Millions of species roam planet Earth, and they have to figure out how to share resources such as food and shelter. Creatures interact with one other in a variety of ways, and these relationships are known all together as symbiosis.

There four main symbiotic relationships: mutualism, commensalism, parasitism, and competition.

To explore these relationships, let's consider a natural ecosystem such as the ocean. Oceanic environments are known for their wide variety of species. Imagine you are on a diving trip to explore the warm waters of the Pacific or Indian Oceans. You would likely spot an excellent example of mutualism: the relationship between clownfish and sea anemones.

Mutualism

In a mutualistic relationship, both species benefit. Sea anemones live attached to the surface of coral reefs. They trap their prey with stinging cells, which are located on their tentacles. The cells release poisons when a small animal touches an anemone's tentacle. This paralyzes the stung animal, allowing the anemone to easily bring the animal into its mouth.

Other fish can be hurt by these toxic stings. However, the clownfish release a substance in the mucus covering their bodies that stops the firing of the stinging cells. This allows the clownfish to swim comfortably between the anemones' tentacles. It creates 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.

Commensalism

As we continue our voyage, we discover the commensalistic relationship between barnacles and humpback whales (Megaptera novaeangliae). Commensalism happens when one species lives with, on or in another species, known as the host. The host species neither benefits from nor is harmed by the relationship. For example, various species of barnacles attach themselves to the skin of whales. 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.

Parasitism

Of course, some symbiotic relationships do cause harm, like 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 relationship, its ability to have babies is greatly diminished.

Competition

The last example of symbiosis we will explore on our imaginary dive is competition, or the struggle among organisms for the same limited resources in an ecosystem. Competition can happen between members of the same species, called intraspecific competition, and between different species, known as interspecific competition. An example of interspecific competition in the ocean is the relationship between coral and sponges. Sponges are ancient sea organisms that are abundant in coral reefs. If they become too successful, they take much of the food and other resources that coral need to survive. Sponges may beat out coral for resources in the short term, but if too many coral die, the reef itself becomes damaged. That is because coral are responsible for building reefs out of calcium, and if coral disappear, the reef will too. Sponges may therefore start to die off until the reef is balanced again.

All this shows how symbiotic relationships can help us understand the health of an ecosystem. Large parts of coral reefs are damaged or dead because of increases in ocean temperature due to climate change. The temperature increase causes coral to eject the algae that live within them. Without their algae, the coral turn white and die. This loss of symbiosis is an early sign of declining coral health, and shows how humans can negatively affect an ecosystem. 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."