (Ahmadjian 132). At intermediate temperatures, the numbers of each species are close to equal. The zoochlorellae excrete only small amounts of fixed carbon, so when the zoochlorellae predominate, the algae must supply something else to make up for it. So, instead of the higher amount of carbon, they excrete nitrogen and phosphorus to stimulate growth in the sea anemone (132). Sea anemones position themselves in a way as to increase light exposure to their symbionts. When their tentacles are relaxed, the algae lie in a single layer. When the anemone contracts its tentacles, the gastrodermal cells shrink and the algae lie on top of each other. The zooxanthellae benefit by receiving carbon dioxide from the respiration of the host, nutrients such as nitrogen and phosphorus from the hosts metabolism which are then recycled back and forth between the host and the symbiont, and a shelter in which to live. The sea anemone benefits by receiving oxygen and food in the form of glycerol, glucose and alanine from photosynthesis.

Reef Building Corals

 The most abundant cnidarians containing dinoflagellates (zooxanthellae) are the stony corals which make up coral reefs in shallow, tropical waters (Smith 33). All reef-building corals contain symbiotic algae (Ahmadjian 133). Stony corals are similar to sea anemones, but coral polyps are smaller (approx. 10 mm in diameter), and they excrete a calcium carbonate shell around their bodies. As the polyp dies, their shells do not decay and new polyps grow over them. After many years of this process, coral reefs are formed (132). Symbiotic algae (zooxanthellae) live within the digestive cavity of the coral polyp, and coral which have symbiotic algae grow much faster than animals without algae. The algae are known to stimulate calcification through their photosynthetic fixation of CO2 . The reaction rate of the calcification process is increased by the removal of CO2 (133). The algae supply the animal host with oxygen, and carbon and nitrogen compounds. The animal host also obtains vitamins, trace elements, and other essential compounds from the digestion of plankton. Animal waste products are converted by the algae to amino acids, which are transferred to the animal host. Pigments produced by the symbiotic corals protect both the host and the algae from ultraviolet radiation. The algae also receives inorganic materials from its host (Sze 1993). Under stressful conditions, such as high temperatures, coral polyps expel their zooxanthellae. The zooxanthellae aid in giving the reef-building corals their striking colors. When the zooxanthellae are expelled, the corals become white masses of calcium carbonate. The polyps can survive for a few months without zooxanthellae. If favorable conditions return, they collect new zooxanthellae, and return to their normal colors and continue growing (Milne 159). If the temperatures remain too high, the coral polyps cannot recover and die (Castro 379). In regards to the question of whether reef-building corals actually require zooxanthellae to survive, the answer would have to be yes, because the corals would have difficulty producing enough calcium carbonate to form coral reefs. They would also be responsible for getting enough nutrients and oxygen themselves in order to survive. The zooxanthellae are a tremendous benefit to the coral reef production.

Class Scyphozoa Jellyfish

 Jellyfish are also exclusively marine animals. The species of jellyfish known as Cassiopeia xamachana has been used in studies regarding how an invertebrate selects its symbiotic algae. During the life cycle of Cassiopeia, algae is found in the sexual medusoid stage, but not found in the asexual polyp stage. The polyps mature after they have obtained a symbiont (Symbiodinium microadriaticum). The study allowed for the scientists to expose the jellyfish to different types of algae to see which kind are able to colonize with the jellyfish. The scientists found that the animal host was able to recognize its symbiont after it was phagocytized (Ahmadjian 132). Cassiopeia xamachana does not swim freely, but instead lies upside down on the sea floor. This allows the zooxanthellae living in the jellyfishes tentacles to receive maximum daylight for photosynthesis.

Class Hydrozoa Hydra

 Hydra are freshwater animals that are found in ponds, lakes, and slow moving rivers. They feed on small animals, such as water fleas and rotifers, in the water column (Douglas 3). They range in size from 0.25 to 2.5 cm long. they are among the simplest in structure of all multicellular animals. Hydra have a symbiotic relationship with a green algae of the genus Chlorella. This gives some hydra their green color. Other hydra tend to be a brown color and are at a disadvantage when food is scarce. It was found that a green hydra, containing the green algae Chlorella can survive even after two to three months without food as long as it was exposed to light. It was concluded that the sugars derived from algal photosynthesis enable green hydra to tolerate starvation (4). Without light, when the algae cannot photosynthesize, the green hydra have no advantage over the brown hydra. Hydra are able to pass Chlorella onto their offspring when they divide so that every individual in the animal population contains algal cells (Ahmadjian 136). This is possible, because the algal cells become a part of the animal during its feeding routine. The algae are consumed with other food, and just as the food digests so does some of the algae. The algae that escapes digestion, is retained within the vacuoles of the host animal (136). It is briefly mentioned in several articles that other members of the class Hydrozoa actually do have symbiotic relationships with dinoflagellates, but not enough information is available to discuss this association.


The symbiotic association between zooxanthellae and animals included in the phylum Cnidaria is most definitely significant in the subject of symbiosis. The relationship between sea anemones and zooxanthellae has been studied at great lengths, and has proven to be very important to both the animal host and the algae. Even more significant, is the necessity of zooxanthellae in the livelihood of coral reefs. It's easy to assume that without zooxanthellae, there would be no coral reefs to explore, no colorful invertebrates growing upon the reefs, and no beautiful sea anemones to view in the intertidal zones. Zooxanthellae has found the ideal living quarters and has given its' hosts the ability to grow at great rates by producing the nutrients needed during photosynthesis.

Literature Cited

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