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Mutualism hot buttons for symbiosis topics in BCCR hot button for commensalism part of B hot button for symbiosis quizzes part of BCCR hot button for mutualism part of BCCR hot button for parasitism part of BCCR
This part of symbioses deals with mutualism, while commensalism and parasitism are accessible via the icons. After viewing these 3 topics, you can test your knowlege by taking the symbiosis quizzes.
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a decorator crab on a gorgonian

"Mutualism is a type of symbiosis where both partners benefit. The decorator crab gains camouflage, while the hydroids have a clean place to live. I can't begin to think what advantage there may be in an orange-icing sponge living with a coral. Perhaps physical support for the sponge in return for protecting the coral." - Turneffe Island, Belize 2000. Video courtesy Andy Stockbridge, Belize.

NOTE Podochela sp.

  Mutualism is a symbiotic relationship where both partners benefit. One of the most important mutualisms to the economy of a reef is the harbouring of photosynthetic plant cells known as zooxanthellae within their gut tissues by a variety of cnidarians, including corals, gorgonians,sea anemones, and others. This topic is considered here, while related topics of OTHER TYPES OF MUTUALISMS and CLEANERS & CLEANING STATIONS are found in their own sections.

Zooxanthella-type mutualisms


photograph of a reef in Bonaire
As noted, a mutualism fundamental to the reef's economy exists between corals and single-celled symbionts known as zooxanthellae. While corals do feed by prey-capture and, in some, by collection of organic particles on mucous sheets, they gain most of their nutrients and energy via photosynthetic activities of their symbionts.

NOTE zooxanthellae (the singular is zooxanthella, pronounced "zoo-zanth-ellah") are single-celled photosynthetic organisms related to dinoflagellate protists. Mutualistic varieties of zooxanthellae are motile outside of their hosts, but lose their swimming ability when enclosed in vacuoles (membrane-bound spaces) in the cells of their host's gut tissues



All the organisms in this photograph save for the fishes rely for
at least part of their nutrition on being in the light. Even the
sponges have photosynthetic symbionts (bacteria) in their tissues

schematic showing location of zooxanthella symbionts in a coral

photograph of coral symbiont Symbiodinium microadriaticum courtesy Max Taylor
A common type of zooxanthellae found in corals is Symbiodinium microadriaticum, the same species thought to associate with giant clams. Photograph courtesy Max Taylor, University of British Columbia.





Symbionts Symbiodinium microadriaticum in the
gastrodermis of a cnidarian 1000X. Note that
several of the cells are in the process of dividing


Given that zooxanthellae require the same, or similar, nutrients as other plants, what do you think is the source of these nutrients? One answer will already be obvious, but at least one other may be unexpected. CLICK HERE for explanations. Ideas from Sebens 1994 Amer Zool 34: 115.

From seawater, including that contained within the host's gut cavity.

From the host's tissues.

From digestion of food particles within vacuoles within the gastrodermal cells.

From the host's digestive wastes.


photograph of brown tube-sponge
Many other reef organisms have photosynthesising symbionts. These will be considered more fully elsewhere in BCCR, but some examples are...sponges, most or all of which photosynthesising bacteria in their cells...






Brown tube-sponge
Ageles conifera 0.33X


photograph of giant sea anemone...sea anemones, which have zooxanthellae symbionts similar or identical to those found in corals...






Sea anemone Condylactis gigantea 1X


photograph of gorgonians...most gorgonians also have symbiotic zoothanthellae.





Sea-rod gorgonian
Plexaurella sp. 0.05X


Zooxanthellae benefit their hosts by providing them with photosynthates (glycerol, sugars, and other nutrients) and, in corals, also aid in calcification of the skeleton, while the only major detriment may be that the hosts are restricted to living in the relatively shallow photic zone. Ideas from Muller-Parker & D'Elia 1997 In, Life and death of coral reefs (Birkeland, ed.). Chapman & Hall, Inc.

NOTE the carbon dioxide produced by the zooxanthellae during night-time respiration is combined with calcium from seawater to form calcium carbonate, the primary constituent of a coral's skeleton

NOTE the depth at which light still affects biological systems. In coral-reef areas this is about 50m for most photosynthetic processes

But, what are the BENEFITS and COSTS to the zooxanthellae? Study the list below and mentally separate the entries, then CLICK HERE. Most are obvious, but the exercise should help clarify the relationships between zooxanthellae and their hosts.

answer box for quiz on cost/benefit for zooxanthellae answer box for quiz on cost/benefit for zooxanthellae
slower growth carbon dioxide & other nutrients provided live in high density risk of expulsion uniform living conditions photosynthates used by host protected from UV protected from grazers maintained in photic zone

photo collage of zooxanthellae
Once thought to be a single species inhabiting a variety of organisms including corals, sea anemones, and giant clams, the dinoflagellate Symbiodinium microadriaticum is now thought to be several related species. Blank & Trench 1985 Proc 5th Int Coral Reef Symp Vol 6: 113.

Within the host the zooxanthellae are non-motile and are contained within vacuoles in the gastrodermal cells lining the digestive cavity. When the symbionts die they are digested and their nutrients absorbed by the host. Sometimes, possibly as part of their normal life cycle, the symbionts are freed from their host, grow flagella, and swim about until they locate another host possibly by following its scent upstream. Photographs courtesy Max Taylor, University of British Columbia.

NOTE sing. "flagellum", is a whip-like tail, as possessed by spermatozoa, used for propulsion


photograph of Coral bleaching is caused by zooxanthellae dying or leaving their hosts in large numbers. Loss of photosynthetic pigments causes the coral's tissues to blanch. Loss of zooxanthellae in this way may lead to death of the polyps and ultimately to death of the coral colony. More on this topic can be found in BCCR at NUTRITION/CORALS/PHOTOSYNTHESIS/BLEACHING.





Bleached plate coral Agaricia sp. 0.67X
Note the invasion of algae onto the
affected parts of the coral

  How do the zooxanthellae get established in a coral? They can do so both via the egg stage and the adult stage. Eggs in corals are produced in the gastrodermal layer of the digestive cavity and are matured there. Depending upon the type of coral, the symbionts gain entry into the eggs either from the parent polyp or from seawater entering via the mouth. If the latter, the zooxanthellae move into the eggs through a cellular lining around the eggs known as a follicle layer.
drawing 1 in a series of 3 showing stages in entry of zooxanthellae into a coral egg drawing 1 in a series of 3 showing stages in entry of zooxanthellae into a coral egg drawing 1 in a series of 3 showing stages in entry of zooxanthellae into a coral egg
In one mode of entry, free-swimming zooxanthellae are carried into the digestive cavity in seawater The zooxanthellae, many of them, move to the eggs that are attached to the gastrodermal layer of the body wall The zooxanthellae impinge on the follicle layer that surrounds the egg
  So many zooxanthellae may be transfered in this way that the egg can become almost filled with them. In a process not fully understood, the eggs are engulfed by individual cells of the follicle layer and move through it towards the egg membrane. By this time the flagellum has been lost. Once through the egg membrane the zooxanthellae take up residence in the egg. During cleavage divisions of early development, the zooxanthellae are partitioned between the cells of the middle part of the embryo, those destined to form the lining of the digestive cavity of the adult. Diagrams modified from Hirose et al. 2001 Coral reefs 20: 273.
drawing 1 in a series of 6 showing actual entry of zooxanthella into an egg of a coral drawing 1 in a series of 6 showing actual entry of zooxanthella into an egg of a coral drawing 1 in a series of 6 showing actual entry of zooxanthella into an egg of a coral drawing 1 in a series of 6 showing actual entry of zooxanthella into an egg of a coral drawing 1 in a series of 6 showing actual entry of zooxanthella into an egg of a coral drawing 1 in a series of 6 showing actual entry of zooxanthella into an egg of a coral
A zooxanthella approaches.. engulfed by follicle cell... ...passes through the cell... ..crosses follicle membrane... ...meets resistance... ...and transits into egg.
  drawings showing production of a zooxanthellate larva
The mature eggs containing the symbionts are then spawned from the mouth, and begin cell division in the plankton. With a day or two the egg hatches to a larval stage known as a planula and then swims freely for a time before settling. Note that the many zooxanthellae are now partitioned into cells in the middle part of the planula - cells that will later form the gastrodermis or lining of the digestive cavity.
  drawings showing different dispersal potentials of zooxanthellate and non-zooxanthellate larvae of coralsWhile virtually all reef-building corals have zooxanthellae, and thus have "zooxanthellate" larvae, there are a few exceptions. These latter have "non-zooxanthellate" larvae, with a shorter pelagic life than the others. The difference is in the provision to the zooxanthellate types of extra nutrients and energy, permitting longer swimming duration and resulting in greater potential for distribution - well beyond the range of non-zooxanthellate species. Richmond 1981 Proc 4th Int Coral Reef Symp Vol. 2: 153.

photograph of coral TubastraeaAs noted above, not all corals have zooxanthellae. Such forms may rely mainly on capture of particulate food for sustenance. Studies in Australia show that non-zooxanthellate species such as Tubastraea micrantha must capture almost twice as much particulate food as comparable zooxanthellate species to achieve similar growth rates. Wilkinson et al. 1988 Proc 6th Int Coral Reef Symp Vol. 3: 27.





Shown here, the closely related Caribbean
species Tubastraea coccinea, which
also may lack zooxanthellae 0.8X


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