Tag Archives: symbiosis

What are symbiotic relationships: nature’s matchmaking

Common Clownfish (Amphiprion ocellaris) swimming in their Sea Anemone (Heteractis magnifica) home on the Great Barrier Reef, Australia. Credit: Wikimedia Commons.

There are millions of different species on Earth, many of which have to share the same habitat and resources. Naturally, these creatures have had to find a way to coexist without driving each other to extinction. Earth’s biodiversity hangs in an intricate but delicate web of dependencies in which some animals prey on others and are, in turn, lunch for others higher up the food chain. But the predator-prey paradigm is just one example of many types of relationships animal and plant life can have.

When two or more unlike organisms live together, biologists refer to this relationship as symbiosis (from the two Greek words for “with” and “living”). This ecological relationship is sometimes, but not always, beneficial to both parties. Perhaps the best word to describe symbiosis is balance, even when the relationship between the two different species sounds highly dysfunctional and one-sided. That’s because the ecosystem at large is balanced thanks to symbiosis.

The main types of symbiotic relationships are:

  • Mutualism — the symbiotic relationship that is mutual to both parties. Think win-win.
  • Commensalism — when only one party seems to benefit, but the other species doesn’t really lose either. Think win-neutral.
  • Parasitism — when one species benefits at the expense of the other. Think win-lose.

Mutualism symbiosis

Credit: Pixabay.

Mutualism symbiosis is a relationship between two different species that cooperate in order to access benefits they wouldn’t be able to on their own. It’s what most people usually think of when they hear the word symbiosis. A great example of mutualism symbiosis is that between the clownfish and sea anemones.

Sea anemones are very shrewd predators. While most predators seek out and hunt their prey, sea anemones are campers. They live a static existence, attached to rocks or corals in the sea, catching food that passes by them using their tentacles. These tentacles have stinging cells called nematocysts that release powerful toxins that paralyze prey, making them easy pickings. Once injected with the paralyzing neurotoxin, the prey is guided into the mouth by the tentacles.

Plankton and small fish comprise the main diet of sea anemones, but not clownfish. These fish secrete a substance in the mucus that covers their body that makes them immune to the anemones’ venom. So the clownfish naturally spend a lot of time swimming between the tentacles of the anemones, where they are protected from potential predators that get stunned by the sea anemones.

The brightly colored clownfish are quite conspicuous, so they attract other small fish, which are then caught and devoured by the anemones. One organism provides shelter, while the other brings in food for the service.

On land, another great example of mutualistic symbiosis is that between bees and flowering plants. The flowers provide bees with sweet nectar and pollen, which the worker bees collect as food to feed their colony. In return, bees spread the pollen from flower to flower, allowing the plants to reproduce through a process known as pollination.

Commensalism symbiosis

Cattle egrets (Bubulcus ibis) stand on the backs of bovines where they pick off parasitic bugs like ticks, fleas, and flies. The cows, in turn, disturb the ground revealing grasshoppers or other insects that are then eaten by the egrets. The two organisms form a commensalism symbiosis. Credit: Wikimedia Commons.

Some organisms are just freeloaders, piggybacking on other species with nothing to show in return. On the bright side, the organism that has nothing to gain isn’t harmed either. Scavengers that trail predators to eat the remains of their kill are an example of such a relationship.

Commensalism can be further broken down depending on where the symbiont lives with, on, or inside another species, which plays the role of a ‘host’. We can thus think of four distinct types of commensalism:

  • Inquilinism, where one symbiont depends on the other for shelter. Think of birds living inside a tree hole.
  • Metabiosis, where one organism forms the habitat of another. A prime example of this type of relationship is the hermit crab that uses the shells of dead gastropods as their home, which doubles as a protective shell when they carry it around. Bacteria that colonize our guts also fall within this category, although some may be considered mutualists since they help break down food and help the host access nutrients, among other important health benefits they may offer.
  • Phoresy refers to organisms that attach to others for transport. Barnacles, for instance, cling to whales, which transport the tiny symbiot to plankton-rich waters, where both organisms can feast. There isn’t evidence to suggest that the whales are harmed or lose anything in particular from this one-sided deal though.

Parasitism symbiosis

Sometimes, symbiotic relationships cause harm. This happens when the symbiont (or parasite in this case) benefits from the symbiotic relationship, at the expense of the host, which is harmed.

Parasites may live inside the host’s body (endoparasitism) or on its surface (ectoparasitism). For instance, tapeworms that live inside the intestines of other animals where they consume partially digested food (and thereby deprive the host of nourishment) are endoparasites. Head lice that live on the scalp, where they suck blood and cause itching on the host, are ectoparasites.

Obligate and facultative symbiosis

In many symbiotic relationships, the host and symbiont may derive benefits or inflict harm that wouldn’t have been possible absent the relationship. But if the symbiosis didn’t occur, each could mind their own business and still find a place in the ecosystem independently of one another. This is known as a facultative symbiosis.  For example, there are many tiny insects that live in bird nests, where they consume waste that the birds produce, keeping the nest clean and decreasing the chance for the build-up of bacteria and disease. In the process, they get a free meal from the birds and the birds get free house-cleaning services. But each organism could theoretically survive without one another.

In contrast, there are some symbiotic relationships in which the symbionts are entirely dependent on each other for survival. For instance, lichens consist of fungal and photosynthetic symbionts that cannot survive without one another. Although the fungal and photosynthetic organisms do occur independently in nature, their physiology and morphology change drastically once they come together to form the lichen. In this particular relationship, the fungus ‘farms’ the autotrophic photosynthetic organism by encapsulating it. The photosynthetic symbiont harnesses the sun to produce food for both parties, while the fungus retains water and provides a footing from which the photobiont can absorb nutrients. These relationships are called obligate symbioses and typically develop over time as each organism adapts to the benefits of depending on each other.

All types of symbiosis can be either obligate or facultative. The lichen example is an obligate mutualistic symbiosis, but many parasites are host-specific and as a result have co-evolved with their hosts, without whom they cannot survive. This also means that it’s in the parasite’s best interest not to completely debilitate its host, otherwise, there will be no one left to exploit. Parasites are huge pests, but they’re almost never fatal.

Almost every creature has at least one symbiotic relationship (think of gut bacteria for example), which makes symbiosis essential to the planet’s ecosystems. 

Researchers look into reviving bleached corals using ‘non-preferred’ algal symbiotes

New research is looking into what makes algae ‘move in’ with their coral hosts — and why the partnership can turn sour, both under normal conditions and when temperatures increase.

Coral polyp.

Coral polyps extending to feed.
Image credits Егор Камелев.

What we know as corals aren’t really alive. They are large exoskeletons built by tiny animals called polyps. Tiny but industrious, these polyps work tirelessly to create the world’s wonderfully colorful coral reefs. A polyp has a sac-like body that ends in a mouth crowned with stinging tentacles called nematocysts (or cnidae). These animals filter calcium and carbonate ions from seawater that they combine to form the limestone (calcium carbonate) they use to build corals that protect their soft, defenseless bodies. If you ever get a chance to visit a coral reef at night, you’ll see these polyps extend their tentacles out to feed.

However, none of this would be possible without the help of various species of single-celled algae we call zooxanthellae, a type of dinoflagellate. These algae live in symbiosis with the polyps, taking up residence inside their cells in a mutually-beneficial relationship: the algae produce nutrients via photosynthesis, while polyps supply the raw materials. The algae are also what gives coral their dazzling colors, which brings us neatly to the subject of:


Warmer mean ocean temperatures (due to anthropic climate change) can apply so much thermal stress on the polyps that they ‘evict’ their symbiotic bacteria in a phenomenon called bleaching. We refer to it this way because, as the algae get expelled, the coral skeletons revert to their natural color: bone-white. If the bleached coral is not recolonized with new algae soon, however, it can die.

“We’re interested in understanding the cellular processes that maintain those preferential relationships,” says Arthur Grossman from the Carnegie Institution for Science, one of the paper’s co-authors.

“We also want to know if it’s possible that more heat tolerant, non-preferred algae could revive bleached coral communities even if the relationship is less efficient.”

The team focused on sea anemones, which are actually closely related to coral (they’re both part of the phylumCnidaria). Sea anemones also host algae, but are easier to work with than corals. The researchers looked at the differences in cellular function that occur when Exaiptasia pallida, a type of anemone, is colonized by two different types of algae — one native strain that is susceptible to thermal bleaching (Breviolum minutum), the other non-native but more resistant to heat (Durusdinium trenchii).

“In this study we hoped to elucidate proteins that function to improve nutrient exchange between the anemone and its native algae and why the anemone’s success is compromised when it hosts the non-native heat resistant algae,” Grossman said.

The anemones colonized by the native algae strain expressed heightened levels of proteins associated with the metabolism of organic nitrogen and lipids. Both are nutrients that get synthesized through the algae’s photosynthetic activity. These anemones also synthesized a protein called NPC2-d, which is believed to underpin the cnidarians’ ability to take in algae and recognize them as a symbiotic partner.

Anemones colonized by non-native algae species expressed proteins associated with stress, the team explains. This is likely indicative of a less-than-ideal integration between the metabolisms of the two organisms, they add.

“Our findings open doors to future studies to identify key proteins and cellular mechanisms involved in maintaining a robust relationship between the alga and its cnidarian host and the ways in which the metabolism of the organisms are integrated,” Grossman concluded.

The results can be used to further our understanding of the biochemical mechanisms that facilitate successful interactions between algae species and the corals that house them. Researchers can explore the metabolic pathways identified in this study to potentially find ways to merge corals with more heat-resistant species — all in a bid to help them both survive in the warmer world we’re creating on Earth.

The paper “Proteomics quantifies protein expression changes in a model cnidarian colonised by a thermally tolerant but suboptimal symbiont” has been published in the journal Nature.

Lichens actually comprise a threesome, not a partnership

When the nature of lichens was discovered 140 years ago, they became the most prominent example of symbiosis, a term that defines a mutually beneficial relationship between two dissimilar organisms.

Image credit Pixabay

Image credit Pixabay

In the case of lichen, the filaments of a single fungus create protection for photosynthetic algae or cyanobacteria, which provide food for the fungus in return. However, a new study reveals that there is actually a third organism involved in this relationship – a yeast that likely provides the structure for “leafy” or “branching lichens.”

“These yeast are sort of hidden just below the surface,” said John McCutcheon, a genome biologist at the University of Montana, and senior author of the study. “People had probably seen these cells before and thought they were seeing something else. But the molecular techniques we used happened to be especially good for spotting the signal of a separate organism, and after years of looking at the data it finally occurred to us what we were seeing.”

McCutcheon’s team made the discovery after studying two lichen species obtained from Missoula, Montana mountains – Bryoria fremontii and B. tortuosa. Despite B. tortuosa possessing a yellow color due to the presence of vulpinic acid, genetic tests revealed identical fungus and alga in both species. However, they also discovered the genetic signature of a third species – a basidiomycete yeast – in both species, although it was more abundant in B. tortuosa.

Additional testing of 56 different lichens from around the world revealed that each one has its own variety of basidiomycete yeast, suggesting that lichens actually comprise a threesome, not a couple, essentially rewriting 150 years of biology.

The team believes that this newly discovered yeast could play a role in creating the large structures seen in macrolichens, which would explain why these particular lichens are hard to grow in the lab when using just a fungus and alga.

“This doesn’t prove that they’re necessary to create the structure of the macrolichens, or that they do anything else for that matter,” McCutcheon said. “But its early days. It took a lot of work just to discover that they were there. We’re interested if the yeast is making these important compounds, or possibly enabling the other fungus to make them. We don’t know, but it’s the obvious next question.”

Journal Reference: Basidiomycete yeasts in the cortex of ascomycete macrolichens. 21 July 2016. 10.1126/science.aaf8287

Unique, hybrid creatures discovered off the coast of Costa Rica

Symbiosis is an absolutely fantastic adaptation in itself, but this case of deep sea symbiosis takes it to a whole new level: basically, a hermit crab uses an anemone as shell; scientists discovered this in a rare place, where to different extreme environments meet.

Researchers discovered a junction of two strange environments off the coast of Costa Rica: hydrothermal vents and cold methane seeps coexist side by side in a swath of the the deep sea, and guess what? Life found a way to adapt two both these environments, as biologists found a swarm of unknown species living there. They found this amazing habitat during a submersion with the Alvin submarine, during a dive in a spectacular tectonic setting, where an underwater mountain is moving under a tectonic plate.

“The most interesting aspects of this site are the presence of vent-like and seep-like features together, along with a vast cover of tubeworms over large areas and a wealth of new, undescribed species,” lead researcher Lisa Levin, of the Scripps Institution of Oceanography at the University of California San Diego, said in a statement.

Hydrothermal vents typically occur deep in the sea where rifts or just cracks in the seafloor through which volcanic substances rich in minerals spew in the water, creating a hydrothermal (hot water) environment, whereas cold seeps, as you can guess by the name, are far less intense and colder environments where hydrocarbon substances (such as methane) cover a large area.

The team coined a new term to describe the environment, calling it a “hydrothermal seep” – they published it in the March 7 issue of the journal Proceedings of the Royal Society B. Although they previously sent unmanned vehicles to the area before, it was only when they went on a manned mission that they found these remarkable creatures.

“It was not until human eyes saw shimmering water coming from beneath a large tubeworm bush that we really understood how special Jaco Scar is,” she said.

Recent expeditions have uncovered other spectacular environments in hydrothermal areas, like in Antarctica or in the Caribbean, where scientists found swarms of eyeless shrimp.

Picture source

Ancient symbiosis between animals and bacteria discovered

As you probably (and should) know already, symbiosis is a close interaction (often long term) between different species, both of which have something to win from this deal. But symbiosis between animals and bacteria… that’s definitely something new.

Marine sandy bottoms

This kind of environment seems dead, desert-like and empty, but if you were to take a closer look, particularly close enough to see the fauna in the space between sand grains, you’d definitely be surprised. A diverse and special fauna thrives in this environment, including bacteria and protozoa, but also numerous animal phyla, some of which are unique to this habitat. Out of these species, perhaps the most interesting is Paracatenula, a mouthless and gutless flatworm, found from tropical areas to the Mediterranean.

Ever since this species was discovered in the 1970s, mystery surrounded it; how on Earth could it feed, with no mouth and no gut ? In order to find the answer to this riddle, researchers had to dig deeper, in the deep ocean hot vents, where giant mouthless tubeworms were found, similar in many ways with Paracatenula. What they found was that these worms live in symbiosis with intracellular bacteria that oxidize reduced sulfur compounds.

Paracatenula and bacteria

Paracatenula indeed has symbionts which are sulphur-reducing bacteria, but even more specifically it’s about Alpha-Proteobacteria, which is a class that includes mitochondria, which are basically the power plants in the cells of all higher organisms.

This symbiosis however is taken to a whole new level; biologists have calculated that bacteria amounts to about half of the tissue of the Paracatenula. Basically, they are all one big team – and they have been so for no less than 500 million years, dating way, way before the dinosaurs were ruling the Earth.