Which of the following pairs of organisms is in a mutualistic relationship?

Step 1: Analyze the question We need the pair of organisms in which both partners benefit (mutualism).

Step 2: Key concept Mutualism is a close association between two different species in which both benefit. The other relationships to compare it with:

• Parasitism: one organism (the parasite) benefits while the host is harmed.
• Commensalism: one benefits while the other is neither helped nor harmed.

Step 3: Examine each option

• A. Hydra and Sea anemone: These are both cnidarians, and there is no known association between them. This is not a mutualistic pair.
• B. Hermit crab and Sea anemone: The sea anemone attaches to the shell the hermit crab lives in. The anemone's stinging tentacles protect the crab from predators, and the anemone gets transported to new feeding areas and picks up scraps of the crab's food. Both benefit, so this is mutualism.
• C. Dodder plant and Cocoa plant: Dodder (Cuscuta) is a parasitic climber that takes nutrients from its host. This is parasitism.
• D. Mistletoe and Kolanut: Mistletoe is a partial parasite that takes water and minerals from the host tree. This is parasitism.

Step 4: Answer The correct option is B. Hermit crab and Sea anemone.

Exam tip: Remember the classic mutualism examples: hermit crab and sea anemone, nitrogen-fixing bacteria (Rhizobium) and legumes, algae and fungi in lichens, and cellulose-digesting protozoa in termites. Dodder, mistletoe, tapeworm, and Plasmodium are parasitism examples.