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Mutualisms By Ronald L. Shimek Aquarium.Net January 1998

Ron Shimek writes on mutulism in aquariums, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist


By Ronald L. Shimek

Amphiprion perideraion in a Heteractis magnifica ; one of the many Indo-Pacific clownfish-sea anemone interactions.

Probably the most enduring and evident icon of the marine aquarium hobby is a picture of a clownfish nestled down in an host sea anemone. The behavior illustrated by such a picture is an example of a mutualism, where organisms of two or more species live together, each providing some benefit to the other. Mutualisms are but one type of symbiosis, a term describing two organisms living in close and intimate proximity to one another. Some of the other types of symbioses include commensalism, where the organisms live together, but only one species benefits; amensalism, where neither species benefits and one is harmed; and three categories, parasitism, competition and predation, where one species benefits and the other is harmed.

Mutualisms are really the basis for coral reefs and, indirectly, the coral reef aquarium hobby. The primary coral-reef mutualism is that found between a dinoflagellate alga, and the coral polyp. In the coral-zooxanthellae mutualism, the coral benefits by being able to utilize some algal byproducts, and the alga benefits by being protected both from herbivores that might eat it, and shallow-water ultraviolet light intensities which might kill it (Lesser, 1996).

Although most commonly found on coral reefs, such algal-animal symbioses are found in all seas. Their great abundance in coral reef areas is probably due to the light intensity in the shallow water tropical areas, which ensures that enough light will pass through the water and the host's tissues to allow for sufficient photosynthesis to occur. A dinoflagellate living in a host is called a zooxanthella (plural = zooxanthellae). There are other mutualisms between an alga and an animal. If the plant is a green alga then the plant is referred to as a zoochlorella (plural = zoochlorellae) (Tsuchida and Potts, 1994a,b). Zoochlorellae are also found in the temperate regions, where in addition to the mutualisms, there are endoparasitic or pathogenic green algae that can kill their host.

It is a common misconception that there is only one type of zooxanthella found in coral. Actually several types may be found in a single coral colony, and the relative abundance of each type can vary depending on the position in the colony, the relative light intensity, the temperature, and the physiological state of the coral animal (Rowan, et al., 1997). Given the basic tenet of ecology that two species with the same limiting factors will compete, one should also be able to see competition within the corals by the different types of zooxanthellae. As conditions change, one or the other types of algae would be favored and would come to be predominant. This is likely the cause of many of the color changes that occur when reef aquarium corals are moved, or when light intensities change. The aquarist in changing conditions has become an unknowing participant in the internal "algae wars" occurring inside the coral.

For some images of zooxanthellae, follow this link. This web page is in Japanese, so unless you have Japanese fonts loaded, the print will be gibberish. The top left color photo is of a zooanthid, Palythoa . The top right are zooxanthellae from the zooanthid. The lower color photos are of zooxanthellae. The top right black and white photo shows zooxanthellae in a cnidarian. The bottom black and white photo shows a transmission electron micrograph view of some zooxanthellae in a host (the zooxanthellae are the roughly circular objects near the middle of the image, and their chloroplasts are the blobby darker structures in them).

Mastigias papuae , this jellyfish lacks a mouth and cannot feed. It gets all of its nutrition from zooxanthellae and dissolved organic material in the water. Photographed in a "Jellyfish lake," a marine lake in Palau.

A second common misconception is that the zooxanthellae can provide all the nutrition that a coral needs. Using only dissolved carbon dioxide, water and light the zooxanthellae can produce only sugars, and in some corals they have been shown to be able to produce sufficient sugar that the caloric needs of the polyp may be met for periods up to about a day (Muscatine, 1973, 1990, Muscatine and Porter, 1977; Davies, 1984). Longer term experiments are few, so there is no conclusive evidence that they can provide those needs for longer periods. Zooxanthellae can also provide their host with amino acids and complex chemicals, however the raw materials for those chemicals must come through the host's tissues as ammonia or other nitrogenous compounds. These materials are basically the byproducts of protein metabolism and they can come from three potential sources, either from dissolved material in the water column, from breakdown of the host's tissues, or as the byproducts of the host's feeding Crossland and Barnes. 1974, Szmant-Froelich and Pilson, 1984). It is interesting to note that there are data that indicate that increases in some of these chemicals, such as nitrate, seem to boost the populations of zooxanthellae and reduce the rate of skeleton formation (Marubini and Davies, 1996). In effect, this indicates that the zooxanthellae may respond to external events in ways that may not be totally beneficial for the corals. With only a few exceptions, such as the medusae of the Indo-Pacific "jellyfish lakes" ( Mastigias papauae ) all zooxanthellate animals must feed. A general rule, the more they feed, the faster they grow or the accumulation of gametes occurs. Nevertheless, the zooxanthellae provide important and critical nutrients to the coral.

The corals appear to use the sugars released by the algae for simple nutrition, but the fate of the other materials is more complex. A simplified explanation is that these more complex organic molecules allow the corals to secrete the organic matrix of their skeleton. Most aquarists know that corals have a calcium carbonate exoskeleton. However, there is a significant organic component to this skeleton, which is manifested in a matrix of organic material upon and around which the calcium carbonate is laid down. The actual process of calcification is incompletely understood, however those corals without zooxanthellae are not able to produce skeleton at anywhere near the rate of those corals that have the algae (Barnes and Chalker, 1990).

The zooxanthellae-cnidarian mutualisms are actually somewhat unusual in that one organism is living completely within the other. Nevertheless, both organisms are capable of free-living existence. Without its zooxanthellae, a reef-building coral grows exceptionally slowly and needs to feed a lot, but if the other physical conditions are okay, it may survive for quite a long time. Without the coral host, the zooxanthellae can become planktonic and survive as more-or-less normal dinoflagellates. Most aquarists know that during coral "bleaching," that zooxanthellae are expelled from corals. Some zooxanthellae, however, also are normally periodically expelled or leave corals during normal behavior for any number of reasons (Titlyanov, et al. 1996). Many aquarists have periodic blooms of dinoflagellate algae in their systems. It is possible that the ultimate source of those algae are some of the mutualistic zooxanthellae populations living in some of the animals in the tanks.

Most mutualisms occur when two organisms live in close proximity, and probably the best example of this for aquarists is the Indo-Pacific clownfish-sea anemone mutualism, where individuals from some genera of relatively closely related fishes ( Amphiprion, Premnas, Dascyllus ) associate with several species of sea anemones from several different and disparate groups (Dunn, 1981; Fautin and Allen, 1992). In this common type of mutualism, is where on party feeds the other in return for incidental or active protection. In these system of mutualisms, the fishes appear to protect the sea anemones from predation by butterfly fishes, and in turn, the anemones provide shelter for the fishes from larger, primarily fish, predators. Additionally, the fishes provide the anemones with supplemental food in the form of fish feces and urine, and as well, at least in aquaria, will bring food to their host.

Many of these anemones appear to be actively capable of feeding on other fishes both in nature and in aquaria, although there is little data from natural populations about the diets of the host anemones. As these hosts can actively kill and eat fish, there is some question as to how the fishes acquire their immunity to the hosts' venomous sting. The jury is still out, but it now appears that at least some of the fishes can intrinsically produce substances that mimic the anemones own mucus, so that the anemone does not perceive the fish as being present as a food source. There are also data that the fishes may acquire some of the anemone's mucus during a period of acclimatization to their host. In any case, however, the anemones do not seem to perceive the anemonefish as food, and the fish are generally safe from predation by their host (Elliott, et al. 1994; Elliott and Mariscal, 1997).

A juvenile painted greenling, Oxylebius pictus , about an inch (2.5 cm) resting on the oral disc of the strawberry anemone, Urticina lofotensis , in the cool waters of an inlet on Vancouver Island.

Numerous other fishes live in close proximity to sea anemones throughout the world's oceans, but until recently it was thought the Indo-Pacific host anemones harbored the only true "anemonefish" mutualism, where the fish was immune to the host's stinging capabilities. Recent work has shown that in the cool waters of the NE Pacific, there is another true anemonefish (Elliott, 1992). This fish is Oxylebius pictus , the convict fish or painted greenling, of the Pacific Coast of North America. The host anemones in this case are found in the genus Urticina . The fish is mostly found with the strawberry anemone, Urticina lofotensis , but occasionally with other species such as the fish-eating anemone, Urticina piscivora , as well. These anemones lack zooxanthellae altogether and are often predatory on fish, and they possess quite virulent stings. The sting of Urticina piscivora is sufficiently potent to cause long-lasting necrotic lesions on humans.

An adult painted greenling, Oxylebius pictus , resting around the base of the strawberry anemone, Urticina lofotensis at night. The fish is about 5 inches (12.5 cm) long.

This association is facultative, as the fish may be found in areas lacking the anemones unlike the Indo-Pacific clownfish, which require a host anemone. If the anemones and fish are present together, however, most of the anemones harbor one or two fish. Larval Oxylebius pictus will metamorphose out of the plankton as juvenile fish directly on the oral disk of the anemone. The strawberry anemones are relatively small, generally no more than about 6 inches (15 cm) across the tentacle span, and the juvenile fishes are less than an inch long (2.5 cm) at metamorphosis. During their life on the oral disk of the anemone, the fish appear to feed on small ectoparasitic or commensal copepods that live on the anemone. If threatened, the fish may swim down through the anemone's mouth into its gut. The major predators on the small fish may be nocturnal octopuses and crustaceans as well as other fishes.

The fishes grow relatively rapidly and as adults they reach lengths of about 10 inches (25 cm) and are far to big to sit on the oral disk of their anemones, but they remain in close proximity to the anemones. At night they lie next to the anemone, often curling their body around the column of the anemone. The long tentacles of the anemone droop down over the fish protecting it from predation.

Sea anemones are odd animals ecologically. They are often very potent predators and yet they don't move fast and their predatory apparatus extends in all directions around the mouth. They seem tailor made for other organisms to take advantage of as protectors, and it is a common practice to assume that all organisms living on anemones are mutualistic. That is not always the case. Animals other than fishes, primarily crustaceans, are often found around sea anemones. It has long been presumed that many of these living-arrangements are mutualisms, but the actual data showing benefits to both parties are often lacking. Recent research indicates that some of the so-called mutualisms are in fact commensal or parasitic/predatory relationships, where the crustaceans, such as the anemone shrimps in the genus Periclimenes , may feed on their host's tissues as well as get protection from them. This would make these shrimps the marine equivalent of fleas...large and attractive, but parasites nonetheless (Fautin, et al. 1995; Guo, et al. 1996).

A protector of its host, the scale worm Arctonoë living the mouth of a temperate sea star, Mediaster aequalis . The worm will actively attack and bite predators of its host.

Mutualisms of the "protection for food" nature do not have to include either fishes or sea anemones. One such mutualism that is common in nature, and sometimes seen in aquaria, is found between sea stars and some polychaete annelid worms (such as scale worms) that live on them. These stars are all predatory, consuming sessile or slow moving prey such as sea pens, clams, and some snails. The worms often live in the sea star's mouth or in the groove between the bases of the tube feet under the arms. The stars are very sloppy eaters and when they eat a lot of tissue debris is often created. The worms feed on this material. These worms often have rather large forcep- or pincer-like jaws. When they feed they used these jaws to grasp or grab pieces of food.

Sea stars are not attacked and eaten by many animals, possibly because they contain some rather nasty chemicals called saponins. Nonetheless, they do have some predators. If the predator is very large relative to the star and worm, such as birds or large fishes, the worms can't help, and likely get eaten along with their host. For many smaller predators, though, the worm can offer significant assistance to the star. Some of these smaller or slower predators are snails or even other sea stars, and when they attempt to attack the host sea star, the worms will often venture over to the predator and actively bite it with their sharp jaws. In many instances, this will cause a disruption of the predator's attack and the host may escape (Davenport, 1950, 1953a,b; Davenport and Hitchcock, 1951).

Another type of mutualism is one where both species assist their partner in escaping predation. One of the best known of this type of mutualism is the well-documented sponge-scallop mutualism which is relatively wide-spread in both temperate and tropical seas. In this symbiosis, the sponge lives on the outside of the scallop shells, and in fact some scallops have shells with hooks and other structures on them that appear to be specifically designed to assist the sponge in adhering to the bivalve (Bloom, 1975; Forester, 1979).

Scallops can swim, and will often swim when disturbed by something touching their shells. This swimming response occurs most frequently when the organism touching the shells is one of the many predators of the tasty clams. However sometimes the scallops take too long to swim when they are touched by the tube-feet of a predatory sea star. Sea stars attach to the substrate when they walk by means of a rapidly setting adhesive secreted on the pad of the tube foot. They do not attach by suction to anything, as they simply don't have the muscles in their tube feet to form the suction. If the scallop shell is lacks sponge, the tube feet may rapidly attach and cling to the scallop until the star eats it. If the scallop is covered in sponge, the attachment by the star takes significantly longer, and the scallop frequently can swim away. Obviously the scallop benefits by having sponge on its shell.

A failure of a mutualism. A small dorid nudibranch Diaulula sandiegensis is eating the encrusting sponge off of the top shell of a scallop, Chlamys hastata . Even the best protective mutualisms fail occasionally.

In turn the sponge benefits as well. The sponges are often the primary food of some grazing predatory snails called dorid nudibranchs. Dorids are relative slow, attacking at the proverbial snail's pace, and they are also blind. They have some chemosensory capability and are able to find the scallops from short distances. When the nudibranchs approach the scallop and attempt to climb on it to eat the sponge, the scallop often swims off, removing sponge from danger. So, in this mutualism, each party protects the other from predation.

Mutualisms seem to be maintained by some of the strongest of all selective pressures, predation and the need for food. As illustrated in the examples, many of the partners benefits by protection from predation or may get food or both from the arrangement. Such mutualisms seem to be very common in marine ecosystems, and may be significantly more common than we presently suspect, as most marine ecosystems are very poorly investigated or understood. We may soon find other mutualisms for our aquarium systems as attractive as that between the Indo-Pacific clownfish and anemones.

For some information on another mutualism of interest to aquarists, the damselfish-algal turf mutualism, follow this link:

There is a significant amount of research into the theoretical aspects of mutualisms, for one of the more readable discussions of some of the properties of mutualisms, follow this link:


References Cited:

Barnes, D. J. and B. E. Chalker. 1990. Calcification and photosynthesis in reef-building corals and algae. In: Dubinsky, Z. Ed. Coral reefs. Elsevier. Amsterdam. pp. 109-131.

Bloom, S. A. 1975. The motile escape response of a sessile prey: a sponge-scallop mutualism. Journal of Experimental Marine Biology and Ecology. 17:311-321.

Crossland, C. J. and D. J. Barnes. 1974. The role of metabolic nitrogen in coral calcification. Marine Biology. 28:325-332.

Davies, P. S. 1984. The role of zooxanthellae in the nutritional energy requirements of Pocillopora eydouxi . Coral Reefs. 2:181-186.

Davenport, D. 1950. Studies in the physiology of commensalism. I. The polynoid genus Arctonoë . Biological Bulletin. 98: 81-93.

Davenport, D. 1953a. Studies in the physiology of commensalism. III. The polynoid genera Achloë, Gattyana and Lepidasthenia . Journal of the Marine Biological Association of the United Kingdom. 32:161-173.

Davenport, D. 1953b. Studies in the physiology of commensalism. IV. The polynoid genera Polynoë, Lepidasthenia and Haromthoë . Journal of the Marine Biological Association of the United Kingdom. 32:273-288.

Davenport, D. and J. F. Hitchcock. 1951. Studies in the physiology of commensalism. 2. The polynoid genera Arctonoë , and Halosydna . Biological Bulletin. 100: 71-83.

Dunn, D. F. 1981. The clownfish sea anemones: Stichodactylidae (Coelenterata: Actinaria) and other sea anemones symbiotic with pomacentrid fishes. Transactions of the American Philosophical Society. 71: 1-115.

Elliott, J. 1992. The role of sea anemones as refuges and feeding habitats for the temperate fish Oxylebius pictus . Experimental Biology of Fishes. 38:381-400.

Elliott, J. K., R. N. Mariscal and K. H. Roux. 1994. Do anemonefishes use molecular mimicry to avoid being stung by host anemones? Journal of Experimental Marine Biology and Ecology. 179:99-113.

Elliott, J. K. and R. N. Mariscal. 1997. Acclimation or innate protection of anemonefishes from sea anemones. Copeia. 122:284-289.

Fautin, D. G. and G. R. Allen. 1992. Field guide to Anemonefishes and their host sea anemones. Western Australian Museum, Perth, Australia. 160 pp.

Fautin, D. G., C. C. Guo and J. S. Hwang. 1995. Costs and benefits of the symbiosis between the anemoneshrimp Periclimenes brevicarpalis and its host Entacmaea quadricolor . Marine Ecology Progress Series. 129:77-84.

Forester, A. J. 1979. The association between the sponge Halichondria panicea (Pallas) and the scallop Chlamys varia (L.): A commensal-protective mutualism. Journal of Experimental Marine Biology and Ecology. 36:1-10.

Guo, C. C., J. S. Hwang and D. G. Fautin. 1996. Host selection by shrimps symbiotic with sea anemones: A field survey and experimental laboratory analysis. Journal of Experimental Marine Biology and Ecology. 202:165-176.

Lesser, M. P. 1996. Elevated temperatures and ultraviolet radiation cause oxidative stress and inhibit photosynthesis in symbiotic dinoflagellates. Limnology and Oceanography. 41:271-283.

Marubini, F. and P. S. Davies. 1996. Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals. Marine Biology. 127:319-328.

Muscatine, L. 1973. Nutrition of corals. In: Jones, O. A. and R. Endean. Eds. Biology 1. Academic Press. New York. pp. 77-115.

Muscatine, L. 1990. The role of symbiotic algae in carbon and energy flux in reef corals. In: Dubinsky, Z. Ed. Coral reefs. Elsevier. Amsterdam. pp. 75-87.

Muscatine L and J. W. Porter. 1977. Reef Corals: Mutualistic symbioses adapted to nutrient-poor environments. Bioscience. 27:454-460.

Rowan, R., N. Knowlton, A. Baker and J. Jara. 1997. Landscape ecology of algal symbionts creates variation in episodes of coral bleaching. Nature 388:265 268.

Szmant-Froelich, A. and M. E. Q. Pilson. 1984. Effects of feeding frequency and symbiosis with zooxanthellae on nitrogen metabolism and respiration of the coral Astrangia danae . Marine Biology. 81:153-162.

Titlyanov, E. A., T. V. Titlyanova, V. A. Leletkin, J. Tsukahara, R. van Woesik and K. Yamazato. 1996. Degradation of zooxanthellae and regulation of their density in hermatypic corals. Marine Ecology Progress Series. 139:167-178.

Tsuchida, C. B. and D. C. Potts. 1994a. The effects of illumination, food, and symbionts on growth of the sea anemone Anthopleura elegantissima (Brandt, 1835). I. Ramet Growth. Journal of Experimental Marine Biology and Ecology. 183:227-242.

Tsuchida, C. B. and D. C. Potts. 1994b. The effects of illumination, food, and symbionts on growth of the sea anemone Anthopleura elegantissima (Brandt, 1835). II. Clonal Growth. Journal of Experimental Marine Biology and Ecology. 183:243-258.

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