22. The relationship between corals and zooxanthellae is best described as ______.

A. Amensalism
B. Mutualism
C. Parasitism
D. Commensalism

B. Mutualism

To determine the relationship between corals and zooxanthellae, we need to understand the definitions and concepts associated with each of the options: amensalism, mutualism, parasitism, and commensalism.

Amensalism refers to a relationship where one organism is harmed or inhibited, while the other is unaffected. Mutualism is a relationship where both organisms benefit from each other. Parasitism occurs when one organism benefits at the expense of the other, causing harm. Commensalism is a relationship in which one organism benefits, and the other is neither helped nor harmed.

Corals and zooxanthellae have a mutualistic relationship. Zooxanthellae are symbiotic algae that live within the tissues of corals. The coral provides a protected environment and nutrients for the zooxanthellae, while the zooxanthellae photosynthesize and provide food for the coral through the transfer of energy-rich compounds. Both organisms benefit from this relationship, making mutualism the best description of their relationship.

Therefore, the correct answer is B. Mutualism.