Practice question

Clownfish and sea anemones illustrate defensive mutualism because each partner can reduce the other’s exposure to enemies while receiving additional benefits. Clownfish acclimate to the anemone’s mucus and nematocysts, allowing them to shelter among tentacles that deter many predators. The fish defend the anemone from butterflyfishes, remove parasites or debris, ventilate it by swimming, and supply nitrogenous waste that can enhance productivity of the anemone and its symbiotic algae. The interaction therefore contains resource exchange, but protection is a prominent reciprocal service. It is not simply commensalism because experiments show benefits to anemone growth, nutrition, or survival in many species pairs. “Obligate” describes dependence rather than function, and dependence varies: some clownfish require host anemones in nature, whereas many anemones survive without clownfish. Calling the interaction resource-based alone overlooks the shelter and enemy-defense mechanisms. Ecological classification follows measured net fitness effects, which can vary among clownfish–anemone combinations and environmental settings.