Symbiosis

Symbiosis is a long-term close relationship between two different species. There are three main types: mutualism (both species benefit), commensalism (one benefits, the other is unaffected), and parasitism (one benefits at the expense of the other). Symbiotic relationships are everywhere in nature. Bees pollinating flowers, bacteria living in your gut, clownfish hiding among anemones, ticks feeding on mammals, are all examples. Many of these partnerships are so close that the two species cannot survive without each other.

  • Three typesMutualism, commensalism, parasitism+/+ , +/0 , +/-
  • Famous mutualismBee + flowerBee gets nectar; flower gets pollination
  • Famous commensalismBarnacle on a whaleBarnacle gets a ride; whale unaffected
  • Famous parasitismTapeworm in a gutWorm gets food; host gets sick
  • Gut bacteria in your bodyapprox. 38 trillionMostly helpful mutualists
  • Mitochondria originAncient symbiosisA bacterium that moved in 1.7 billion years ago

Mutualism: everyone wins

Mutualism is a partnership where both species benefit. Both sides put something in and both sides get something out. Common examples:

  • Bees and flowers: bees collect nectar (food); flowers get their pollen carried to other flowers.
  • Cleaner fish and bigger fish: small wrasses and shrimp clean parasites off the bodies and gills of bigger fish, even sharks. The cleaners get food; the bigger fish get parasites removed.
  • Oxpecker birds and large mammals: oxpeckers ride on the backs of buffalo, zebra and rhino, eating ticks and other parasites. Both win.
  • Clownfish and sea anemones: clownfish shelter inside the stinging tentacles of an anemone (which does not sting them, due to a chemical mucus on their skin). The clownfish get protection; the anemone gets food scraps and protection from its own predators.
  • Lichens: a partnership between a fungus and an algae (or cyanobacteria). The fungus provides shelter and minerals; the algae does photosynthesis and shares the food.
  • You and your gut bacteria: trillions of friendly bacteria in your intestines help digest your food and produce vitamins; in exchange they get a warm, food-rich home.

Commensalism: one wins, the other does not care

Commensalism is a partnership where one species benefits and the other is not measurably affected (neither helped nor harmed).

  • Barnacles on whales: barnacles get a free ride through nutrient-rich waters; the whale is barely affected.
  • Remora fish and sharks: remoras attach to sharks and feed on scraps from the shark's meals. The shark mostly ignores them.
  • Birds nesting in trees: birds get shelter; trees neither gain nor lose much from a nest in their branches.
  • Pseudoscorpions hitching rides: tiny harmless pseudoscorpions grab onto the legs of beetles or flies to travel further than they could on their own. The host insect barely notices.

In practice, completely neutral commensalism is rare. Often the host species is slightly affected one way or the other; biologists just have not yet measured how.

Parasitism: one wins, the other loses

Parasitism is a partnership where one species benefits at the expense of the other. The parasite gets food, shelter or transport; the host loses resources, suffers damage, or even dies. Parasitism is the dark side of symbiosis but it is incredibly common: roughly 40% of all known animal species live as parasites at some point in their life cycle.

  • Tapeworms: live in the guts of mammals (including humans, occasionally) and absorb partly digested food.
  • Ticks and fleas: drink blood from larger animals.
  • Mistletoe: lives on tree branches and steals nutrients from the host tree.
  • Cuckoos: lay their eggs in other birds' nests so the unwitting host raises the cuckoo chick at the expense of its own babies.
  • Cordyceps fungi: infect insects (especially ants and beetles), take over their brains, force them to climb to a high spot, then sprout a fruiting body from the dead insect's body to release spores. The basis of many a horror story (and the videogame and TV series "The Last of Us").
Fact Your body is home to roughly 38 trillion bacteria, almost as many as your own human cells. Most of them live in your gut, where they help digest food, produce vitamins (especially vitamin K and several B vitamins) and crowd out harmful bacteria. They are not really "you", but you cannot live without them. The whole community is called your microbiome, and it is one of the largest mutualistic symbioses on Earth.

Symbiosis and evolution

Some of the most important symbioses are so old that the two species can no longer be separated. Every plant and animal cell, for example, contains tiny structures called mitochondria that produce energy. Mitochondria have their own DNA, divide on their own schedule, and behave a lot like bacteria. That is because they ARE bacteria, or rather, descendants of bacteria.

Around 1.7 billion years ago, a small bacterium got swallowed by a larger one. Instead of being digested, it took up permanent residence inside its host, providing extra energy through respiration. The two cells eventually fused into a single organism: the ancestor of every modern eukaryotic cell. Plant cells did the same trick later with photosynthetic bacteria (now called chloroplasts). These ancient mutualisms gave rise to all complex life on Earth, including us.

Did you know? Some symbioses are so specific that the two partners are completely dependent on each other and cannot survive alone. The Madagascar star orchid is pollinated only by a single species of moth with a tongue exactly 30 cm long. The fig tree is pollinated only by tiny fig wasps that crawl deep into the developing fruit. If either partner went extinct, the other one would follow.
Deeper dive: parasites that control their hosts' minds

Some of the strangest stories in biology come from parasites that have evolved the ability to control the behaviour of their hosts, often in ways that help the parasite spread to new hosts.

The parasitic fungus Ophiocordyceps infects certain ants in tropical forests. The fungus releases chemicals into the ant's brain that make the ant climb up to a high spot on a plant, bite down with its jaws and refuse to let go. The ant then dies, and the fungus grows out of its body into a long stalk that releases spores onto the forest floor below, where they infect more ants. The behaviour is completely unnatural for the ant; it has been hijacked.

The protozoan Toxoplasma gondii infects rodents and reproduces in cats. To get from rodent to cat, Toxoplasma changes the rodent's brain so the rodent loses its natural fear of cat smells and even becomes attracted to them. Cat-curious mice get eaten more often, and Toxoplasma gets to reproduce. (Toxoplasma can also infect humans, although the effects on us are subtle and still being researched.)

The hairworm grows inside crickets and grasshoppers. When the worm is ready to reproduce, it forces its host to find water and jump in, which the cricket would never normally do. The cricket drowns; the worm wriggles out into the water to mate and lay eggs.

These extreme examples of mind-controlling parasites are some of the most fascinating (and disturbing) cases of co-evolution in nature. They show just how powerful symbiotic relationships can become when shaped by natural selection over long enough time.

For broader ecology, see what is an ecosystem. For predator and prey, see predator and prey.