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 (Anthozoa). Sea Anemone (Anthozoa) and Jellyfish(Scyphozoa) also belong to the phylum Cnidaria and are hosts to zooxanthellae.

Corals, anemones and jellies are related and all classified in the phylum Cnidaria or “stinging needles.” The organisms in this group are aquatic, possess stinging cells within their tissues and have a body plan characterized by radial symmetry, which allows all parts of their bodies to be equally receptive and responsive to predator and prey. Anthozoa consists of 10 orders and thousands of species. Adults are attached to the seabed, but their larvae are free-floating and can drift to new settlements. Anthozoans can secrete a nonliving substance around the outside of the body to support and protect their soft body tissues. Coloration can vary from red, pink and purple to yellow, blue and orange.


Order Scleractinia

Order Scleractinia, Stony Corals: Stony or reef-building corals form a skeleton made of calcium carbonate under the polyps to create the hard structure that most people recognize as coral. These corals are responsible for forming the base structure of coral reefs. As older polyps die off, new polyps continue to build calcifications on the old skeletons, allowing for the huge scale of reefs in the Caribbean and the Great Barrier Reef. Not all stony corals are reef-building, though, as some are not able to produce enough calcium carbonate to facilitate reef formation.

Subclass Octocorallia

Subclass Octocorallia, Octocorals: Despite sharing a similar appearance with stony corals, soft corals, sea pens, gorgonians and sea fans do not build the hard, calcium-carbonate skeleton of stony corals. Instead, these corals may create some internal structural supports that allow them to grow vertically but still sway with ocean currents. Soft corals are always colonial and grow with eight-fold symmetry, which means their tentacles come in groups of eight — hence the name Octocoral. Octocorals include the orders Alcyonacea and Helioporace.

Order Corallimorpharia

Order Corallimorpharia, Anemone Corals or Mushroom Corals: Members of this order are sometimes called anemone corals or mushroom corals because they resemble anemones more closely than other types of corals due to their large, flat, disc-like shape and short tentacles. They grow like wheel spokes, radiating from a center and forming concentric circles. The diameter of the circle increases as they grow. This order is extremely popular in home aquariums.

Order Zoantharia

Order Zoantharia, Zoanthids: Zoanthids have long, prominent tentacles arranged in two rows. Unlike stony and soft corals, Zoanthids incorporate sand and other substrates into their colonies for structure. They may live as individual polyps or in colonial groups.

Order Actiniaria

Order Actiniaria, Anemones: Larger anemones tend to be solitary while smaller species may use asexual reproduction to propagate and live in large concentrations when there is suitable habitat. Anemones come in a wide range of colors, some owing their coloration to the zooxanthellae, microscopic algae, they host. They also have a disc-shaped bottom they use to attach themselves to rocks, in crevices and on other suitable surfaces, including the shells of other marine invertebrates.

Subclass Ceriantharia

Subclass Ceriantharia, Tube-dwelling Anemones: This subclass looks similar to sea anemones, but tube-dwelling anemones are known for being solitary and living buried in soft sediments. They live inside tubes made of secreted mucus and organelles, and can recede into these tubes for protection. Ceriantharia includes the orders Spirularia and Penicillaria.

Algae + Coral

The most abundant cnidarians containing dinoflagellates (zooxanthellae) are the stony corals which make up coral reefs in shallow, tropical waters (all reef building corals contain symbiotic algae). 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. Symbiotic algae (zooxanthellae) live within the digestive cavity of the coral polyp, and coral which have the 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.

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. 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.

(Stony coral)

Algae + Anemone

Sea anemones are known to have symbiotic algae living within their gastrodermal cells. They 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. At intermediate temperatures, the numbers of each species are close to equal.

The zoochlorellae excrete only small amounts of fixed carbon, so when they predominate, the algae must supply something else to make up for it. Therefore, instead of the higher amount of carbon, they excrete nitrogen and phosphorus to stimulate growth in the sea anemone. 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 they contract their 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.

Actinia tenebrosa
(Sea anemone)

Algae + 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. They found that the animal host was able to recognize its symbiont after it was phagocytized. Cassiopeia xamachana does not swim freely, but instead lies upside down on the seafloor. This allows the zooxanthellae living in the jellyfish tentacles to receive maximum daylight for photosynthesis.

Cassiopeia xamachana
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