Without symbiosis, living organisms would be quite different from what they are today. This is true not only because symbiotic relationships were fundamental to the separation of eukaryotes (organisms whose cells have true nuclei) from prokaryotes (cellular organisms lacking a true nucleus), but also because they represent a unique biological process without which many organisms could not exist. All herbivorous mammals and insects, for example, would starve without their cellulose-digesting mutualists; coral reefs could not form if corals were not associated with algae; and the human immune system would not be as complex as it is today if humans had not been infested so often by parasites over the course of their evolution.
The English word “symbiosis” is derived from two Greek words, sym, meaning “with,” and bios, meaning “life.” The term was introduced into scientific usage in 1879 by Heinrich Anton de Bary, professor of botany at the University of Strasbourg. De Bary used symbiosis in a global sense to refer to any close association between two heterospecific organisms. He explicitly referred to parasitism as a type of symbiosis, but excluded associations of short duration. According to de Bary’s definition of symbiosis, the infection of humans by Plasmodium falciparum, the agent that causes malaria, is an instance of symbiosis whereas the pollenization of flowering plants by insects is not. As of 2003, “symbiosis” is used in two basic ways. First, the term can be used to refer to a close association between two organisms of different species that falls into one of three categories—mutualism, commensalism, or parasitism. Second, symbiosis may be used in a more restricted sense as a synonym of mutualism, to identify a relationship in which the two associated species derive benefits from each other.


Symbiosis always implies a biological interaction between two organisms. The first organism is called the host, and is generally larger than the other or at least supports the other. The second organism is called the symbiont. It is usually the smaller of the two organisms and always derives benefits from its host. The symbiont is called an ectosymbiont when it lives on the surface of the host; it is called an endosymbiont when it is internal— that is, when it lives inside the host’s digestive system, coelom, gonads, tissues or cells. The symbiosis is beneficial to the host in mutualism, neutral in commensalism, and harmful in parasitism.
These three categories are often abbreviated by the signs “+/+” for mutualism; “0/+” for commensalism; and “-/+” for parasitism, where the symbols to the left and right of the slash represent the primary effect of the association on the host and symbiont respectively. The “+” indicates that the relation is beneficial, “0″ that it is neutral, and “-” that it is harmful. A symbiosis is defined as facultative if the host is not necessary over the full course of the symbiont’s life cycle; it is defined as obligatory if the symbiont is dependent on the host throughout its life cycle. In addition, symbionts are described as either specific or opportunistic. A specific symbiont is associated with a few host species, the highest specificity being assigned to symbionts that infest only one species. Opportunistic symbionts, on the other hand, are associated with a wide range of hosts belonging to a wide variety of different taxonomic groups.
Some symbiotic associations are difficult to place within one of these three categories. Biologists often prefer to speak of the existence of a symbiotic continuum along which mutualism, commensalism, and parasitism shade into one another without strict dividing lines. Suckerfishes are an illustrative example of the problem of precise categorization. The suck-erfish is an organism that attaches itself to large marine vertebrates (a host) by means of an anterior sucker. Some authors consider this symbiosis an example of mutualism because the suckerfishes eat ectoparasites located on the skin of the vertebrates to which they are attached; the suckerfishes are also able to conserve energy because while they are attached to a host, they allow their hosts to swim for them. But other researchers regard suckerfishes as ectocommensals because they eat the remains of their hosts’ prey. They are even considered inquilines (symbionts that live as “tenants” in a host’s nest, burrow, fur, etc. without deriving their nourishment from the host) on occasion because some of them live inside the buc-cal (cheek) cavities of certain fishes.
Life cycles are another factor that complicates the categorization of symbiotic relationships, in that some organisms move from one symbiotic state to another over the course of their life cycle. Myzostomids, for example, are tiny marine
An example of parasitism: a gastroid of the genus Stilifer living on the arm of a sea star.
An example of parasitism: a gastroid of the genus Stilifer living on the arm of a sea star.
worms associated with the comatulid crinoids, organisms related to sea stars. Most myzostomids are parasitic when they are young and cause deformities on the skin of their hosts. They develop, however, into ectocommensals that do not harm the crinoids except for stealing their food. Myzostomids are the oldest extant animal parasites currently known; deformities attributed to these strange worms have been identified on fossil crinoids from the Carboniferous Period, 360-286 million years ago.
The problem of categorization becomes even more complicated when organisms change their symbiotic relationships according to environmental conditions. This is the case in the mutualism between the freshwater cnidarian Hydra, which lives in ponds and slowly moving rivers, and the alga Chlorella, which lives in the cnidarian’s cells. Under normal environmental conditions, the algae perform photosynthesis and release substantial amounts of carbon to the animal’s cells in the form of a sugar known as maltose. In darkness, however, the flow of carbon-based compounds is reversed, with the nutrients coming from the feeding of Hydra being diverted by the algae. As a result, the growth of the cnidarians is reduced and the mutualist algae have become parasites.


Commensalism (from the Latin com, or “with,” and mensa, or “table”) literally refers to “eating together” but encompasses a wide range of symbiotic interactions. A commensal symbiont feeds at the same place as its host or steals the food of its host. This narrower definition is restricted to a very few organisms; most of the time, commensalism covers all associations that are neutral for the hosts, in which the commensal organisms benefit from the acquisition of a support, a means of transport, a shelter, or a food source. There are three major types of commensal relationships: phoresy (from the Greek phoros, “to carry”), in which the host carries or transports the phoront; aegism (from the Greek aegidos, or aegis, the shield of Athena), in which the host protects the aegist; and inquilism (from the Latin incolinus, “living inside”), in which the host shelters the inquiline in its body or living space without negative effects. Inquilism has been described by some researchers as a form of “benign squatting.”
The loosest symbiotic associations are certainly the facultative phoresies. The modified crustacean Lepas anatifera, which is often attached to the skins of cetaceans (whales, dolphins, and porpoises) or the shields of turtles, is an instructive example of a phoresy. These crustaceans can be found hanging from floating pieces of wood as well as from members of other species. If other organisms often serve L. anatifera as a substrate, the association is not at all obligatory over the course of the crustacean’s life cycle. The polychaete worm Spirorbis is a similar instance of a phoresy; its tube can be found sticking either to various types of organisms or to rocks in inter-tidal zones.
A stronger association exists between aegist symbionts and their hosts. Aegist coeloplanids are tiny flat marine invertebrates found on the spines of sea urchins or the skin of sea stars. These organisms are related to comb jellies or sea gooseberries, which are planktonic organisms. The coeloplanids are protected from potential predators by their host’s defensive organs or structures. They eat plankton and organic materials from the water column trapped by their sticky fishing threads. The coeloplanids do not harm their echinoderm hosts even when hundreds of individuals are living on a single host. Many aegist relationships involve marine invertebrates, especially poriferans (sponges), cnidarians, and echinoderms. The associated organisms include polychaete worms; such crustaceans as crabs and shrimps; brittle stars; and even fishes.
The relationships between aegists and their hosts are often quite close. For example, the snapping shrimp Synalpheus lives and mates on comatulid crinoids. The crinoids have feathery rays or arms that hide the shrimps from predators. The shrimps often leave their hosts in order to feed, but are able to relocate them by smell as well as sight; they recognize the odor of a substance secreted by their hosts. This behavior is also found in some crabs, like Harrovia longipes, which also lives on comatulid crinoids.
Inquiline symbionts are particularly interesting to researchers. The most extraordinary inquilines, however, are the symbiotic pearlfishes. Pearlfishes belong to the family Carap-idae, which includes both free-living and symbiotic fishes. The latter are associated with bivalves and ascidians. They can also be found in the digestive tubes of sea stars and the respiratory trees of sea cucumbers. Pearlfishes that have been extracted from their hosts cannot live more than a few days. They are totally adapted to their symbiotic way of life: their bodies are spindle-shaped; their fins are reduced in size; and their pigmentation is so poorly developed in some species that their internal organs are visible to the naked eye. Pearlfishes are often specific, and make use of olfaction (sense of smell) as well as vision to recognize their hosts. They must have some type of physiological adaptation that protects them against their hosts’ internal fluids, such as the digestive juices of sea stars; however, these adaptations are not understood as of 2003.


In parasitic relationships (from the Greek para, “beside,” and sitos, “food”), the parasitic organism first acquires a biotic substrate where it lives for part of its life cycle. This biotic substrate is often a food source for the parasite and sometimes a source of physiological factors essential to its life and growth. The parasite then seeks out a host.
Most parasites do not kill the hosts they infest even if they are pathogenic and cause disease. Diseases are alterations of the healthy state of an organism. Parasitic diseases may be divided into two types: structural diseases, in which the parasite damages the structural integrity of the host’s tissues or organs; and functional diseases, in which the parasite affects the host’s normal growth, metamorphosis, or reproduction. Parasitism is by far the most well-known symbiotic category, as many parasites have a direct or indirect impact on human health and economic trends. The causes of disease have always fascinated people since ancient times. Early humans thought that diseases were sent by supernatural forces or evil spirits as punishment for wrongdoing. It was not until the nineteenth century that scientific observations and studies led to the germ theory of disease. In 1807, Benedicte Prevost demonstrated that the bunt disease of wheat was produced by a fungal pathogen. Prevost was the first to demonstrate the cause of a disease by experimentation, but his ideas were not accepted at that time as most people clung to the notion of spontaneous generation of life. By the end of the nineteenth century, however, Anton de Bary’s work with fungi, Louis Pasteur’s with yeast, and Robert Koch’s with anthrax and cholera closed the debate on spontaneous generation and established the germ theory of disease.
In parasitic associations, animals are either parasites or hosts. There are three animal phyla, Mesozoa, Acantho-cephala, and Pentastomida, that are exclusively parasitic. In addition, parasites are commonly found in almost all large phyla, including Platyhelminthes, Arthropoda and Mollusca. Many gastropod mollusks, for example, are parasites of such echinoderms as brittle stars and sea stars. Stilifer linckiae buries itself so deeply in the body wall of some sea stars that only the apex of the shell remains visible at the center of a small round hole. The gastropod’s proboscis pierces the sea star’s tissues and extends into the body cavity of its host, where it sucks the host’s internal fluids and circulating cells. The morphology of Stilifer linckiae closely resembles that of free-living mollusks. In most cases, however, the body plans of parasites have been modified from those of their free-living relatives. The parasites tend to lose their external appendages and their organs of locomotion; in addition, their sense organs are commonly reduced or absent.
The rhizocephalan sacculines are a remarkable example of the evolutionary modification of a crustacean body plan. They are so profoundly adapted to parasitism that only an understanding of their early larval form allows them to be recognized as crustaceans. Sacculina carcini, for example, can often be observed as an orange sac on the ventral side of a crab. The early larval stages of this organism are free-living nau-plii that move into the water column until they find a crab. Only female larvae seek out and attach themselves to the base
An example of commensalism: the crab Harrovia longipes associated with a crinoid.
An example of commensalism: the crab Harrovia longipes associated with a crinoid.
of one of the crab’s bristles. Once attached, the female larvae molt and lose their locomotory apparatus, giving rise to new forms known as kentrogon larvae. Kentrogon larvae are masses of cells, each armed with a hollow stylet or thin probe. The stylet pierces the body wall of the crab as far as the body cavity; the cell mass then passes through the stylet into the host’s body. In this way the kentrogon injects itself into the crab. The internal mass proceeds to grow and differentiate into two main parts: an internal sacculina that absorbs nutrients through a complex root system gradually extending throughout the crab’s body; and an external sacculina that forms after the root system, emerges from the ventral side of the crab, and develops into the true body of the female parasite. The female reproductive system opens to the outside through a pore that allows the entry of a male larva. The male larva injects its germinal cells, which eventually become spermatozoa capable of fertilizing the ova. S. carcini reproduces throughout the year on the crab Carcinus maenas, but more frequently between August and December.


In mutualism (from the Latin mutuus, “reciprocal”), the interactions between the symbiotic organisms can be as simple as a service exchange or as complex as metabolic exchanges. Mutualistic animals are associated with a range of different organisms, including bacteria, algae, or other animals. Many marine fishes, for example, are cleaned regularly of ectoparasites and damaged tissues by specialized fishes or shrimps called cleaners. The cleaners provide a valuable service by keeping the fishes free of parasites and disease; in turn, they acquire food and protection from predators. Cleaning mutualisms occur throughout the world, but are most commonly found in tropical waters. The cleaning fishes or shrimps involved in this type of mutualism establish cleaning stations on such exposed parts of the ocean floor as pieces of coral. The cleaner organisms are generally brightly colored and stand out against the background pattern of the coral. The bright colors, along with the cleaners’ behavioral
Sea anemones of the genus Heteractis use stinging cells to capture prey such as small fish.
Sea anemones of the genus Heteractis use stinging cells to capture prey such as small fish. The commensal anemonefish (Amphiprion sp.) have developed a way to mimic the anemone’s own membrane, so that the anemone does not know that the anemonefish is a foreign animal. The fish gets protection from other fish, and since the anemonefish is territorial and tidy, it appears to keep the anemone safe and clean as well.
displays, attract fishes to the cleaning stations. The cleaners are then allowed to enter the mouth and gill chambers of such species as sharks, parrotfishes, grunts, angelfishes, and moray eels. Most cleaning fishes belong to the genus Labroides. Parasites that are removed from the cleaned fishes include copepods, isopods, bacteria, and fungi. Beside fishes, cleaning shrimps are also common in the tropics. The best-known species are the Pederson cleaner shrimp, Periclimenes pedersoni, and the banded coral shrimp, Stenopus hispidus. When the fishes approach their cleaning stations, these shrimps wave their antennae back and forth until the fishes get close enough for the shrimps to climb on them. Experiments have shown that the cleaners control the spread of parasites and infections among members of their host species. Cleaning symbioses also occur between land organisms: for example, various bird species remove parasites from crocodiles, buffalo and cattle; and the red rock crab Grapsus grap-sus cleans the iguana Amblyrhynchus subcristatus.
The most evident mutualism between oceanic species is the one that exists between sea anemones and clown fishes. Fishes of the genera Amphiprion, Dascyllus, and Premnas are commonly called clown fishes due to their striking color patterns. The symbiosis is obligatory for the fish but facultative for the anemones. The brightly colored clown fishes attract larger predator fishes that sometimes venture too close to the anemones; they can be stung by the anemone tentacles, killed, and eaten. Clown fishes share in the meal and afterward remove wastes and fragments of the prey from the anemone. A number of experiments have been conducted in order to understand why the clown fishes are immune to the stinging tentacles of the sea anemones when other fishes are not. It is known that the clown fishes must undergo a period of acclimation before they are protected from the anemones. Further studies showed that the mucous coating of the clown fishes changes during this period of acclimation, after which the anemones no longer regard them as prey. The change in the mucous coating was first thought to result from fish secretions, but researchers were able to demonstrate that it results from the addition of mucus from the anemones themselves.

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