The imperial shrimp, Periclimenes imperator, is sometimes seen riding on the back of large sea cucumbers. The shrimp gets to move around to areas with food and expend very little energy, while the sea cucumber is unaffected. What type of relationship is this, and why?

A. Commensalism, because one species benefits
B. Mutualism, because both species benefit
C. Parasitism, because one species is harmed
D. Predation, because one species dies