Perhaps the most common form of symbiosis in any ecosystem is mutualism. In these associations, both creatures benefit from one another in a number of ways. A common example is the case of clownfish, which dwell within anemones in order to be protected from predators. The anemone benefits from the clownfish by gaining nutrients from their excrement.

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In the deep sea, mutualism occurs on a much grander scale, and plays a fundamental role in the structure of the entire deep sea ecosystem. This is due to the mutualism that forms at deep sea hydrothermal vents, where chemosynthetic bacteria are the primary producers. Here, the water that wells up from below the sea-floor is rich in minerals for the bacteria to convert into food and energy. To be specific, these compounds are hydrogen sulphide and methane. Both are toxic to animals, but certain specialised bacteria are able to use the chemicals to make the organic matter that animals rely on. This process is known as chemosynthesis. Think of it as an alternative to photosynthesis, but where nutrients are made by converting chemical energy rather than light energy.

Gigantic, odd-looking tube worms grow in clusters around vents and cold seeps; unusual creatures, for they have no stomach, gut, or mouth. This is because they instead contain a trophosome, a large organ containing billions of the chemosynthetic bacteria. The tube worms are specifically adapted to being a symbiont in this relationship; the stable nature of the vents has allowed the worms to evolve over millions of years to possess these important physical characteristics - including a long ‘root’ that collects hydrogen sulphide from the sediment, and ‘plumes’ that collect oxygen from the water. Both the oxygen and the chemicals are transported to the long, central trophosome, where the bacteria use them and carbon dioxide to produce organic molecules. Without the tube worm’s trophosome providing them with a constant supply of these key ingredients, the single-celled bacteria would have a hard time gathering it all in one place.

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100% of the tube worm’s nutrition is provided by the bacteria undergoing this process. Symbiosis is so important to these organisms, that they simply would not be able to survive without it. Life needs energy. Without bacteria taking refuge within towering tube worms, the worms would obtain no energy, and life here would be impossible, especially as photosynthesis is not an option down where sunlight cannot reach.

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This example of mutualism is not only vital for the survival of the tube worms and mussels. It also supports an abundance of other organisms that rely on the tube worms, and thus form complex communities at vents and cold seeps that simply would not exist if not for the ‘worm-bacteria’ symbiosis. Crabs, eels, isopods, and fishes all use tube worms and mussels as sources of food, or shelter. These animals are called associates.