More species that have symbiotic relationships with algae. Click here to read the full article or an excerpt below.



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).


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