Which of the following have species that can establish a symbiotic relationship with other organisms?
1. Cnidarians
2. Fungi
3. Protozoa
Select the correct answer using the code given below:

Correct Option: 1, 2 and 3

A symbiotic relationship refers to a close, long-term biological interaction between two different biological organisms. Symbiosis can be mutualistic (both organisms benefit), commensal (one benefits without harming the other), or parasitic (one benefits at the expense of the other).

Let us analyze each of the given organisms to determine whether they can form symbiotic relationships:

1. Cnidarians:
Yes, many cnidarians form symbiotic relationships. A classic and highly important example is reef-building corals (which are cnidarians) living in a mutualistic relationship with photosynthetic microalgae known as zooxanthellae (dinoflagellates). The algae provide nutrients via photosynthesis, while the coral provides a protected environment and essential compound nutrients. Additionally, sea anemones (also cnidarians) frequently form mutualistic relationships with clownfish.

2. Fungi:
Yes, fungi are famous for establishing symbiotic relationships. The most well-known examples include:
- Lichens: A symbiotic partnership between a fungus and a photosynthetic partner (an alga or a cyanobacterium).
- Mycorrhizae: A mutualistic association between fungi and the roots of vascular plants, where the fungus assists in nutrient and water absorption while receiving carbohydrates from the plant.

3. Protozoa:
Yes, many species of protozoa enter into symbiotic associations with other organisms. For instance, cellulose-digesting protozoa live inside the digestive tracts of termites and ruminants (like cows), allowing them to digest complex plant materials. Additionally, some protozoa (such as Paramecium bursaria) host endosymbiotic green algae (zoochlorellae).

Since all three groups (Cnidarians, Fungi, and Protozoa) contain species capable of establishing symbiotic relationships, the correct choice is 1, 2 and 3.