Symbiosis

Understanding the ways in which different species of animals interact in nature is one of the fundamental goals of biology. Predator-prey relationships, competition between species for limited resources, and symbiosis are the major forms of species interactions. These have profoundly influenced the diversity and ecology of all forms of life. Significant advances have been made in understanding how organisms interact, but in studies of symbiosis (which literally means “living together”), one finds the most complex, interesting, and important examples of both cooperation and exploitation known in the living world.

Symbiosis involves many types of dependent or interdependent associations between species. In contrast to predator-prey interactions, however, symbioses are seldom rapidly fatal to either associating species (symbiotes) and are often of long duration. Except for grazing animals that do not often entirely consume or destroy their plant “prey,” most predators quickly kill and consume their prey. While a predator may share its prey with other individuals of the same species, an example of living together, such intraspecific behavior is not considered a symbiosis. Fleas, some ticks, mites, mosquitoes, and other bloodsucking flies are viewed as micro predators rather than parasites.

All organisms are involved in some form of competition. The abundance and availability of environmental resources are finite, and competition for resources occurs between members of the same species and individuals and populations of different species. When the number of individuals in a population increases, competition for limited food, water, shelter, space, and other resources necessary for survival and reproduction also increases. Thus, competition plays a major role in populations of free-living animals (those not inhabiting the body of other organisms) and in populations living on or in other animals. For example, both tapeworms and whales must compete for resources, and both have evolved habitat-specific adaptations to accomplish this goal. Whales compete with whales, fishes, and other predators for food; tapeworms compete with tapeworms and other symbiotes (such as roundworms) for food and space; and tapeworms and whales compete for food in the whale’s gut.

Symbiosis is a term used to describe nonaccidental, nonpredatory associations between species. When used by itself, the term symbiosis does not provide information on how or why species live together, or the biological consequences of their interactions. Recognizably different forms of symbioses all have one or more characteristics in common. All involve living together; most involve food sharing; many involve shelter; and some involve damage to one or both symbiotes.

Hosts and Symbiotes

Host species may be thought of as landlords. Hosts provide their symbiotes (also called symbionts) with transportation, shelter, protection, space, some form of nutrition, or some combination. Host species are generally larger and structurally more complex than their symbiotes, and different parts of a host’s body (skin, gills, and gut, for example) may provide habitats for several different kinds of symbiotes at the same time. The three primary categories of symbiosis most referred to in popular and scientific works are commensalism, mutualism, and parasitism.

Symbiotes that share a common food source are known as commensals (literally, “mess-mates”). In the usual definition of commensalism, one species (usually referred to as the commensal, although both species are commensals) is said to benefit from the relationship, while the other (usually referred to as the host) neither benefits nor is harmed by the other. Adult tapeworms live in the intestinal tracts of vertebrate hosts and provide a classic example of commensals. Adult tapeworms share the host’s food, usually with little or no effect on otherwise healthy hosts. As in all species, however, too large a tapeworm population may result in excessive competition, lower fitness, or disease in both the host and the tapeworms. For example, the broad fish tapeworm of man, Diphyllobothrium latum, may cause a vitamin B₁₂ deficiency and anemia in humans when the worm burden is high. In addition to tapeworms, many human symbiotes called “parasites” are, in fact, commensals.

External commensals (those living on the skin, fur, scales, or feathers of their hosts) are called epizoites. A good example of an epizoite is the fish louse (a distant relative of the copepod), which feeds on mucus of the skin and scales of fishes. Another type of commensalism is called phoresis (phoresy), which involves passive transportation of the commensal (phoront) by its host. Examples of phoreses include barnacles carried by whales and sea turtles, and remoras (sharksuckers), which, in the absence of sharks, may temporarily attach themselves to human swimmers. In inquilinism, the transported commensal (inquiline) shares, or more accurately, steals, food from the host, or may even eat parts of the host. Perhaps the best-known inquilines are the glass- or pearlfishes, which take refuge in the cloacae of sea cucumbers and often eat part of the host’s respiratory system. A unique type of commensalism, known as symphilism, is found in certain ants and some other insects (hosts) which “farm” aphids (symphiles) and induce them to secrete a sugary substance which the ants eat.

Mutualism

The most diverse type of commensalism is mutualism. In some studies, particularly those dealing with animal behavior, mutualism is used as a synonym of symbiosis; hence, the reader must use caution in order to determine an author’s usage of these terms. As used here, mutualism is a special case of commensalism, a category of symbiosis. The relationship between mutuals may be obligatory on the part of one or both species, but it is always reciprocally beneficial, as the following examples illustrate. Some species of hermit crabs place sea anemones on their shells or claws (sea anemones are carnivores which possess stinging cells in their tentacles). Hermit crabs without anemones on their shells or claws may be more vulnerable to predators than those with an anemone partner. Hermit crabs, which shred their food in processing it, lose some of the scraps to the water, which the anemones intercept, and eat. Thus, the crab provides food to the anemone, which in turn protects its provider. Such relationships, which are species-specific, are probably the result of a long period of coevolution.

A different type of mutualism, but one having the same outcome as the crab-anemone example, is found in associations between certain clownfish and sea anemones. Clownfish appear to be fearless and vigorously attack intruders of any size (including scuba divers) that venture too close to “their” anemone. When threatened or attacked by predators, these small fishes dive into an anemone’s stinging tentacles, where they find relative safety. Anemones apparently share in food captured by clownfish, which have been observed to drop food on their host anemone’s tentacles.

Cleaning symbiosis is another unique type of mutualism found in the marine environment. In this type of association, marine fishes and shrimp of several species “advertise” their presence by bright and distinctive color patterns or by conspicuous movements. Locations where this behavior occurs are called “cleaning stations.” Instead of being consumed by predatory fishes, these carnivores approach the cleaner fish or shrimp, stop swimming, and sometimes assume unusual postures. Barracudas, groupers, and other predators often open their mouths and gill covers to permit the cleaners easy entrance and access to the teeth and gills. Cleaners feed on epizoites, ectoparasites, and necrotic tissue that they find on host fishes, to the benefit of both species. Some studies have shown that removal of cleaning symbiotes from a coral reef results in a significant decrease in the health of resident fishes.

Parasitism

Parasitism is a category of symbiosis involving species associations that are very intimate and in which competitive interactions for resources may be both acute and costly. The extreme intimacy (rather than damage) between host and parasite is the chief difference between parasitism and other forms of symbiosis. Parasites often, but not always, live within the cells and tissues of their hosts, using them as a source of food. Some types of commensals also consume host tissue, but in such cases (pearl fishes and sea cucumbers, for example) significant damage to the host rarely occurs. Commensalism is associated with nutritional theft.

Some, but not all, parasites harm their hosts, by tissue destruction (consumption or mechanical damage) or toxic metabolic by-products (ammonia, for example). Commonly, however, damage to the host is primarily the result of the host’s own immune response to the presence of the parasite in its body, cells, or tissues. In extreme cases, parasites may directly or indirectly cause the host’s death. When the host dies, its parasites usually die as well. It follows that the vast majority of host-parasite relationships are sublethal. Some parasites are actually beneficial or crucial to the survival of their hosts. The modern and biologically reasonable definition of parasitism as an intimate type of symbiosis, rather than an exclusively pathogenic association between species, promotes an ecological-evolutionary understanding of interspecies associations. Most nonmedical ecologists and other scientists who study symbiogenesis agree that two distinct forms of intimate associations, or parasitisms (with many intermediate types), occur in nature. The most familiar are those involving decreased fitness in humans and in their domestic animals and crops.

Among animal parasites, malarial parasites, hookworms, trypanosomes, and schistosomes (blood flukes) cause death and disease in millions of people each year. The degree to which these parasites are pathogenic, however, is partly the result of preexisting conditions of ill health, malnutrition, other diseases, unsanitary living conditions, overcrowding, or lack of education and prevention. Parasites which frequently kill or prevent reproduction of their hosts do not survive in an evolutionary sense, because both the parasites and their hosts perish. Both members of intimate symbiotic relationships constantly adapt to their environments, and to each other. Over time, evolutionary selection pressures result in coadaptation (lessening of pathogenicity) or destruction or change in form of the symbiosis.

Nonpathogenic or beneficial host-parasite associations are among the most highly evolved of reciprocal interactions between species. The extreme degree of intimacy of the symbiotes (not lack of pathogenicity) distinguishes this type of parasitism from mutualism. Parasitic dinoflagellates (relatives of the algae that cause “red tides”) are found in the tissues of all reef-building corals. These photosynthetic organisms use carbon dioxide and other waste products produced by corals. In turn, the dinoflagellates (genus Symbiodinium) provide their hosts with oxygen and nutrients that the corals cannot obtain or produce by themselves. Without parasitic dinoflagellates, reef-building corals starve to death. Similar host-parasite relationships occur in termites, which, without cellulose-digesting parasitic protozoans in their gut, would starve to death.

Studying Symbiosis

Early studies of symbiosis focused primarily on the discovery and description of commensals and parasites found in humans and their domestic food animals. Malarial parasites and some of the important trematodes (flatworms known as flukes) and nematodes (roundworms) of humans were described by the ancient Greeks, and references to the guinea worm (Dracunculus medinensis) are found in the Bible, where it is called “the fiery serpent.” Some of the dietary conventions or laws observed in modern cultures have the side effect of preventing harmful symbioses, although it is still debated whether these proscriptions actually have their basis in early observations. It is widely known, for example, that pork products, if eaten at all, should never be consumed without thorough cooking. Swine are intermediate hosts for two very pathogenic human parasites, the trichina worm (the nematode Trichinella spiralis) and the bladderworm (the infective stage of the tapeworm Taenia solium). Much research in parasitology involves the description of symbiotes, particularly those of potential medical, veterinary, or agricultural importance.

The life cycles of many commensals and parasites are extremely complex and often involve two or more intermediate hosts living in different environments, as well as free-living developmental stages. Except for symbiotes of medical importance, relatively few complete life cycles have been worked out. Knowledge of life cycles remains as one of the most important areas of research in parasitology and is usually the phase of research following the description of new species.

Scientists have long recognized that “chemical warfare” (antibiotics, antihelminthics, insecticides) against microbial and animal parasites, and their insect and other vectors, provides only short-term solutions to the control or eradication of symbiotes of medical importance. Research attempts are being made to find ways of interrupting life cycles, sometimes with the use of parasites of other parasites. This research requires sophisticated ecological and biochemical knowledge of both the host-parasite relationship and the parasite mix. Studies of the parasite mix are ecological (parasite-parasite and host-parasite competition), immunological (host defense mechanisms and parasite avoidance strategies), and ethological (host and symbiote behavioral interactions) in nature. Investigators involved in this kind of research must be well-trained in many of the biological disciplines, including epidemiology (the distribution and demographics of disease).

Immunology is a promising modern research area in parasitology. Not only have specific diagnostic tests for the presence of cryptic (hidden or hard to find) parasites been developed but also vaccines may be discovered that can protect people from destructive protozoan diseases like malaria.

There is evidence that symbiotic relationships existed among species during the time of the dinosaurs. One example comes from the remains of beetle larvae captured in fossilized plant resin. It is believed these larvae consumed feathers that were detached from Theropods, or feathered dinosaurs. Other relationships may have been more parasitic. Fossil records indicate that blood parasites occurred in Cerapods, Thyreophorans, and Theropods. In 2016, a seventy-seven-million-year-old dinosaur was found with patterns in its stomach that indicated the presence of a parasitic worm. Scientists have speculated that the Tyrannosaurus rex may have struggled with bird parasites. Some fossils reveal markings with the appearance of lesions in dinosaurs similar to those in species of modern birds known to be caused by parasites.

Various symbiotic relationships have become apparent in twenty-first-century research. Coral reefs eat the algae growing on their surface if nutrient availability in their environment is low. The African turquoise killifish (Nothobranchius furzeri) has the shortest vertebrate life span, with most living around two months in the wild. Their short life span is linked to the bacteria in their gut, which is the same as that of humans but in different amounts. When altered, their lifespan changes, indicating the microbials’ relationship with life spans in vertebrates. Similarly, mice living in laboratories have gut bacteria very different from wild mice, which play a role in their immune system.

The Interrelationship of Species

All species are involved in complex interrelationships with other species that live in or on their bodies, or with which they intimately interact behaviorally or ecologically. Such interactions may play a minor role in the life and well-being of one or both of the associates, or they may be necessary for the mutual survival of both. In relatively few symbiotic relationships, one or both species may suffer damage or death. Pathogenic associations are relatively rare because disease or death of one symbiote generally results in corresponding disease or death of the other. Such relationships, which cannot persist over evolutionarily long periods, may nevertheless cause catastrophic loss of life in nonadapted host populations.

Domestic animals and animals that live in captivity often become ill if they move to some regions, such as Central Africa or Southeast Asia, because they have little or no resistance to parasites of wild species in those regions. Native species, which are the normal hosts and are not harmed, have coadapted with the parasites. This situation presents a moral dilemma to humans. In the face of human needs for space and other resources, should native animals be displaced or killed? Or should human populations proactively slow their reproductive rates? History shows that humanity has often chosen to take the former course.

Scientists have long recognized the symbiotic relationship of microbial organisms with invertebrates and plants. However, it was only with the advent of early twenty-first-century technology that their pervasive existence became known. All jawed vertebrates, at least at some point, share a direct or indirect symbiotic relationship with microorganisms. This understanding of symbiosis impacts most areas of scientific study, and research perspectives began changing in the early twenty-first century to incorporate these relationships appropriately.

The common view that animals that live inside other animals are degenerate creatures that take advantage of more deserving forms of life is understandable but inaccurate. Symbiotes are highly specialized animals, and many do not live cost-free, or aways to their hosts’ detriment. Symbiotic relationships between species have vastly increased the living world's diversity, complexity, and beauty.

Principal Terms

Commensalism: symbiotic associations based chiefly on some form of food sharing, which may also involve shelter, protection, or cleaning

Host: by convention, the larger of two species involved in a symbiotic association

Intermediate host: an animal species in which nonsexual developmental stages of some commensals and parasites occur

Mutualism: a type of commensalism in which both symbiotes benefit from the association in terms of food, shelter, or protection

Parasite: a symbiote that must live in intimate contact with its host to survive; a parasite may be pathogenic or beneficial to the host

Parasite mix: all the individuals and species of symbiotes living in a host concurrently

Reservoir host: a host species other than the one of primary interest in a given research study

Symbiosis: all forms of evolved, nonaccidental, nontrivial, interspecies associations, excluding predator-prey relationships

Symbiote: a species involved in any form of symbiotic association with another species

Bibliography

Boothroyd, John C., and Richard Komuniecki, editors. Molecular Approaches to Parasitology. Wiley-Liss, 1995.

Brusatte, Stephen. "Could Dinosaurs Have Had Symbiotic Relationships with Other Animals?" BBC Science Focus, 1 July 2023, www.sciencefocus.com/nature/symbiotic-relationships-in-dinosaurs. Accessed 13 July 2023.

Brusowankin, Aryeh. "What Is Symbiosis?" PBS, 14 July 2022, www.pbs.org/articles/what-is-symbiosis. Accessed 13 July 2023.

Caullery, Maurice. Parasitism and Symbiosis. Sidgwick and Jackson, 1952.

Margulis, Lynn. “Symbiosis and Evolution.” Scientific American, vol. 225, no. 2, 1971, pp. 48–57, doi:10.1038/scientificamerican0871-48.

Margulis, Lynn, and Dorion Sagan. Slanted Truths: Essays on Gaia, Symbiosis, and Evolution. Copernicus, 1997.

McFall-Ngai, M. "Symbiosis Takes a Front and Center Role in Biology." PLoS Biology, vol. 22, no. 4, 2024, doi.org/10.1371/journal.pbio.3002571.

Noble, Elmer, et al. Parasitology: The Biology of Animal Parasites. 6th ed., Lea & Febiger, 1989.

Osterloff, Emily. "Mutualism: Eight Examples of Species That Work Together to Get Ahead." Natural History Museum, nhm.ac.uk/discover/mutualism-examples-of-species-that-work-together.html. Accessed 13 Sept. 2024.

Paracer, Surindar, and Vernon Ahmadjian. Symbiosis: An Introduction to Biological Associations. 2nd ed., Oxford UP, 2023.

Peñalver, Enrique, et al. "Symbiosis between Cretaceous Dinosaurs and Feather-Feeding Beetles." PNAS, 17 Apr. 2023, www.pnas.org/doi/10.1073/pnas.2217872120. Accessed 13 July 2023.

Toft, Catherine Ann, et al., editors. Parasite-Host Associations: Coexistence or Conflict? Oxford UP, 1991.

Whitefield, Philip. The Biology of Parasitism: An Introduction to the Study of Associating Organisms. University Park, 1979.

Zann, Leon P. Living Together in the Sea. TFH Publications, 1980.

Zinsser, Hans. Rats, Lice, and History. Reprint, Bantam, 2000.