Planet Earth is inhabited by millions of species—at least! Because different species often inhabit the same spaces and share—or compete for—the same resources, they interact in a variety of ways, known collectively as symbiosis. There are five main symbiotic relationships: mutualism, commensalism, predation, parasitism, and competition.
To explore these relationships, let’s consider a natural ecosystem such as the ocean. Oceanic environments are known for their species diversity. Imagine you are on a diving expedition to explore the worlds beneath the waves. If we were in the warm waters of the Pacific or Indian Oceans, we’d likely spot an excellent example of mutualism: the relationship between clownfish and sea anemones. In a mutualistic relationship, both species benefit. Sea anemones live attached to the surface of coral reefs. They trap their prey with stinging cells called nematocysts, which are located on their tentacles. Nematocysts release toxins when a small animal contacts an anemone’s tentacle. This paralyzes the stung animal, allowing the anemone to easily bring the animal into its mouth for ingestion.
While other fish succumb to these toxic stings, clownfish secrete a substance in the mucus covering their bodies that suppresses the firing of nematocysts. This allows the clownfish to swim comfortably between the tentacles of anemones, creating a protected environment in which potential predators are killed off by anemone stings. This clearly benefits the clownfish, but how about the sea anemones? The brightly-colored clownfish attract other fish looking for a meal. These unsuspecting would-be predators are then caught and eaten by the anemones.
As we continue in our imaginary deep-sea voyage, we may observe the commensalistic relationship that exists between barnacles and humpback whales. Commensalism happens when one species lives with, on, or in another species, known as the host. The host species neither benefits nor is harmed from the relationship. In our imagined example, various species of barnacles attach themselves to the skin of whales. Scientists have not discovered the exact mechanism by which barnacles are able to do this, but it does not appear to bother the whales. How do the barnacles benefit from this unlikely relationship? The huge whales transport the tiny barnacles to plankton-rich waters, where both species feast upon the abundant microorganisms that live there.
Of course, some symbiotic relationships do cause harm. In predation, one species (the predator) hunts and kills another species (the prey). One of the better studied predators in the oceans is the orca, or killer whale. Found in every ocean on Earth, orcas are categorized as apex predators. Though they hunt and eat numerous other organisms—over 140 species—orcas themselves are not hunted by any other predator. In other words, they are at the top of the food chain!
Another harmful relationship is parasitism. This happens when one species (the parasite) lives with, on, or in a host species, at the expense of the host species. Unlike in predation, the host is not immediately killed by the parasite, though it may sicken and die over time. Examples of common parasites found in the ocean include nematodes, leeches, and barnacles. That’s right—though barnacles exist commensally with whales, they are parasites for swimming crabs. A barnacle may root itself within a crab’s reproductive system. While the crab does not die from this interaction, its reproductive capabilities are greatly diminished.
The last example of symbiosis we will explore on our imaginary dive is competition—the struggle among organisms for the same limited resources in an ecosystem. Competition can happen between members of the same species (intraspecific competition) and between different species (interspecific competition). An example of interspecific competition in the ocean is the relationship between corals and sponges. Sponges are very abundant in coral reefs. If they become too successful, however, they take needed food and other resources from the corals that make up the reef. Sponges may outcompete corals for resources in the short term, but if too many corals die, the reef itself becomes damaged. This is bad for the sponges, which may themselves begin to die off until the reef is balanced again.
Symbiotic relationships can be useful measures of an ecosystem’s health. For example, large tracts of coral reefs are severely damaged or dead because of recent increases in ocean temperature due to climate change. The temperature increase induces coral to expel the algae that live mutualistically within them. Without their algae, the coral turn white and die. This loss of symbiosis is an early sign of declining coral health and speaks to the importance not only of studying symbiosis within marine environments, but also of examining the negative impacts that humans can have on these interactions. In the words of National Geographic Explorer Sylvia Earle: “We need to respect the oceans and take care of them as if our lives depend on it. Because they do.”
In nature, species will sometimes form unexpectedly close bonds and work to their mutual benefit.
Symbiotic relationships are the close associations formed between pairs of species. They come in a variety of forms, such as parasitism (where one species benefits and the other is harmed) and commensalism (where one species benefits and the other is neither harmed nor helped).
Mutualism is a type of symbiotic relationship where all species involved benefit from their interactions. While mutualism is highly complex, it can be roughly broken down into two types of relationship. In some cases, the species are entirely dependent on each other (obligate mutualism) and in others, they derive benefits from their relationship but could survive without each other (facultative mutualism).
Here are eight examples of mutualistic relationships.
1. Pistol shrimps and gobies
Gobies and pistol shrimps stay close together when they are outside their shared burrow © Francesco_Ricciardi/ Shutterstock
True gobies (Gobiidae) are a family of about 2,000 species of fishes. Most of them are quite small and live on the seafloor. In some cases, gobies will form mutualistic relationships with pistol shrimps of the family Alpheidae.
Pistol shrimp are burrowers, digging holes in the sandy seafloor that they will maintain and sometimes share with a goby. Outside the burrow, the pair stay close together, often with the shrimp maintaining physical contact by resting its sensitive antennae on the fish.
When the goby spots a potential predator, it uses chemical cues and bolts for cover in the shared burrow. The shrimp relies on these tactile and chemical cues to know when it needs to hide, too. When the goby is active, it signals to the shrimp that it's relatively safe to be outside the burrow.
A 2019 study showed that, as predicted by their role as lookouts, the goby - in this case the fierce shrimpgoby (Ctenogobiops feroculus) - was always first to venture outside. It seems that the shrimp's decision to leave the safety of its home only begins once its partner has exited the burrow.
The shrimps are also thought to benefit from their relationship with the fish through an increase in food, such as the fish's faeces or any parasites on its body.
2. Aphids and ants
Ants feed on the honeydew produced by aphids and may offer them protection in return © Jmalik at English Wikipedia via Wikimedia Commons (CC BY-SA 3.0)
Aphids are little sap-sucking insects that secrete honeydew, a sugary liquid that is the waste product of their diet. Many aphid species are known to engage in a mutualistic relationship with ants that feed on the honeydew by 'milking' the aphids with their antennae.
In return, some species of ants will protect the aphids from predators and parasites. Some will move aphid eggs and nymphs underground to their nest, which ultimately makes harvesting their honeydew more efficient - like an ant equivalent of a dairy farm.
However, some aphids have evolved to take advantage of the honeydew-seeking ants. Paracletus cimiciformis aphids come in two morphs: the round morph, which is milked, and a flat, ant-mimicking morph. When the ants carry the flat individuals to their brood chamber, the aphids will drink the body fluid of the ants' larvae.
Honeydew is produced by a variety of insects, including scale insects and some caterpillars, and is appealing to species other than ants. In Madagascar, some geckos have been observed lapping up the honeydew produced by plant hoppers. This may be mutualism, with the gecko's presence keeping predators of planthoppers away, but scientists aren't sure yet.
3. Woolly bats and pitcher plants
Pitcher plants are carnivores that use nectar at the rim of their tube-like structure to attract prey such as insects and small vertebrates. A slippery substance at the rim causes these animals to fall into the digestive juices contained in the plant's equivalent of a stomach.
While you might think it would be prudent for animals to avoid these plants where possible, some bats voluntarily clamber inside them.
Woolly bats are known to roost in Nepenthes hemsleyana, a tropical pitcher plant found in Borneo.
While the bat gets a hidey-hole to rest in, the plant benefits by catching the guano (faeces) that the little mammal produces. This provides the plant with the nutrients it needs to survive.
A similar relationship occurs between tree shrews and another Bornean pitcher plant, Nepenthes lowii. The shrews climb onto the pitcher's rim to feed on the nectar. In return, with the plant's hollow body acting a bit like a toilet bowl, the shrews drop their nutritional faeces into the plant's stomach.
Find out more about carnivorous plants.
4. Coral and algae
Corals may look like rocks or plants, but they are actually marine animals. The bright colours of reef-building corals come from the zooxanthellae algae they have a mutualistic relationship with.
Coral starts life as a tiny, free-swimming larva which eventually fixes itself to a hard surface and metamorphoses into a polyp. The polyp replicates and expands to form a colony by producing many identical polyps, growing one on top of each other and secreting a hardened skeleton around themselves.
As corals grow, they acquire zooxanthellae from their surrounding environment. The coral provides shelter and essential nutrients for the zooxanthellae to use during photosynthesis, while the zooxanthellae produce synthetised sugars, which the coral feeds on, and oxygen as a by-product.
Pollution and heat stress can cause corals to expel their algae which turns the coral ghostly white - this is known as coral bleaching. Going too long without algae can be fatal to the coral, as it usually cannot grab enough food particles from its surroundings to fulfil its energy demand.
5. Oxpeckers and large mammals
Oxpeckers feed on parasites, such as ticks and blood-sucking flies © AndreAnita/ Shutterstock
There are two species of oxpecker: the red-billed oxpecker (Buphagus erythrorhynchus) and yellow-billed oxpecker (Buphagus africanus). Both regularly spend time clinging to large grazing mammals such as wildebeest, rhinos and zebras.
The birds pick at parasites on the mammal's body, including ticks and blood-sucking flies. This may help keep the mammal's parasite load under control, and the birds get an easy meal.
Like a number of other species, oxpeckers will raise the alarm and warn their hosts of impending danger. People have observed that the birds will help hosts such as rhinos (which are short-sighted) evade humans.
However, mammals and oxpeckers may not be a perfect example of mutualism, as the birds can harm their hosts. The birds remove parasites and seem to prefer hosts with large numbers of them, but they will also dig into wounds. While the mammals appear relatively tolerant of this behaviour, it's not beneficial to them.
6. Clownfish and anemones
It's thought that mucus plays a role in protecting a clownfish from an anemone's sting © cbpix/Shutterstock
Anemones are flowerlike marine animals with neurotoxin filled stinging tentacles. They use these to help them subdue their prey, which are mostly plankton, crabs and fish, though larger species take larger prey such as starfish and jellyfish.
Anemones associate with many fish species, but they are particularly close with one group. Clownfish, also known as anemonefish, are immune to anemone stings, though scientists aren't exactly sure how. It's thought that the layer of mucus on the fish's body is involved in protecting them. This means clownfish can safely nestle into the anemone's tentacles to hide from predators.
In return, clownfish help the anemone in multiple ways. They keep the anemones free of parasites and provide them with nutrients through their faeces, which may also stimulate the growth-beneficial symbiotic algae within the anemone. Clownfish may also drop food onto the anemone and also drive off anemone-eating intruders that stray too close. It's also thought that the movement of clownfish helps to circulate the water, and in turn helps to oxygenate the anemone. It's possible that the bright colours of clownfish also helps to lure meals of small animals to within reach of the anemone.
Anemones that harbour clownfish appear to have faster growth rates, higher rates of asexual reproduction and lower mortality than those without fish.
7. Honeyguides and humans
Greater honeyguides and humans have a relationship that strecthes back through many generations © Dominic Sherony via Flickr (CC BY-SA 2.0)
The eggs, larvae and beeswax contained in bee nests are a key food source for greater honeyguides (Indicator indicator). One of the ways these birds gain easy access to a nutritious meal is by leading other honey-coveting species to the nest and allowing them to do the hard work of breaking into it.
The human-honeyguide relationship is the best-documented of these partnerships. The wild honeyguides recruit people with a demanding call, indicating that they have found a bee nest. The honey-hunting humans reply with calls passed down through generations and follow the bird.
When they reach the nest, the humans subdue the bees, such as with smoke, break into the nest and help themselves to the sugar-rich honey contained within. The Hadza people of Tanzania are one group known to work with honeyguides. It has been estimated that up to 10% their diet is acquired with the help of the birds.
With the bees dispatched and the humans satisfied, the honeyguides are left to dine on the beeswax, eggs and larvae left behind.
8. The senita cactus and senita moth
When the sun sets on North America's Sonoran Desert, the night-blooming flowers of senita cacti (Lophocereus schottii) are visited by tiny senita moths (Upiga virescens).
The female moths collect pollen on specialised abdominal scales and transfer it from flower to flower, pollinating cacti as she goes. The senita moth is the only nocturnal pollinator of this cactus and is responsible for 75-95% of its pollination. The rest is attributed to other insects that are active during the day.
During her visits, the female moth will lay one egg on a flower petal. When the flower closes and the larva hatches, it will bore into the top of the developing fruit, spending about six days feeding on the seeds and fruit tissue.
The moth larvae don't eat all the seeds or fruit - it's been found that they only destroy about 21% of the developing fruit, which means the cactus can continue to prosper.
There are several similar mutualistic relationships, such as yuccas and yucca moths, figs and fig wasps, and Phyllanthaceae and Epicephala moths. Senita moths differ from these in that although the relationship is highly specialised, they are not the sole pollinator of their host plant, yet their relationship with the cactus clearly plays an important role in the cactus's survival.