Symbiosis. Many would say this word describes two living things that benefit each other. While this is a common understanding of the term, in biology, symbiosis has a slightly more broad definition. Symbiosis (as defined by Western science), is an intimate association between two or more organisms, typically a very specialized association that has evolved over millions of years. For example, many people are familiar with the mutually beneficial association between clownfish and anemones. However, there also exists a clownfish parasite that eats the fish’s tongue and mooches off its food. This too is symbiosis; there must be a close relationship, but not everyone necessarily benefits.
There are 3 main types of symbiosis. Categorizing a relationship into these 3 types depends on who gets something out of the deal.
- (+/+) mutualism (what most people think of when they hear symbiosis)
- (+/o) commensalism
- (+/-) parasitism
The plus (+), minus (-), and zero (o) symbols represent the impact on each of the organisms involved. A plus means that the organism benefits from the relationship, a minus means they are negatively affected by it, and a zero means they are not significantly affected by it either way.
You may have noticed that there are more possible combinations of pluses, minuses, and zeros not listed above. Below are the rest of them, though these are debated as to whether they are truly forms of symbiosis or not. This is because they are too broad or unspecific; just because a seagull may benefit from eating your french fries does not mean they have a close evolutionary relationship with potatoes.
When I first heard about symbiosis in school, I was ecstatic. Not only was I learning about amazing coadaptations, but every symbiotic relationship could be satisfyingly placed into one of those neat little categories, or so I thought. It turned out that the more I learned, the more I researched in the lab, and the more I talked to other scientists, the messier these categorizations became (we will explore some of these “exceptions to the rule” in the lesson below). These counterexamples were important because I realized something that to me was quite profound, and something that I think we should teach every student: your textbook is lying to you.
Textbooks and other learning tools can be helpful, but they are also biased and simplified. As an educator, I want my students to learn earlier than I did that biology is messy and breaks the rules. Below are a couple activities you can use to guide students through this concept, best for students who have already been introduced to the idea of symbiosis.
Symbiosis is a great topic for a card sorting activity (you can find an example at https://learn.islandwood.org/ab-sorting/).
Make cards with different symbiotic relationships (examples below), along with some context regarding the nature of the relationship with text and/or pictures. Students may also brainstorm ideas and create the cards themselves. Some examples can be classic or “easy” relationships to sort, others can be a bit trickier or in some of the “contested” categories of symbiosis. Students will sort the cards into the 3 main categories of symbiosis (mutualism, commensalism, parasitism). This is best done in groups to facilitate discussion, but students can also complete their sorts individually and then compare afterwards.
Below are some classic examples to get started:
- Epiphytes on trees (o/+)
- Barnacles and whales (+/o)
- Clownfish and anemones (+/+)
- Bees and flowers (+/+)
- Lichen/algae and fungus (+/+)
- Wasps and caterpillars (+/-)
- Dogs and fleas (+/-)
These are some much more contested relationships that will be explored in the next activity:
- Water buffalo and oxpeckers (?)
- Tapeworms and humans (?)
- Leaf cutter ants (?)
- Bacteria and humans (?)
Hopefully, the card sort will spark active discussion. There may be disagreement as to how things should be categorized, and that’s great, because it means students will have to dig deeper into their reasoning. During and after the sort, ask students to justify how they sorted their cards, and ask students with different answers to figure out why they disagree. Then, you can open it up to a large group discussion with the next activity to further challenge student thinking.
The following are case studies of classic symbiotic relationships that are more complicated than they may first appear. Each case includes 1) theme or lesson to discuss, 2) the example relationship, and 4) an explanation of the nuance behind why it is a contested example. You might have students re-sort their cards from activity 1 after they have discussed these case studies.
Theme 1: Categorization is subjective and depends on the observer
Example: Oxpecker and Bison. A supposed poster child of mutualism, the oxpecker bird receives nutrition by eating insects and other pests off of the skin of bison, giving it some relief and health benefits.
Nuance: Scientists have found that the oxpeckers are in fact not eating parasites, but instead picking at the bison's wounds and drinking their blood, perhaps making the birds a parasite themselves. (P.S. the same goes for remoras and sharks; watch a video about this at https://youtu.be/yzDWWzDenZQ)
Theme 2: Defining the nature of symbiotic relationships means defining values of good and bad
Example: Tapeworms. These are a well known parasite that live in the intestines of vertebrates such as ourselves, where they absorb food through their skin and deprive the host of nutrients.
Nuance: Tapeworms were used in the early 1900s as a weight loss treatment. While this practice is no longer recommended for a number of reasons, tapeworms were at one point seen as a helpful remedy--a mutualistic companion. Nothing about their nature changed, it was simply our perception of them that shifted. This really highlights the impact of societal norms on the perception of the natural world. Weight loss can be a sensitive subject for some people, so be sure that you present this example with care and compassion, and do not use it if you know any of your students would be uncomfortable. The use of leeches for medical purposes is a similar example that can be used as a substitute to illustrate the same point.
Theme 3: Symbiosis is not always simple partnership between two organisms
Example: Leaf cutter ants. Also known as attine ants, these insects cut leaves off of plants and bring them back to their colony. Without further observation, this may appear to be a simple case of herbivory, one of the contested cases of symbiosis listed at the top of this article.
Nuance: This example of symbiosis actually involves at least 6 different organisms, as diagrammed below—and this doesn’t even include the plants the ants eat! A red arrow on the diagram represents a negative or parasitic relationship, and a green arrow represents a positive or mutualistic one.
Diagram by Michael Poulsen
Microbes are great at breaking the rules. There are many surprising examples of bacteria switching between mutualism and parasitism with their hosts, or acting as both a parasite and a partner at the same time. This (https://courses.lumenlearning.com/boundless-microbiology/chapter/microbial-symbioses/) is a great resource to get started, but I highly recommend further exploring the world of symbiotic microbes with your students!
Categories can be useful--they make complicated concepts more simple, universal, and easier to understand--but there are often exceptions. In biology, the exceptions are sometimes the most interesting things to study. Really, every example of symbiosis could be more carefully examined. Is one organism both a benefit and a detriment to another? If it’s mutualism, does one organism benefit more? What environmental factors might change the nature of this relationship? How did this relationship evolve and how might it continue to change?
This uncertainty is not proof that science is broken, it’s proof that science works. To me, science is about continuing to learn, and it cannot be successful unless it is routinely questioned, retried, and modified. When a discovery breaks the scientific norm, it should be a moment of triumph and growth for the field. When this happens, it is also a chance for us to question our own assumptions and biases. It is critical that educators challenge the idea that relationships are defined and finite no matter the discipline. We will always have something to learn from the natural world, and we should always be thinking about how we can better understand and protect it. Now that is symbiosis.