Symbiosis: when living together is win-win

There are many cases in nature where species team up to help each other, a behaviour known as symbiosis. Resources or services that may be scarce for one organism may be cheap and easy for another to provide.

Lactobacillus and humans

Biologist Cameron Currie from the University of Wisconsin in Madison, USA, cites bacteria – such as the Lactobacillus – cosily dwelling inside humans as a classic example. “Our own bodies have hundreds or even thousands of species of symbiotic microbes inside them – we couldn’t survive without their beneficial effects,” he said.

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Sea anemones and hermit crabs

Sea anemones (Calliactus spp) hitchhike on the back of hermit crabs, scoring a ride across the seabed and extending their tentacles to eat the crab’s leftovers. Crabs actively recruit these passengers. After poking an anemone with its pincers – causing it to release its grip from its current home – the crab holds it in place so the anemone can reattach to the crab’s own shell.

In return, the anemones fend off hungry octopuses and other predators using their barbed tentacles. The crabs return the favour by driving away creatures that eat anemones, such as starfish and fireworms.

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Goby fish and snapping shrimp

Danger! That’s what the frantically flapping tail of a goby fish says to the near-blind snapping shrimp (Alfeus spp). In a crafty collaboration, snapping shrimps construct and maintain burrows in the seabed, while the fish stands guard. During construction, shrimps leave the burrow to deposit excavated sand.

Throughout this hazardous venture, shrimps maintain constant contact with their gobies using their antennae. In some cases, gobies even hover above their shrimp, allowing it to take its load further from the burrow’s entrance. Sighting potential threats, the fish waggles its tail against the shrimps’ antennae or into the burrow entrance, warning the shrimp of the danger. In return, the fish can call the burrow home, sleeping in it with the shrimp at night and using it as a convenient bolthole in the face of peril.

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African oxpeckers

African oxpeckers (Buphagus africanus and Buphagus erythrorhynchus) feed on the backs of zebra, elephants, hippopotamuses and other large African animals. Once thought to be friendly tick-eating helpers, they’re actually vampire birds, sucking blood out of open tick-wounds. This shows how the line between symbiotic assistant and parasite can be blurred. Oxpeckers do eat ticks as well, and some animals may be happy to sacrifice a bit of blood for this service. Oxpeckers may also be tolerated because they produce a hissing scream when startled – like a personal danger alarm.

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Cells and mitochondria

Over a billion years ago, one type of bacteria ate another – or tried to. Surviving this ordeal, the prey became a permanent house guest in the wet, sheltered, food-rich environment of the predator’s body. Like an internal battery, the smaller bacteria adapted to turn food and oxygen into chemical energy for the larger one.

Eventually, by swapping segments of DNA, the two bacteria merged into a single, inseparable, complex cell. This ultimate partnership is the ancestor of all multicellular life, including our own species. These mitochondrial descendants of bacterial ancestors power each and every cell in our bodies.

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Ants and fungi

Did you think we invented agriculture? Think again. Ants have been farming fungi for around 50 million years – weeding, mulching and fertilising their crops. Fungus-farming ants originated in South America, spreading throughout the New World tropics, from Argentina to the southern USA. One well-known example is the leafcutter ant (species in the Acromyrmex and Atta genera).

They build their fungus farms in sheltered underground nests, feeding them on chewed-up leaves. The fungi is the ant’s only food. Although benefitting from free food and protection, these species of fungi occasionally escape enslavement and become free-living.

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Coral and algae

Corals have a fairly easy life: they just sit back and let specially adapted algae make much of their energy for them. The algae live in the coral, feasting on waste products. Crucially, the algae can photosynthesise, using sunlight to turn carbon dioxide into sugars. After taking what it needs, the algae produces a little extra sugar for the coral. Completing the cycle, carbon dioxide from the coral is used by the algae.

The partnership may be the coral’s undoing, however: worldwide, many species are losing their algae due to climate change. In a process called coral bleaching, algae are ejected when the temperature gets too warm, leaving ghostly-white reefs of dead coral in its wake.

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Cleaner fish

Fish don’t go to the health centre. Instead, they frequent ‘cleaning stations’ – neutral zones where small cleaner fish – including wrasses, catfish and gobies – wait for larger clients. When visiting a station, client fish – such as parrotfish, damselfish and sharks – adopt a distinctive pose, signalling they want to be cleaned (and won’t eat the cleaner).

Cleaner fish then gorge themselves on parasites, mucous and dead tissues from the surface of their client. In addition to a spick and span skin, client fish enjoy a good tickle. It’s partly this rewarding sensation that stops the client fish gobbling up the cleaner fish.

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Bees and orchids

Male orchid bees (also known as Euglossine bees) collect perfume from a wide variety of South- and Central American neotropical orchids, and turn this into chemical signals called pheromones. It’s a messy procedure, which involves scraping brush-like foreleg tips all over orchid flowers before transferring the heady scent to storage sacs on the back legs.

In the process, orchid pollen is conveniently attached to the bee’s back, where it can subsequently rub off onto female parts of other flowers. The process is vital for orchid reproduction. Scientists aren’t yet sure what orchid bees do with their perfume-based pheromones. The potent concoction may attract females, be used to mark territories, or it may just smell awfully good.

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Related reading: Symbiotic connections of plants and fungi