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Getting Up-To-Date on Zooxanthellae

Getting Up-To-Date on Zooxanthellae

By Eric Borneman


Coral reefs are largely the product of the accretions of masses of tiny polyps called corals. The success of corals in exploiting the harsh environment of clear tropical oceans is largely attributed to their symbiotic union of a heterotrophic animals with an autotrophic plant. These symbiotic corals form the vast majority of the hermatypic, or reef building, corals. The polyps of symbiotic corals house large numbers of tiny algae within their tissues which contribute to the energy requirements of the coral through the photosynthesis of the algae. Excess algae can also be used as a direct food source upon voluntary digestion by the animal. In return, the algae is able to reside in the protection of coral tissue while absorbing nutrients released by its host animal. The success of this symbiosis is one of the major reasons for the existence of coral reefs.

The Discovery and Past Assumptions of Symbiosis

Brandt first discovered the existence of microalgae within the cells of corals in 1881. The name Zooxanthella nutricula was proposed as a name for the algal inhabitants. Later, the names were changed, when Hovasse identified the algae as dinoflagellates in 1922. However, Hovasse believed them to be parasitic to the coral, and named them Endodinium sp. Later still, Kawaguti recognized they were not parasitic, and proposed they be placed into a new category of Gymnodinium sp. Finally, Freudenthal placed them back into a parasitic category, calling them Symbiodinium sp. It was the identification of the symbiotic algae from the jellyfish, Cassiopeia sp., as Symbiodinium microadriaticum that started an assumption for more than twenty years that all zooxanthellae belonged to this single category based on general similarities between species, both coral and non-coral. S. microadriaticum was alternately referred to as Gymnodinium microadriaticum in the literature, with the more loose term, zooxanthellae, being applied frequently to describe the symbiotic golden dinoflagellates of various marine animals. A new category, Zooxanthella microadriatica was also proposed in the late 1970’s.

Nomenclature aside, the assumptions that were made, and continue to be made outside the much of the scientific community, is that all corals possess a single species of zooxanthellae. Despite the early work of Robert Trench in the 1970’s, there was a reluctance to accept the fact that corals harbored more than one type of symbiotic algae. At one point, it was proposed that every coral harbored a separate specific type of zooxanthellae, though this was quickly shown to be untrue. However, the amount of study now being done on zooxanthellae has led, in the past five to ten years, to conclusions that still, to this time, are mostly unknown by aquarists. The implications of such study and findings can play a very important role in the husbandry of corals in the aquarium.

Current Knowledge of Zooxanthellae

During the 1970’s and 1980’s, it became increasingly apparent that there were at least several species of zooxanthellae that inhabited various marine animals, although it was still largely assumed that only one inhabited corals. Over time, the recognition of, perhaps, “strains” of the species might inhabit different corals. This was especially evident in corals which were found in various light regimes, and those which were of different species. Soon, increasingly sophisticated research techniques proved the existence of at least two other species of zooxanthellae in corals; S. pilosum in zoanthids, and S. kawaguti in Montipora verrucosa. About this time, it became more widely recognized that there were various strains, but the degree of variability had yet to be elucidated. As more data came in, the degree of genetic disparity between the different zooxanthellae inhabiting different corals in the same location and the same corals in different locations became obvious. Because there is still little known about the reproduction of these algae outside the host environment or a laboratory setting, traditional species concepts in terms of reproductive fitness are strained. It is difficult to interpret degrees of genotypic variation in terms of when species are truly separate. Even so, to date, more than eighty “strains” have been isolated, with true species belonging to eight genera and 4-5 orders. Three large groups, or clades, of zooxanthellae have been assigned, Types A, B, and C, in the scleractinia. Anywhere from 2-7 clades have been identified within regions thus far. Atlantic stony corals seem to be able to draw from any of the clades, and seem to harbor a larger diversity of zooxanthellae than Pacific corals. The Pacific corals studied thus far have only been found to draw from clade Type C.

Some Zooxanthellae and Cnidarian Associations
(Trench 1997)
Symbiodinium sp. Oculina divisa
Symbiodinium sp. Millepora dichotoma (high light)
S. microadriaticum Cassiopeia xamachana
S. goreauii Ragactis lucida
S. kawaguti Montipora verrucosa
S. pilosum Zoanthus sociatus
S. meandrinae Meandrina meandrites
S. pulchorum Aiptasia pulchella
S. cariborum Condylactus gigantea
S. bermudense Aiptasia pallida
S. californicum Anthopleura elegantissima
Symbiodinium sp. Acropora cervicornis (high light)
Symbiodinium sp. Acropora cervicornis (low light)
Gleodinium visucm Millepora dichotoma (low light)
Clades and Corals (Baker, Rowan 1997)
Type A Type B Type C
Montastrea cavernosa
Montastea annularis
Montastrea faveolata
Porites asteroides
Porites furcata
Porites divaricata
Porites porites

Siderastrea siderea
Acropora cervicornis

Diploria labyrinthiformis

Diploria clivosa
Mycetophyllia ferox
Leptoseris cucullata
Isophyllastria rigida
Agaricia agaracites
Agaricia tenuifolia
Agaricia lamarcki
Agaricia danae
Stephanocoenia michelinii

Montastrea annularis
Montastrea faveolata

Diploria labyrinthiformis
Diploria strigosa

Colpophyllia natans
Eusmilia fastigiata
Meandrina meandrites
Favia fragum
Montastrea annularis
Montastrea faveolata
Porites asteroides
Porites colonensis

Acropora cervicornis
Acropora palmata

Madracis mirabilis
Madracis decactis


Pocillopora damicornis
Pocillopora elegans
Pocillopora eydouxi
Psammocora stellata
Psammocora superficialis
Gardinoseris planulata
Porites panamensis
Porites lobata
Pavona gigantea
Pavona clavus
Pavona varians

There are currently 5 other unidentified clades (V,W,X,Y & Z) isolated from octocorals by Coffroth . In the Pacific, octocorals are dominated by clade X, and in the Atlantic gorgonians by clade B. Coffroth’s research group is finding no patterns of clade zonation with depth in Octocorals. They find both strong geographic patterns (e.g., Clade B dominating the Caribbean & Clade X dominating the Pacific), and also some strong taxonomic patterns (e.g., Alcyoniidae dominated by Clade X; Plexuriidae & Gorgoniidae dominated by Clade B, regardless of geographic zone of origin). This is even more interesting because it is completely different than the pattern showing up in the scleractinians (Tonnen pers comm.).

It is now well established that various species of corals can harbor more than one type of zooxanthellae. Some corals of the same species can harbor different species of zooxanthellae according to the depth or conditions in which they are found. At least some corals are even capable of harboring more than one species of zooxanthellae within the same coral colony. The degree of specificity, number of species, subspecies, strains, and variability has yet to be completely understood, although it may well be that the variation between corals and species of zooxanthellae is quite large.

The Implications of Current Knowledge to Corals

What are the implications of such information to corals in the wild? Each strain of zooxanthellae seems to be somewhat more adaptive to various conditions of light, temperature, environment, and coral species. As such, corals can maximize the success of their symbiosis by containing a species of zooxanthellae that is most conducive to maximizing photosynthetic production. Zooxanthellae populations and densities are largely controllable by the coral animals. They not only regulate the amount of waste material they provide to the algae for their growth, but also can increase or decrease the levels of photosynthesis in other ways. The degree to which they expand their polyps is a control over the amount of zooxanthellae exposed to light. Furthermore, corals can expel zooxanthellae if the rate of photosynthetic production is too high. This process is known as bleaching, and was proposed to be an adaptive mechanism to environmental change over five years ago by Buddemeir and Fautin (1993), as opposed to an exclusively harmful process that is undergone by corals under stress. Finally, corals are able to control the amount and types of photosynthetic products that are released to them from the algae by the use of host release factors. At least a few of such factors have been discovered, though the exact mechanism of product release is not yet fully understood.

It has also been theorized that corals which are able to choose or harbor multiple species of zooxanthellae will also be more likely to survive bleaching events, and that they will be better able to photoacclimate in response to long and short term changes in light, temperature, and other related environmental parameters. As evidence, the more frequent but less severe bleaching episodes in the Caribbean have been contrasted to the less frequent Pacific bleaching episodes that generally have a higher rate of coral mortality (Baker and Rowan 1997). In summary, it appears that the ability of corals to harbor multiple species and strains of zooxanthellae is not only much more specific and variable than had previously been assumed, but also that the ability is largely one of superior adaptation that ensures their success and survival.

A Brief Description of the Symbiosis

The importance of zooxanthellae in providing some of the energy needs of symbiotic corals can not be overstated. Shallow water corals have been shown to receive well in excess of 100% of their daily carbon needs through photosynthesis. There are many products that the zooxanthellae make and package for the corals, including glycerol and various amino acids. Even shade adapted corals or low light corals can modify various aspects of the zooxanthellae and the photosynthetic opportunity. While such corals, including shallow water corals, are never fully autotrophic, the input from zooxanthellae is still significant. Correspondingly lower levels of light force corals to gain more energy from feeding and absorption of organic and inorganic nutrients, and these inputs can vary widely depending on the nutritional status of the coral, variations in the light field, and the availability of external nutrients.

How do the dinoflagellates get inside coral tissue? In corals which brood their planulae, some of the parent coral’s zooxanthellae may be incorporated as a “starter culture” for the young polyp once released. Corals which are broadcast spawners, releasing sperm and/or eggs into the water for external fertilization, must incorporate free living dinoflagellates from the water column. Despite a depauperate body of evidence addressing the abundance and uptake of zooxanthellae from the water column, it is apparent that newly formed planula get to pick from an available pool prior to settling. The uptake of the algae into coral cells usually occurs by ingestion, with a cellular membrane forming around the algae. This intracellular “cage”, known as a vacuole, is special for several reasons. Material that is ingested and which has a vacuole formed around it is almost invariably digested. Yet, the coral not only doesn’t digest the algae, an unknown mechanism moves the vacuole enclosed algae within the tissue to its final residing place. Zooxanthellae are generally found in the gastroderm of corals, although some amounts are also found in the epidermis and tentacles of corals. High densities usually are found in the polyp’s gastric cavity and in areas easily exposed to light. The density of zooxanthellae can be extraordinary in coral tissue, with densities generally varying from 1,000,000 to 5,000,000 algal cells per square centimeter of stony coral tissue. Some soft corals, such as Xenia spp., Sarcophyton spp., and Litophyton sp. can have densities that are significantly higher.

What prevents the coral from digesting the algae? It has been postulated that this symbiosis is one of the key aspects to early evolvement of immunity in multicellular animals. There appears to be cell recognition events that allow the coral to both recognize the algae as “not-self” and also as “not a problem.” There may be both physical and chemical signals that determine whether or not a given type of algae is considered “acceptable” by a coral, with some corals considering several species as acceptable symbionts. There is clearly a fairly high degree of specificity involved. Furthermore, when zooxanthellae are expelled, the very same vacuole that has been treated so specially inside the coral becomes a modified secretory vacuole, and the algae are released, usually completely intact, into the environment. The process by which this is accomplished is not yet fully understood. Finally, corals may occasionally opt to digest zooxanthellae for their nutritional value. At such times, this vacuole is then specifically made to release the contents of the zooxanthellae into the gastric cavity where, contrary to its original uptake, is no longer bound safely in a membrane but is digested like an ordinary food item.

What it Means to Aquarists

What are the implications of all this information to someone keeping corals and other symbiotic organisms in aquaria. Practically, knowing that certain corals are potentially able to be more photoadaptive allows one to better ascertain their requirements in captivity. Corals known to possess a single strain of zooxanthellae and which are found in high light environments will not be suitable candidates or a tank that cannot provide a certain amount of light. The same can be said for those corals that are adapted to various nutrient levels, etc. Furthermore, it helps to explain some of the adaptive changes commonly witnessed upon placement of a coral into a new environment. It has long been assumed that changes in coral color and morphology were due almost entirely to light, water flow, and nutrient levels in a tank. To a large degree, this is still the case. However, some cases of bleaching or adjustment may, in fact, be due to redistribution or even swapping in various zooxanthellae populations. The abundance of free living zooxanthellae in aquariums is really questionable, especially in tanks employing strong foam fractionation. However, every day, most corals renew or release certain numbers of zooxanthellae normally. The amounts can vary, but generally more are released around mid-day to late afternoon, during and immediately following periods of highest light and photosynthetic maximum production. The ability of corals to draw from the pools of recently released zooxanthellae is certainly a possibility.

Perhaps more than anything, such information serves to further emphasize the need to base reef aquaria around a specific habitat. Zonation is, in my opinion, an extremely important concept of which reef aquarists are only beginning to take advantage. Providing conditions, or species appropriate to given conditions, not only makes good sense, but improves survivability, decreases competitive incompatibilities, and certainly allows for a more natural and aesthetically attractive display. At the very least, the more we can learn and understand about the biology of corals, the better prepared we are to attend to their needs and limit unnecessary losses from wild communities.

Literature cited and used:

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Last modified 2006-11-19 02:11