Symbiosis in the animal kingdom. Mutualism, commensalism and parasitism

Sometimes, completely different species or organisms forge close or interdependent relationships, to the advantage of at least one of the parties involved. This interaction is called ‘symbiosis’.

It’s not easy to come up with a narrow definition of symbiosis. In fact, symbiotic relationships can be divided into several groups:

Clownfish swimming among anemones

Symbiosis

All organisms or species involved in the relationship not only benefit from it, but it’s also essential or indispensable for their survival.

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Mutualism

All parties involved in the relationship benefit from it, as it improves their biological fitness, but it’s not strictly necessary for their survival.

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Commensalism

Only one party involved in the relationship benefits from it, without harming the other one.

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Parasitism

When a species or organism (parasite or host) benefits from the relationship at the expense of the other party (host), usually harming them.

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Apart from groups based on behaviour, symbiosis may also have different levels of integration: from the initial level, in which different organisms or species benefit from coexisting, to a higher level, where they end up sharing genetic material. The latter case can explain the appearance of animals and plants on our planet, but that’s a whole other debate. In this article, we’ll go through some fascinating examples of symbiotic relationships in the animal kingdom.

Ants and aphids

A clear example of mutualism is the relationship established by some types of ants with aphids. The latter secrete a dense liquid that is very rich in carbohydrates called ‘honeydew’. Ants feed on it, so they literally herd the aphid colonies to protect them from potential predators. By doing so, they allow aphids to procreate in a safer environment.

Termites, protozoa and bacteria

Termites feed on wood and other organic substances, but their bodies aren’t ready to digest them. To do so, they need the help of another species: protozoa. These unicellular organisms live in the stomach of termites and are in charge of digesting wood cellulose, which they feed on. Protozoa also have bacteria that transform the remaining cellulose into nitrogen, which is then absorbed by the termites. Additionally, the termite offspring consume the feces of adult termites to gain the protozoa and bacteria they need. Three parties are involved in this relationship, and none of them would survive without the rest.

Sloths, algae and moths

Sloths are extremely slow animals. They’re most vulnerable when they descend to the ground to defecate, only once per week. Why do they put themselves at such high risk? A type of moth that inhabits their fur takes advantage of the descent to the ground to lay its eggs in the sloth’s droppings. After they hatch, the new moths fly up to the crowns of the trees and settle in the fur of sloths. They then produce a natural fertilizer that boosts nitrogen levels, allowing algae and fungi to grow on the fur. Sloths have a number of cracks in their fur; water accumulates in them, allowing the algae to grow. Sloths feed on these organisms to supplement their low-energy diet.

A sloth with his calf

Crocodiles and Egyptian plovers

Another clear example of mutualism is the relationship between these two species. Crocodiles have the strongest and fastest bite in the animal kingdom. To keep their most precious asset in top condition, they’re constantly replacing the 80 teeth in their snout (they do this 2 or 3 times per year). The food debris that accumulates between their teeth can cause infections and severe health issues, so they allow plovers to feed on this lodged food.

Sharks and remoras (sucker fish)

The relationship between sharks and remoras is a clear example of commensalism. Instead of having a dorsal fin, remoras have developed a powerful sucker that adheres to the body of sharks, from which they obtain food and protection. This allows sharks to get rid of certain parasites that live on their skin, but they get a lot less out of this relationship than remoras do.

Spiny-tailed lizards and the fat-tailed scorpion

Desert lizards are extremely territorial when it comes to their dens. These are cool, shady places that are often raided by foxes and other predators. Fat-tailed scorpions like to be in the shade and offer the lizards protection from predators in exchange for living with them in their dens. This is another clear example of mutualism, from which both species have obtained an advantage that’s key for their survival.

Goby fish and blind shrimp

Goby fish have excellent vision, while blind shrimp, as their name implies, can barely see. Shrimp keep their dens clean and in perfect condition, and they share them with goby fish for protection. In exchange, the goby fish will stay by the shrimp at all time, and will give a slight tail flick as a sign for them to hide when it sees a potential threat.

Narrow-mouthed toads and tarantulas

Tarantulas usually hunt small toads, but they make an exception for narrow-mouthed toads. These small amphibians keep tarantula eggs pest and insect-free, in exchange for their protection and for shelter. Scops owls usually hunt narrow-mouthed toads, but they’ll think twice if there’s a tarantula guarding their prey.

Tarantula and narrow-mouthed toad. BBC Earth.

Snails and green-banded broodsacs

That of green-banded broodsacs is one of the most shocking examples of parasitism in the animal kingdom. This parasite invades the body of the snail and grows until it adopts a tubular or worm-like shape inside the animal’s antennae. The thick antennae develop an eye-catching colour and make a pulsating movement to attract birds. The parasite also invades the snail’s brain, forcing it to wander around the leaves like a zombie in plain sight, without the ability to hide. When a bird eats one of these antennae, the parasite lays its eggs in the bird’s digestive system. The eggs will then infect other snails, as they feed, amongst other things, on bird feces. If a snail is lucky enough to survive, it’ll grow a new infected antenna.

Caterpillars and Glyptapanteles wasps

The relationship between parasitic species can be extremely cruel, as the case of the Glyptapanteles wasp shows. This wasp introduces its eggs in the body of a butterfly caterpillar. When they hatch, the larvae feed on the caterpillar’s fluids – without killing it – until they reach a considerable size. Then, they exit its body by making a hole through its skin. Once outside, they form a cocoon to undergo metamorphosis. But the nightmare doesn’t end there. The wasp previously introduced part of its DNA in the caterpillar, forcing it to follow its commands. The caterpillar will remain by the larvae until they complete their metamorphosis, even using its silk to protect them. Once the process is complete, the caterpillar dies of dehydration and starvation.

There are many other examples of symbiosis in the animal kingdom, such as clown fish and anemones, buffaloes and oxpeckers, moray eels and shrimp, hippos and black sharkminnows, scops owls and blind snakes, flukes and certain types of fish, etc. These are some of the best-known examples and they help us to understand how species can adapt in order to survive, even if that means turning to beings that are completely different to them.

 

Translated by Carlos Heras